\section{Dynamic graph algorithms}
-In many applications, we often need to solve a~certain graph problem for a~sequence
-of graphs that differ only a~little, so recomputing the solution from scratch for
-every graph would be a~waste of time. In such cases, we usually turn our attention
-to \df{dynamic graph algorithms.} A~dynamic algorithm is in fact a~data structure
-that remembers a~graph and offers operations that modify the structure of the graph
-and also operations that query the result of the problem for the current state
-of the graph. A~typical example of a~problem of this kind is dynamic
-maintenance of connected components:
+In many applications, we often need to solve a~certain graph problem for a~sequence of graphs that
+differ only a~little, so recomputing the solution for every graph from scratch would be a~waste of
+time. In such cases, we usually turn our attention to \df{dynamic graph algorithms.} A~dynamic
+algorithm is in fact a~data structure that remembers a~graph. It offers operations that modify the
+structure of the graph and also operations that query the result of the problem for the current
+state of the graph. A~typical example of a~problem of this kind is dynamic maintenance of connected
+components:
\problemn{Dynamic connectivity}
Maintain an~undirected graph under a~sequence of the following operations:
\:$\<Insert>(G,u,v)$ --- Insert an~edge $uv$ to~$G$ and return its unique
identifier. This assumes that the edge did not exist yet.
\:$\<Delete>(G,e)$ --- Delete an~edge specified by its identifier from~$G$.
-\:$\<Connected>(G,u,v)$ --- Test if $u$ and~$v$ are in the same connected component of~$G$.
+\:$\<Connected>(G,u,v)$ --- Test if vertices $u$ and~$v$ are in the same connected component of~$G$.
\endlist
\para
Kruskal's algorithm in Section \ref{classalg}. At that time, we did not need to delete
any edges from the graph, which makes the problem substantially easier. This special
case is customarily called an~\df{incremental} or \df{semidynamic} graph algorithm.
-We mentioned the Disjoint Set Union data structure of Tarjan and van Leeuwen (Theorem \ref{dfu})
-which can be used for that: Connected components are represented as an~equivalence classes.
+We mentioned the Disjoint Set Union data structure of Tarjan (Theorem \ref{dfu})
+which can be used for that: Connected components are represented by equivalence classes.
Queries on connectedness translate to \<Find>, edge insertions to \<Find>
followed by \<Union> if the new edge joins two different components. This way,
a~sequence of $m$~operations starting with an~empty graph on $n$~vertices is
bound in the cell-probe model which is stronger than RAM with $\O(\log n)$-bit
words.
-The edges that have triggered the \<Union>s form a~spanning forest of the current graph.
-So far, all known algorithms for dynamic connectivity maintain some sort of a~spanning
-tree. This suggests that a~dynamic MST algorithm could be obtained by modifying the
-dynamic connectivity algorithms. This will indeed turn out to be true. Semidynamic MST
-is easy to achieve even in the few pages of this section, but making it fully dynamic will require
-more effort, so we will review some of the required building blocks before going into that.
+\paran{Dynamic MSF}%
+In this chapter, we will focus on the dynamic version of the minimum spanning forest.
+This problem seems to be intimately related to the dynamic connectivity. Indeed, all known
+algorithms for dynamic connectivity maintain some sort of a~spanning forest. For example, in the
+incremental algorithm we have just mentioned, this forest is formed by the edges that have
+triggered the \<Union>s. This suggests that a~dynamic MSF algorithm could be obtained by modifying
+the mechanics of the data structure to keep the forest minimum. This will really turn out to be true, although we cannot
+be sure that it will lead to the most efficient solution possible --- as of now, the known lower
+bounds are very far.
+
+Incremental MST will be easy to achieve even in the few pages of this section, but making it fully
+dynamic will require more effort, so we will review some of the required building blocks before
+going into that.
We however have to answer one important question first: What should be the output of
-our MSF data structure? Adding an~operation that would return the MSF of the current
-graph is of course possible, but somewhat impractical as this operation has to
+our MSF data structure? Adding an~operation that returns the MSF of the current
+graph would be of course possible, but somewhat impractical as this operation would have to
spend $\Omega(n)$ time on the mere writing of its output. A~better way seems to
be making the \<Insert> and \<Delete> operations report the list of modifications
of the MSF implied by the change in the graph.
Let us see what happens when we \<Insert> an~edge~$e$ to a~graph~$G$ with its minimum spanning
forest~$F$, obtaining a~new graph~$G'$ with its MSF~$F'$. If $e$~connects two components of~$G$ (and
therefore also of~$F$), we have to add~$e$ to~$F$. Otherwise, one of the following cases happens:
-Either $e$~is $F$-heavy and so the forest~$F$ is also the MSF of the new graph. Or it is $F$-light
+Either $e$~is $F$-heavy and thus the forest~$F$ is also the MSF of the new graph. Or it is $F$-light
and we have to modify~$F$ by exchanging the heaviest edge~$f$ of the path $F[e]$ with~$e$.
-Correctness of the former case follows immediately from the Theorem on Minimality by order
-(\ref{mstthm}), because all $F'$-light would be also $F$-light, which is impossible as $F$~was
+Correctness of the former case follows immediately from the Minimality Theorem (\ref{mstthm}),
+because any $F'$-light would be also $F$-light, which is impossible as $F$~was
minimum. In the latter case, the edge~$f$ is not contained in~$F'$ because it is the heaviest
on the cycle $F[e]+e$ (by the Red lemma, \ref{redlemma}). We can now use the Blue lemma
(\ref{bluelemma}) to prove that it should be replaced with~$e$. Consider the tree~$T$
of~$F$ that contains both endpoints of the edge~$e$. When we remove~$f$ from~$F$, this tree falls
-apart to two components $T_1$ and~$T_2$. The edge~$f$ was the lightest edge of the cut~$\delta_G(T_1)$
+apart to two components $T_1$ and~$T_2$. The edge~$f$ was the lightest in the cut~$\delta_G(T_1)$
and $e$~is lighter than~$f$, so $e$~is the lightest in~$\delta_{G'}(T_1)$ and hence $e\in F'$.
+\looseness=1 %%HACK%%
A~\<Delete> of an~edge that is not contained in~$F$ does not change~$F$. When we delete
an~MSF edge, we have to reconnect~$F$ by choosing the lightest edge of the cut separating
disconnected.
The idea of reporting differences in the MSF indeed works very well. We can summarize
-what we have shown in the following lemma and use it to define the dynamic MSF.
+what we have shown by the following lemma and use it to define the dynamic MSF.
\lemma
An~\<Insert> or \<Delete> of an~edge in~$G$ causes at most one edge addition, edge
and its minimum spanning forest under a~sequence of the following operations:
\itemize\ibull
\:$\<Init>(n)$ --- Create a~graph with $n$~isolated vertices $\{1,\ldots,n\}$.
-\:$\<Insert>(G,u,v)$ --- Insert an~edge $uv$ to~$G$. Return its unique
+\:$\<Insert>(G,u,v,w)$ --- Insert an~edge $uv$ of weight~$w$ to~$G$. Return its unique
identifier and the list of additions and deletions of edges in $\msf(G)$.
\:$\<Delete>(G,e)$ --- Delete an~edge specified by its identifier from~$G$.
Return the list of additions and deletions of edges in $\msf(G)$.
\endlist
-\paran{Semidynamic MSF}%
-To obtain a~semidynamic MSF algorithm, we need to keep the forest in a~data structure that
+\paran{Incremental MSF}%
+To obtain an~incremental MSF algorithm, we need to keep the forest in a~data structure that
supports search for the heaviest edge on the path connecting a~given pair
of vertices. This can be handled efficiently by the Link-Cut trees of Sleator and Tarjan:
There is a~data structure that represents a~forest of rooted trees on~$n$ vertices.
Each edge of the forest has a~weight drawn from a~totally ordered set. The structure
supports the following operations in time $\O(\log n)$ amortized:\foot{%
-The Link-Cut trees offer many other operations, but we do not mention them
-as they are not needed in our application.}
+The Link-Cut trees can offer a~plethora of other operations, but we do not mention them
+as they are not needed for our problem.}
\itemize\ibull
\:$\<Parent>(v)$ --- Return the parent of~$v$ in its tree or \<null> if $v$~is a~root.
\:$\<Root>(v)$ --- Return the root of the tree containing~$v$.
\:$\<Weight>(v)$ --- Return the weight of the edge $(\<Parent(v)>,v)$.
-\:$\<PathMax>(v)$ --- Return the vertex~$w$ closest to $\<Root>(v)$ such that the edge
- $(\<Parent>(w),w)$ is the heaviest of those on the path from the root to~$v$.
+\:$\<PathMax>(v)$ --- Return the vertex~$u$ closest to $\<Root>(v)$ such that the edge
+ $(\<Parent>(u),u)$ is the heaviest of those on the path from the root to~$v$.
If more edges have the maximum weight, break the tie arbitrarily.
If there is no such edge ($v$~is the root itself), return \<null>.
-\:$\<Link>(u,v,x)$ --- Connect the trees containing $u$ and~$v$ by an~edge $(u,v)$ of
- weight~$x$. Assumes that $u~$is a tree root and $v$~lies in a~different tree.
+\:$\<Link>(u,v,w)$ --- Connect the trees containing $u$ and~$v$ by an~edge $(u,v)$ of
+ weight~$w$. Assumes that $v~$is a tree root and $u$~lies in a~different tree.
\:$\<Cut>(v)$ --- Split the tree containing the non-root vertex $v$ to two trees by
removing the edge $(\<Parent>(v),v)$. Returns the weight of this edge.
-\:$\<Evert>(v)$ --- Modify the orientations of the edges in the tree containing~$v$
- to make~$v$ the tree's root.
+\:$\<Evert>(v)$ --- Modify the orientations of edges to make~$v$ the root of its tree.
\endlist
%% \>Additionally, all edges on the path from~$v$ to $\<Root>(v)$ can be enumerated in
Once we have this structure, we can turn our ideas on updating of the MSF to
an~incremental algorithm:
-\algn{\<Insert> in a~semidynamic MSF}
+\algn{\<Insert> in an~incremental MSF}
\algo
\algin A~graph~$G$ with its MSF $F$ represented as a~Link-Cut forest, an~edge~$uv$
with weight~$w$ to be inserted.
\:$\<Evert>(u)$. \cmt{$u$~is now the root of its tree.}
\:If $\<Root>(v) \ne u$: \cmt{$u$~and~$v$ lie in different trees.}
-\::$\<Link>(u,v,w)$. \cmt{Connect the trees.}
+\::$\<Link>(v,u,w)$. \cmt{Connect the trees.}
\::Return ``$uv$ added''.
\:Otherwise: \cmt{both are in the same tree}
\::$y\=\<PathMax>(v)$.
\::$x\=\<Parent>(y)$. \cmt{Edge~$xy$ is the heaviest on $F[uv]$.}
\::If $\<Weight>(y) > w$: \cmt{We have to exchange~$xy$ with~$uv$.}
-\:::$\<Cut>(y)$, $\<Evert>(v)$, $\<Link>(v,y,w)$.
+\:::$\<Cut>(y)$, $\<Evert>(v)$, $\<Link>(u,v,w)$.
\:::Return ``$uv$~added, $xy$~removed''.
\::Otherwise return ``no changes''.
\algout The list of changes in~$F$.
Deletion of edges that do not belong to the MSF is trivial (we do not
need to change anything) and so is deletion of bridges (we just remove the bridge
from the Link-Cut tree, knowing that there is no edge to replace it). The hard part
-is the search for replacement edges after an~edge of the MSF is deleted.
+is the search for replacement edges after an~edge belonging to the MSF is deleted.
-As this very problem has to be solved by algorithms for fully dynamic connectivity,
-we will take a~look on them first.
+This very problem also has to be solved by algorithms for fully dynamic connectivity,
+we will take a~look at them first.
%--------------------------------------------------------------------------------
\section{Eulerian Tour trees}
+An~important stop on the road to fully dynamic algorithms has the name \df{Eulerian Tour trees} or
+simply \df{ET-trees}. It is a~representation of forests introduced by Henzinger and King
+\cite{henzinger:randdyn} in their randomized dynamic algorithms. It is similar to the Link-Cut
+trees, but it is much simpler and instead of path operations it offers efficient operations on
+subtrees. It is also possible to attach auxiliary data to vertices and edges of the original tree.
+
+\defn\id{eulseq}%
+Let~$T$ be a~rooted tree. We will call a~sequence of vertices of~$T$ its \df{Eulerian Tour sequence (ET-sequence)}
+if it lists the vertices visited by the depth-first traversal of~$T$.
+More precisely, it can be generated by the following procedure $\<ET>(v)$
+when it is invoked on the root of the tree:
+\algo
+\:Record~$v$ in the sequence.
+\:For each son~$w$ of~$v$:
+\::Call $\<ET>(w)$.
+\::Record~$w$.
+\endalgo
+\>A~single tree can have multiple ET-sequences, corresponding to different orders in which the
+sons can be enumerated in step~2.
+
+In every ET-sequence, one of the occurrences of each vertex is defined as its \df{active occurrence} and
+it will be used to store auxiliary data associated with that vertex.
+
+\obs
+An~ET-sequence contains a~vertex of degree~$d$ exactly $d$~times except for the root which
+occurs $d+1$ times. The whole sequence therefore contains $2n-1$ elements. It indeed describes the
+order of vertices on an~Eulerian tour in the tree with all edges doubled. Let us observe what happens
+to an~ET-sequence when we modify the tree. (See the picture.)
+
+When we \em{delete} an~edge $uv$ from the tree~$T$ (let $u$~be the parent of~$v$), the sequence
+$AuvBvuC$ (with no~$u$ nor~$v$ in~$B$) splits to two sequences $AuC$ and $vBv$.
+If there was only a~single occurrence of~$v$, then $v$~was a~leaf and thus the sequence
+transforms from $AuvuC$ to $AuC$ and $v$~alone.
+
+\em{Changing the root} of the tree~$T$ from~$v$ to~$w$ changes its ET-sequence from $vAwBwCv$ to $wBwCvAw$.
+If $w$~was a~leaf, the sequence changes from $vAwCv$ to $wCvAw$. If $vw$ was the only edge of~$T$,
+the sequence $vw$ becomes $wv$. Note that this works regardless of the possible presence of~$w$ inside~$B$.
+
+\em{Joining} the roots of two trees by a~new edge makes their ET-sequences $vAv$ and~$wBw$
+combine to $vAvwBwv$. Again, we have to handle the cases when $v$ or~$w$ has degree~1 separately:
+$v$~and~$wBw$ combine to $vwBwv$, and $v$~with~$w$ makes $vwv$.
+
+\float{\valign{\vfil#\vfil\cr
+\hbox{\epsfbox{pic/ettree.eps}}\cr
+\noalign{\qquad\quad}
+ \halign{#\hfil\cr
+ $T_1: 0121034546474308980$,~~$T_2: aba$. \cr
+ $T_1-34: 01210308980, 4546474$. \cr
+ $T_1\hbox{~rooted at~3}: 3454647430898012103$. \cr
+ $T_1+0a+T_2$: $0121034546474308980aba0$. \cr
+ }\cr
+}}{Trees and their ET-sequences}
+
+If any of the occurrences that we have removed from the sequence was active, there is always
+a~new occurrence of the same vertex that can stand in its place and inherit the auxiliary data.
+
+\para
+The ET-trees will store the ET-sequences as $(a,b)$-trees with the parameter~$a$ set upon
+initialization of the structure and with $b=2a$. We know from the standard theorems of $(a,b)$-trees
+(see for example \cite{clrs}) that the depth of a~tree with $n$~leaves is always $\O(\log_a n)$
+and that all basic operations including insertion, deletion, search, splitting and joining the trees
+run in time $\O(b\log_a n)$ in the worst case.
+\looseness=-1 %%HACK%%
+
+We will use the ET-trees to maintain a~spanning forest of the dynamic graph. The auxiliary data of
+each vertex will hold a~list of edges incident with the given vertex, that do not lie in the
+forest. Such edges are usually called the \df{non-tree edges.}
+
+\defn
+\df{Eulerian Tour trees (ET-trees)} are a~data structure that represents a~forest of trees and a~set of non-tree
+edges associated with the vertices of the forest. To avoid confusion, we will distinguish between
+\df{original} vertices and edges (of the given trees) and the vertices and edges of the
+data structure. The structure consists of:
+\itemize\ibull
+\:A~collection of $(a,b)$-trees of some fixed parameters $a$ and~$b$.
+ Each such tree corresponds to one of the original trees~$T$. Its
+ leaves (in the usual tree order) correspond to the elements
+ of an~ET-sequence for~$T$. Each two consecutive leaves $u$ and~$v$ are separated
+ by a~unique key stored in an~internal vertex of the $(a,b)$-tree. This key is used to represent
+ the original edge~$uv$. Each original edge is therefore kept in both its orientations.
+\:Mappings \<act>, \<edge> and \<twin>:
+ \itemize\icirc
+ \:$\<act>(v)$ maps each original vertex to the leaf containing its active occurrence;
+ \:$\<edge>(e)$ of an~original edge~$e$ is one of the internal keys representing~it;
+ \:$\<twin>(k)$ pairs an~internal key~$k$ with the other internal key of the same original edge.
+ \endlist
+\:A~list of non-tree edges placed in each leaf. The lists are allowed to be non-empty only
+ in the leaves that represent active occurrences of original vertices.
+\:Boolean \df{markers} in the internal vertices that signal presence of a~non-tree
+ edge anywhere in the subtree rooted at the internal vertex.
+\:Counters $\<leaves>(v)$ that contain the number of leaves in the subtree rooted at~$v$.
+\endlist
+
+\defn
+The ET-trees support the following operations on the original trees:
+\itemize\ibull
+\:\<Create> --- Create a~single-vertex tree.
+\:$\<Link>(u,v)$ --- Join two different trees by an~edge~$uv$ and return a~unique identifier
+ of this edge.
+\:$\<Cut>(e)$ --- Split a~tree by removing the edge~$e$ given by its identifier.
+\:$\<Connected>(u,v)$ --- Test if the vertices $u$ and~$v$ lie in the same tree.
+\:$\<Size>(v)$ --- Return the number of vertices in the tree containing the vertex~$v$.
+\:$\<InsertNontree>(v,e)$ --- Add a~non-tree edge~$e$ to the list at~$v$ and return a~unique
+ identifier of this edge.
+\:$\<DeleteNontree>(e)$ --- Delete a~non-tree edge~$e$ given by its identifier.
+\:$\<ScanNontree>(v)$ --- Return a~list of non-tree edges associated with the vertices
+ of the $v$'s tree.
+\endlist
+
+\impl
+We will implement the operations on the ET-trees by translating the intended changes of the
+ET-sequences to operations on the $(a,b)$-trees. The role of identifiers of the original vertices
+and edges will be of course played by pointers to the respective leaves and internal keys of
+the $(a,b)$-trees.
+
+\<Cut> of an~edge splits the $(a,b)$-tree at both internal keys representing the given edge
+and joins them back in the different order.
+
+\<Link> of two trees can be accomplished by making both vertices the roots of their trees first
+and joining the roots by an~edge afterwards. Re-rooting involves splits and joins of $(a,b)$-trees.
+As we can split at any occurrence of the new root vertex, we will use the active occurrence
+which we remember. Linking of the roots is translated to joining of the $(a,b)$-trees.
+
+\<Connected> follows parent pointers from both $u$ and~$v$ to the roots of their trees.
+Then it checks if the roots are equal.
+
+\<Size> finds the root~$r$ and returns $\<leaves>(r)$.
+
+\<InsertNontree> finds the leaf $\<act>(v)$ containing the list of $v$'s non-tree edges
+and inserts the new edge there. The returned identifier will consist from the pointer to
+the edge and the vertex in whose list it is stored. Then we have to recalculate the markers
+on the path from $\<act>(v)$ to the root. \<DeleteNontree> is analogous.
+
+Whenever any other operation changes a~vertex of the tree, it will also update its marker and
+counter and, if necessary, the markers and counters on the path to the root.
+
+\<ScanNontree> traverses the tree recursively from the root, but it does not enter the
+subtrees whose roots are not marked.
+
+Analysis of time complexity of the operations is now straightforward:
+
+\thmn{Eulerian Tour trees, Henzinger and Rauch \cite{henzinger:randdyn}}\id{etthm}%
+The ET-trees perform the operations \<Link> and \<Cut> in time $\O(a\log_a n)$, \<Create>
+in $\O(1)$, \<Connected>, \<Size>, \<InsertNontree>, and \<DeleteNontree> in $\O(\log_a n)$, and
+\<ScanNontree> in $\O(a\log_a n)$ per edge reported. Here $n$~is the number of vertices
+in the original forest and $a\ge 2$ is an~arbitrary constant.
+
+\proof
+We set $b=2a$. Our implementation performs $\O(1)$ operations on the $(a,b)$-trees
+per operation on the ET-tree, plus $\O(1)$ other work. We apply the standard theorems
+on the complexity of $(a,b)$-trees \cite{clrs}.
+\qed
+
+\examplen{Connectivity acceleration}\id{accel}%
+In most cases, the ET-trees are used with $a$~constant, but sometimes choosing~$a$ as a~function
+of~$n$ can also have its beauty. Suppose that there is a~data structure which maintains an~arbitrary
+spanning forest of a~dynamic graph. Suppose also that the structure works in time $\O(\log^k n)$
+per operation and that it reports $\O(1)$ changes in the spanning forest for every change
+in the graph. If we keep the spanning forest in ET-trees with $a=\log n$, the updates of the
+data structure cost an~additional $\O(\log^2 n / \log\log n)$, but connectivity queries accelerate to $\O(\log
+n/\log\log n)$.
+
+\paran{ET-trees with weights}
+In some cases, we will also need a~representation of weighted graphs and enumerate the non-tree
+edges in order of their increasing weights (in fact, it will be sufficient to find the
+lightest one, remove it and iterate). This can be handled by a~minute modification of the
+ET-trees.
+
+The tree edges will remember their weight in the corresponding internal keys of the ET-tree.
+We replace each list of non-tree edges by an~$(a,b)$-tree keeping the edges sorted by weight.
+We also store the minimum element of that tree separately, so that it can be accessed in constant
+time. The boolean \em{marker} will then become the minimum weight of a~non-tree edge attached to the
+particular subtree, which can be recalculated as easy as the markers can. Searching for the
+lightest non-tree edge then just follows the modified markers.
+
+The time complexities of all operations therefore remain the same, with a~possible
+exception of the operations on non-tree edges, to which we have added the burden of
+updating the new $(a,b)$-trees. This however consists of $\O(1)$ updates per a~single
+call to \<InsertNontree> or \<DeleteNontree>, which takes $\O(a\log_a n)$ time only.
+We can therefore conclude:
+
+\corn{Weighted ET-trees}\id{wtet}%
+In weighted ET-trees, the operations \<InsertNontree> and \<DeleteNontree> have time
+complexity $\O(a\log_a n)$. All other operations take the same time as indicated by Theorem
+\ref{etthm}.
+
+%--------------------------------------------------------------------------------
+
+\section{Dynamic connectivity}
+
+The fully dynamic connectivity problem has a~long and rich history. In the 1980's, Frederickson \cite{frederickson:dynamic}
+has used his topological trees to construct a~dynamic connectivity algorithm of complexity $\O(\sqrt m)$ per update and
+$\O(1)$ per query. Eppstein et al.~\cite{eppstein:sparsify} have introduced a~sparsification technique which can bring the
+updates down to $\O(\sqrt n)$. Later, several different algorithms with complexity on the order of $n^\varepsilon$
+were presented by Henzinger and King \cite{henzinger:mst} and also by Mare\v{s} \cite{mares:dga}.
+A~polylogarithmic time bound was first reached by the randomized algorithm of Henzinger and King \cite{henzinger:randdyn}.
+The best result known as of now is the $\O(\log^2 n)$ time deterministic algorithm by Holm,
+de~Lichtenberg and Thorup \cite{holm:polylog}, which will we describe in this section.
+
+The algorithm will maintain a~spanning forest~$F$ of the current graph~$G$, represented by an~ET-tree
+which will be used to answer connectivity queries. The edges of~$G\setminus F$ will be stored as~non-tree
+edges in the ET-tree. Hence, an~insertion of an~edge to~$G$ either adds it to~$F$ or inserts it as non-tree.
+Deletions of non-tree edges are also easy, but when a~tree edge is deleted, we have to search for its
+replacement among the non-tree edges.
+
+To govern the search in an~efficient way, we will associate each edge~$e$ with a~level $\ell(e) \le
+L = \lfloor\log_2 n\rfloor$. For each level~$i$, we will use~$F_i$ to denote the subforest
+of~$F$ containing edges of level at least~$i$. Therefore $F=F_0 \supseteq F_1 \supseteq \ldots \supseteq F_L$.
+We will maintain the following \em{invariants:}
+
+{\narrower
+\def\iinv{{\bo I\the\itemcount~}}
+\numlist\iinv
+\:$F$~is the maximum spanning forest of~$G$ with respect to the levels. (In other words,
+if $uv$ is a~non-tree edge, then $u$ and~$v$ are connected in~$F_{\ell(uv)}$.)
+\:For each~$i$, the components of~$F_i$ have at most $\lfloor n/2^i \rfloor$ vertices each.
+(This implies that it does not make sense to define~$F_i$ for $i>L$, because it would be empty
+anyway.)
+\endlist
+}
+
+At the beginning, the graph contains no edges, so both invariants are trivially
+satisfied. Newly inserted edges enter level~0, which cannot break I1 nor~I2.
+
+When we delete a~tree edge at level~$\ell$, we split a~tree~$T$ of~$F_\ell$ to two
+trees $T_1$ and~$T_2$. Without loss of generality, let us assume that $T_1$ is the
+smaller one. We will try to find the replacement edge of the highest possible
+level that connects the spanning tree back. From I1, we know that such an~edge cannot belong to
+a~level greater than~$\ell$, so we start looking for it at level~$\ell$. According
+to~I2, the tree~$T$ had at most $\lfloor n/2^\ell\rfloor$ vertices, so $T_1$ has
+at most $\lfloor n/2^{\ell+1} \rfloor$ of them. Thus we can move all level~$\ell$
+edges of~$T_1$ to level~$\ell+1$ without violating either invariant.
+
+We now start enumerating the non-tree edges incident with~$T_1$. Each such edge
+is either local to~$T_1$ or it joins $T_1$ with~$T_2$. We will therefore check each edge
+whether its other endpoint lies in~$T_2$ and if it does, we have found the replacement
+edge, so we insert it to~$F_\ell$ and stop. Otherwise we move the edge one level up. (This
+will be the grist for the mill of our amortization argument: We can charge most of the work on level
+increases and we know that the level of each edge can reach at most~$L$.)
+
+If the non-tree edges at level~$\ell$ are exhausted, we try the same in the next
+lower level and so on. If there is no replacement edge at level~0, the tree~$T$
+remains disconnected.
+
+\impl
+For each level~$\ell$, we will use a~separate ET-tree ${\cal E}_\ell$ with~$a$ set to~2,
+which will represent the forest~$F_\ell$ and the non-tree edges at that particular level.
+Besides operations on the non-tree edges, we also need to find the tree edges of level~$\ell$
+when we want to bring them one level up. This can be accomplished either by modifying the ET-trees
+to attach two lists of edges attached to vertices instead of one, or by using a~second ET-tree.
+
+\algn{Insertion of an~edge}
+\algo
+\algin An~edge $uv$ to insert.
+\:$\ell(uv) \= 0$.
+\:Ask the ET-tree ${\cal E}_0$ if $u$ and~$v$ are in the same component. If they are:
+\::Add $uv$ to the list of non-tree edges in ${\cal E}_0$ at both $u$ and~$v$.
+\:Otherwise:
+\::Add $uv$ to~$F_0$.
+\endalgo
+
+\algn{Deletion of an~edge}
+\algo
+\algin An~edge $uv$ to delete.
+\:$\ell \= \ell(uv)$.
+\:If $uv$ is a~non-tree edge:
+\::Remove $uv$ from the lists of non-tree edges at both $u$ and~$v$ in~${\cal E}_{\ell}$.
+\:Otherwise:
+\::Remove $uv$ from~$F_\ell$ and hence also from $F_0,\ldots,F_{\ell-1}$.
+\::Call $\<Replace>(uv,\ell)$ to get the replacement edge~$f$.
+\::Insert $f$ to~$F_0,\ldots,F_{\ell(f)}$.
+\endalgo
+
+\algn{$\<Replace>(uv,i)$ -- Search for replacement for edge~$uv$ at level~$i$}
+\algo
+\algin An~edge~$uv$ to replace and a~level~$i$ such that there is no replacement
+at levels greater than~$i$.
+\:Let $T_1$ and~$T_2$ be the trees in~$F_i$ containing $u$ and~$v$ respectively.
+\:If $n(T_1) > n(T_2)$, swap $T_1$ with~$T_2$.
+\:Find all level~$i$ edges in~$T_1$ using ${\cal E}_i$ and move them to level~$i+1$.
+\:Enumerate non-tree edges incident with vertices of~$T_1$ and stored in ${\cal E}_i$.
+ For each edge~$xy$, $x\in T_1$, do:
+\::If $y\in T_2$, remove~$xy$ from~${\cal E}_i$ and return it to the caller.
+\::Otherwise increase $\ell(xy)$ by one.
+ \hfil\break
+ This includes deleting~$xy$ from~${\cal E}_i$ and inserting it to~${\cal E}_{i+1}$.
+\:If $i>0$, call $\<Replace>(xy,i-1)$.
+\:Otherwise return \<null>.
+\algout The replacement edge.
+\endalgo
+
+\>As foretold, time complexity will be analysed by amortization on the levels.
+
+\thmn{Fully dynamic connectivity, Holm et al.~\cite{holm:polylog}}\id{dyncon}%
+Dynamic connectivity can be maintained in time $\O(\log^2 n)$ amortized per
+\<Insert> and \<Delete> and in time $\O(\log n/\log\log n)$ per \<Connected>
+in the worst case.
+
+\proof
+The direct cost of an~\<Insert> is $\O(\log n)$ for the operations on the ET-trees
+(by Theorem \ref{etthm}). We will also have the insertion pre-pay all level increases of the new
+edge. Since the levels never decrease, each edge can be brought a~level up at most
+$L=\lfloor\log n\rfloor$ times. Every increase costs $\O(\log n)$ on the ET-tree
+operations, so we pay $\O(\log^2 n)$ for all of them.
+
+A~\<Delete> costs $\O(\log^2 n)$ directly, as we might have to update all~$L$
+ET-trees. Additionally, we call \<Replace> up to $L$ times. The initialization of
+\<Replace> costs $\O(\log n)$ per call, the rest is paid for by the edge level
+increases.
+
+To bring the complexity of the operation \<Connected> from $\O(\log n)$ down to $\O(\log n/\log\log n)$,
+we apply the trick from Example \ref{accel} and store~$F_0$ in an~ET-tree with $a=\max(\lfloor\log n\rfloor,2)$.
+This does not hurt the complexity of insertions and deletions, but allows for faster queries.
+\qed
+
+\rem\id{dclower}%
+An~$\Omega(\log n/\log\log n)$ lower bound for the amortized complexity of the dynamic connectivity
+problem has been proven by Henzinger and Fredman \cite{henzinger:lowerbounds} in the cell
+probe model with $\O(\log n)$-bit words. Thorup has answered by a~faster algorithm
+\cite{thorup:nearopt} that achieves $\O(\log n\log^3\log n)$ time per update and
+$\O(\log n/\log^{(3)} n)$ per query on a~RAM with $\O(\log n)$-bit words. (He claims
+that the algorithm runs on a~Pointer Machine, but it uses arithmetic operations,
+so it does not fit the definition of the PM we use. The algorithm only does not
+need direct indexing of arrays.) So far, it is not known how to extend this algorithm
+to fit our needs, so we omit the details.
+
+%--------------------------------------------------------------------------------
+
+\section{Dynamic spanning forests}\id{dynmstsect}%
+
+Let us turn our attention back to the dynamic MSF.
+Most of the early algorithms for dynamic connectivity also imply $\O(n^\varepsilon)$
+algorithms for dynamic maintenance of the MSF. Henzinger and King \cite{henzinger:twoec,henzinger:randdyn}
+have generalized their randomized connectivity algorithm to maintain the MSF in $\O(\log^5 n)$ time per
+operation, or $\O(k\log^3 n)$ if only $k$ different values of edge weights are allowed. They have solved
+the decremental version of the problem first (which starts with a~given graph and only edge deletions
+are allowed) and then presented a~general reduction from the fully dynamic MSF to its decremental version.
+We will describe the algorithm of Holm, de Lichtenberg and Thorup \cite{holm:polylog}, who have followed
+the same path. They have modified their dynamic connectivity algorithm to solve the decremental MSF
+in $\O(\log^2 n)$ and obtained the fully dynamic MSF working in $\O(\log^4 n)$ per operation.
+
+\paran{Decremental MSF}%
+Turning the algorithm from the previous section to the decremental MSF requires only two
+changes: First, we have to start with the forest~$F$ equal to the MSF of the initial
+graph. As we require to pay $\O(\log^2 n)$ for every insertion, we can use almost arbitrary
+MSF algorithm to find~$F$. Second, when we search for an~replacement edge, we need to pick
+the lightest possible choice. We will therefore use the weighted version of the ET-trees (Corollary \ref{wtet})
+and scan the lightest non-tree edge incident with the examined tree first. We must ensure
+that the lower levels cannot contain a~lighter replacement edge, but fortunately the
+light edges tend to ``bubble up'' in the hierarchy of levels. This can be formalized
+in form of the following invariant:
+
+{\narrower
+\def\iinv{{\bo I\the\itemcount~}}
+\numlist\iinv
+\itemcount=2
+\:On every cycle, the heaviest edge has the smallest level.
+\endlist
+}
+
+\>This immediately implies that we always select the right replacement edge:
+
+\lemma\id{msfrepl}%
+Let $F$~be the minimum spanning forest and $e$ any its edge. Then among all replacement
+edges for~$e$, the lightest one is at the maximum level.
+
+\proof
+Let us consider any two edges $f_1$ and~$f_2$ replacing~$e$. By minimality of~$F$ and the Cycle
+rule (Lemma \ref{redlemma}), each $f_i$ is the heaviest edge on the cycle~$C_i = F[f_i] + f_i$.
+In a~moment, we will show that the symmetric difference~$C$ of these two cycles is again a~cycle.
+This implies that if $f_1$ is heavier than~$f_2$, then $f_1$~is the heaviest edge on~$C$, so
+$\ell(f_1) \le \ell(f_2)$ by I3. Therefore the lightest of all replacement edges must have
+the maximum level.
+
+Why is~$C$ a~cycle:
+Let $F^a$ and~$F^b$ be the trees of~$F-e$ in which the endpoints of~$e$ lie, and for
+every edge~$g$ going between $F^a$ and~$F^b$ let $g^a$ and~$g^b$ be its respective endpoints.
+We know that $C_i$ consists of the path $F[f_i^a,e^a]$, the edge~$e$, the path $F[e^b,f_i^b]$,
+and the edge~$f_i$. Thus~$C$ must contain the paths $F[f_1^a,f_2^a]$ and $F[f_1^b,f_2^b]$ and
+the edges $f_1$ and~$f_2$, which together form a~simple cycle.
+\qed
+
+We now have to make sure that the additional invariant is really observed:
+
+\lemma\id{ithree}%
+After every operation, the invariant I3 is satisfied.
+
+\proof
+When the structure is freshly initialized, I3 is obviously satisfied, as all edges
+are at level~0. Sole deletions of edges (both tree and non-tree) cannot violate I3, so we need
+to check only the replaces, in particular the place when an~edge~$e$ gets its level increased.
+
+For the violation to happen for the first time, $e$~must be the heaviest on
+some cycle~$C$, so by the Cycle rule, $e$~must be non-tree. The increase of
+$\ell(e)$ must therefore take place when~$e$ is considered as a~replacement
+edge incident with some tree~$T_1$ at level $\ell=\ell(e)$. We will pause the
+computation just before this increase and we will prove that all other edges
+of~$C$ already are at levels greater than~$\ell$, so the violation cannot occur.
+
+Let us first show that for edges of~$C$ incident with~$T_1$. All edges of~$T_1$ itself
+already are at the higher levels as they were moved there at the very beginning of the
+search for the replacement edge. The other tree edges incident with~$T_1$ would have
+lower levels, which is impossible since the invariant would be already violated.
+Non-tree edges of~$C$ incident with~$T_1$ are lighter than~$e$, so they were already considered
+as~candidates for the replacement edge, because the algorithm always picks the lightest
+candidate first. Such edges therefore have been already moved a~level up.
+
+The case of edges of~$C$ that do not touch~$T_1$ is easy to handle: Such edges do not exist.
+If they did, at least one more edge of~$C$ besides~$e$ would have to connect~$T_1$ with the other
+trees of level~$\ell$. We already know that this could not be a~tree edge. If it were a~non-tree
+edge, it could not have level greater than~$\ell$ by~I1 nor smaller than~$\ell$ by~I3. Therefore
+it would be a~level~$\ell$ edge lighter than~$e$, and as such it would have been selected as the
+replacement edge before $e$~was.
+\qed
+
+We can conclude:
+
+\thmn{Decremental MSF, Holm et al.~\cite{holm:polylog}}
+When we start with a~graph on $n$~vertices with~$m$ edges and we perform a~sequence of
+edge deletions, the MSF can be initialized in time $\O((m+n)\cdot\log^2 n)$ and then
+updated in time $\O(\log^2 n)$ amortized per operation.
+
+\paran{Fully dynamic MSF}%
+The decremental MSF algorithm can be turned to a~fully dynamic one by a~blackbox
+reduction whose properties are summarized in the following theorem:
+
+\thmn{MSF dynamization, Holm et al.~\cite{holm:polylog}}
+Suppose that we have a~decremental MSF algorithm with the following properties:
+\numlist\ndotted
+\:For any $a$,~$b$, it can be initialized on a~graph with~$a$ vertices and~$b$ edges.
+\:Then it executes an~arbitrary sequence of deletions in time $\O(b\cdot t(a,b))$, where~$t$ is a~non-decreasing function.
+\endlist
+\>Then there exists a~fully dynamic MSF algorithm for a~graph on $n$~vertices, starting
+with no edges, that performs $m$~insertions and deletions in amortized time:
+$$
+\O\left( \log^3 n + \sum_{i=1}^{\log m} \sum_{j=1}^i \; t(\min(n,2^j), 2^j) \right) \hbox{\quad per operation.}
+$$
+
+\proofsketch
+The reduction is very technical, but its essence is the following: We maintain a~logarithmic number
+of decremental structures $A_0,\ldots,A_{\lfloor\log n\rfloor}$ of exponentially increasing sizes. Every non-tree
+edge is contained in exactly one~$A_i$, tree edges can belong to multiple structures.
+
+When an~edge is inserted, we union it with some of the $A_i$'s, build a~new decremental structure
+and amortize the cost of the build over the insertions. Deletes of non-tree edges are trivial.
+Delete of a~tree edge is performed on all $A_i$'s containing it and the replacement edge is
+sought among the replacement edges found in these structures. The unused replacement edges then have
+to be reinserted back to the structure.
+
+The original reduction of Henzinger et al.~\cite{henzinger:twoec} handles these reinserts by a~mechanism of batch insertions
+supported by their decremental structure, which is not available in our case. Holm et al.~have
+replaced it by a~system of auxiliary edges inserted at various places in the structure.
+We refer to the article \cite{holm:polylog} for details.
+\qed
+
+\corn{Fully dynamic MSF}\id{dynmsfcorr}%
+There is a~fully dynamic MSF algorithm that works in time $\O(\log^4 n)$ amortized
+per operation for graphs on $n$~vertices.
+
+\proof
+Apply the reduction from the previous theorem to the decremental algorithm we have
+developed. This results in an~algorithm of amortized complexity $\O(\log^4\max(m,n))$ where~$m$
+is the number of operations performed. This could exceed $\O(\log^4 n)$ if
+$m$~is very large, but we can rebuild the whole structure after $n^2$~operations,
+which brings $\log m$ down to $\O(\log n)$. The $\O(n^2\log^4 n)$ cost of the
+rebuild then incurs only $\O(\log^4 n)$ additional cost on each operation.
+\qed
+
+\rem
+The limitation of MSF structures based on the Holm's algorithm for connectivity
+to only edge deletions seems to be unavoidable. The invariant I3 could be easily
+broken for many cycles at once whenever a~very light non-tree edge is inserted.
+We could try increasing the level of the newly inserted edge, but we would quite
+likely hit I1 before we managed to skip the levels of all the heaviest edges on the
+particular cycles.
+
+On the other hand, if we decided to drop I3, we would encounter different obstacles. The ET-trees can
+bring the lightest non-tree incident with the current tree~$T_1$, but the lightest replacement edge
+could also be located in the super-trees of~$T_1$ at the lower levels, which are too large to scan
+and both I1 and I2 prevent us from charging the time on increasing levels there.
+
+An~interesting special case in which insertions are possible is when all non-tree
+edges have the same weight. This leads to the following algorithm for dynamic MSF
+on~graphs with a~small set of allowed edge weights. It is based on an~idea similar
+to the $\O(k\log^3 n)$ algorithm of Henzinger and King \cite{henzinger:randdyn},
+but have adapted it to use the better results on dynamic connectivity we have at hand.
+
+\paran{Dynamic MSF with limited edge weights}%
+Let us assume for a~while that our graph has edges of only two different weights (let us say
+1~and~2). We will forget our rule that all edge weights are distinct for a~moment and we recall
+the observation in \ref{multiweight} that the basic structural properties of
+the MST's from Section \ref{mstbasics} still hold.
+
+We split the graph~$G$ to two subgraphs~$G_1$ and~$G_2$ according to the edge
+weights. We use one instance~$\C_1$ of the dynamic connectivity algorithm to maintain
+an~arbitrary spanning forest~$F_1$ of~$G_1$, which is obviously minimum. Then we add
+another instance~$\C_2$ to maintain a~spanning forest~$F_2$ of the graph $G_2\cup F_1$
+such that all edges of~$F_1$ are forced to be in~$F_2$. Obviously, $F_2$~is the
+MSF of the whole graph~$G$ --- if any edge of~$F_1$ were not contained in~$\msf(G)$,
+we could use the standard exchange argument to create an~even lighter spanning tree.
+
+When a~weight~2 edge is inserted to~$G$, we insert it to~$\C_2$ and it either enters~$F_2$
+or becomes a~non-tree edge. Similarly, deletion of a~weight~2 edge is a~pure deletion in~$\C_2$,
+because such edges can be replaced only by other weight~2 edges.
+
+Insertion of edges of weight~1 needs more attention: We insert the edge to~$\C_1$. If~$F_1$
+stays unchanged, we are done. If the new edge enters~$F_1$, we use a~Sleator-Tarjan tree
+kept for~$F_2$ to check if the new edge covers some tree edge of weight~2. If this is not
+the case, we insert the new edge to~$\C_2$ and hence also to~$F_2$ and we are done.
+Otherwise we exchange one of the covered weight~2 edges~$f$ for~$e$ in~$\C_2$. We note
+that~$e$ can inherit the level of~$f$ and $f$~can become a~non-tree edge without
+changing its level. This adjustment can be performed in time $\O(\log^2 n)$, including
+paying for the future level increases of the new edge.
+
+Deletion of weight~1 edges is once more straightforward. We delete the edge from~$\C_1$.
+If it has no replacement, we delete it from~$\C_2$ as well. If it has a~replacement,
+we delete the edge from~$\C_2$ and insert the replacement on its place as described
+above. We observe than this pair of operations causes an~insertion, deletion or
+a~replacement in~$\C_2$.
+
+This way, we can handle every insertion and deletion in time $\O(\log^2 n)$ amortized.
+This construction can be iterated in an~obvious way: if we have $k$~distinct edge weights,
+we build $k$~connectivity structures $\C_1,\ldots,\C_k$. The structure~$\C_i$ contains edges of
+weight~$i$ together with the MSF edges from~$\C_{i-1}$. Bounding the time complexity is then easy:
+
+\thmn{MSF with limited edge weights}
+There is a~fully dynamic MSF algorithm that works in time $\O(k\cdot\log^2 n)$ amortized
+per operation for graphs on $n$~vertices with only $k$~distinct edge weights allowed.
+
+\proof
+A~change in the graph~$G$ involving an~edge of weight~$w$ causes a~change in~$\C_w$,
+which can propagate to~$\C_{w+1}$ and so on, possibly up to~$\C_k$. In each~$\C_i$,
+we spend time $\O(\log^2 n)$ by updating the connectivity structure according to
+Theorem \ref{dyncon} and $\O(\log n)$ on operations with the Sleator-Tarjan trees
+by Theorem \ref{sletar}.
+\qed
+
+%--------------------------------------------------------------------------------
+
+\section{Almost minimum trees}\id{kbestsect}%
+
+In some situations, finding the single minimum spanning tree is not enough and we are interested
+in the $K$~lightest spanning trees, usually for some small value of~$K$. Katoh, Ibaraki
+and Mine \cite{katoh:kmin} have given an~algorithm of time complexity $\O(m\log\beta(m,n) + Km)$,
+building on the MST algorithm of Gabow et al.~\cite{gabow:mst}.
+Subsequently, Eppstein \cite{eppstein:ksmallest} has discovered an~elegant preprocessing step which allows to reduce
+the running time to $\O(m\log\beta(m,n) + \min(K^2,Km))$ by eliminating edges
+which are either present in all $K$ trees or in none of them.
+We will show a~variant of their algorithm based on the MST verification
+procedure of Section~\ref{verifysect}.
+
+In this section, we will require the edge weights to be numeric, because
+comparisons are certainly not sufficient to determine the second best spanning tree. We will
+assume that our computation model is able to add, subtract and compare the edge weights
+in constant time.
+
+Let us focus on finding the second lightest spanning tree first.
+
+\paran{Second lightest spanning tree}%
+Suppose that we have a~weighted graph~$G$ and a~sequence $T_1,\ldots,T_z$ of all its spanning
+trees. Also suppose that the weights of these spanning trees are distinct and that the sequence
+is ordered by weight, i.e., $w(T_1) < \ldots < w(T_z)$ and $T_1 = \mst(G)$. Let us observe
+that each tree is similar to at least one of its predecessors:
+
+\lemman{Difference lemma}\id{kbl}%
+For each $i>1$ there exists $j<i$ such that $T_i$ and~$T_j$ differ by a~single edge exchange.
+
+\proof
+We know from the Monotone exchange lemma (\ref{monoxchg}) that $T_1$ can be transformed
+to~$T_i$ by a~sequence of edge exchanges which never decrease tree weight. The last
+exchange in this sequence therefore obtains~$T_i$ from a~tree of the desired properties.
+\qed
+
+\para
+This lemma implies that the second best spanning tree~$T_2$ differs from~$T_1$ by a~single
+edge exchange. It remains to find which exchange it is. Let us consider the exchange
+of an~edge $f\in E\setminus T_1$ with an~edge $e\in T_1[f]$. We get a~tree $T_1-e+f$
+of weight $w(T_1)-w(e)+w(f)$. To obtain~$T_2$, we have to find~$e$ and~$f$ such that the
+difference $w(f)-w(e)$ is the minimum possible. Thus for every~$f$, the edge~$e$~must be always
+the heaviest on the path $T_1[f]$. We can apply the algorithm from Corollary \ref{rampeaks}
+and find the heaviest edges (peaks) of all such paths and thus examine all possible choices of~$f$
+in linear time. So we get:
+
+\lemma
+For every graph~$H$ and a~MST $T$ of~$H$, linear time is sufficient to find
+edges $e\in T$ and $f\in H\setminus T$ such that $w(f)-w(e)$ is minimum.
+
+\nota
+We will call this procedure \df{finding the best exchange in $(H,T)$.}
+
+\cor
+Given~$G$ and~$T_1$, we can find~$T_2$ in time $\O(m)$.
+
+\paran{Third lightest spanning tree}%
+Once we know~$T_1$ and~$T_2$, how to get~$T_3$? According to the Difference lemma, $T_3$~can be
+obtained by a~single exchange from either~$T_1$ or~$T_2$. Therefore we need to find the
+best exchange for~$T_2$ and the second best exchange for~$T_1$ and use the better of them.
+The latter is not easy to find directly, so we will make a~minor side step.
+
+We know that $T_2$ equals $T_1-e+f$ for some edges $e$ and~$f$. We define two auxiliary graphs:
+$G_1 := G/e$ and $G_2 := G-e$. The tree~$T_1/e$ is obviously the MST of~$G_1$
+(by the Contraction lemma) and $T_2$ is the MST of~$G_2$ (all $T_2$-light edges
+in~$G_2$ would be $T_1$-light in~$G$).
+
+\obs\id{tbobs}%
+The tree $T_3$~can be obtained by a~single edge exchange in either $(G_1,T_1/e)$ or $(G_2,T_2)$:
+
+\itemize\ibull
+\:If $T_3 = T_1-e'+f'$ for $e'\ne e$, then $T_3/e = (T_1/e)-e'+f'$ in~$G_1$.
+\:If $T_3 = T_1-e+f'$, then $T_3 = T_2 - f + f'$ in~$G_2$.
+\:If $T_3 = T_2-e'+f'$, then this exchange is found in~$G_2$.
+\endlist
+
+\>Conversely, a~single exchange in $(G_1,T_1/e)$ or in $(G_2,T_2)$ corresponds
+to an~exchange in either~$(G,T_1)$ or $(G,T_2)$.
+Even stronger, a~spanning tree~$T$ of~$G$ either contains~$e$ and then $T\sgc
+e$ is a~spanning tree of~$G_1$, or $T$~doesn't contain~$e$ and so it is
+a~spanning tree of~$G_2$.
+
+Thus we can run the previous algorithm for finding the best edge exchange
+on both~$G_1$ and~$G_2$ and find~$T_3$ again in time $\O(m)$.
+
+\paran{Further spanning trees}%
+The construction of auxiliary graphs can be iterated to obtain $T_1,\ldots,T_K$
+for an~arbitrary~$K$. We will build a~\df{meta-tree} of auxiliary graphs. Each node of this meta-tree
+carries a~graph\foot{This graph is always derived from~$G$ by a~sequence of edge deletions
+and contractions. It is tempting to say that it is a~minor of~$G$, but this is not true as we
+preserve multiple edges.} and its minimum spanning tree. The root node contains~$(G,T_1)$,
+its sons have $(G_1,T_1/e)$ and $(G_2,T_2)$. When $T_3$ is obtained by an~exchange
+in one of these sons, we attach two new leaves to that son and we let them carry the two auxiliary
+graphs derived by contracting or deleting the exchanged edge. Then we find the best
+edge exchanges among all leaves of the new meta-tree and repeat the process. By Observation \ref{tbobs},
+each spanning tree of~$G$ is generated exactly once. The Difference lemma guarantees that
+the trees are enumerated in the increasing order.
+
+Recalculating the best exchanges in all leaves of the meta-tree after generating each~$T_i$
+is of course not necessary, because most leaves stay unchanged. We will rather remember
+the best exchange for each leaf and keep the weight differences of these exchanges in a~heap. In every step, we will
+delete the minimum from the heap and use the exchange in the particular leaf to generate
+a~new spanning tree. Then we will create the new leaves, calculate their best exchanges and insert
+them into the heap. The algorithm is now straightforward and so will be its analysis:
+
+\algn{Finding $K$ best spanning trees}\id{kbestalg}%
+\algo
+\algin A~weighted graph~$G$, its MST~$T_1$ and an~integer $K>0$.
+\:$R\=$ a~meta tree whose vertices carry triples $(G',T',F')$. Initially
+ it contains just a~root with $(G,T_1,\emptyset)$.
+ \hfil\break
+ \cmt{$G'$ is a~graph, $T'$~is its MST, and~$F'$ is a~set of edges of~$G$
+ that are contracted in~$G'$.}
+\:$H\=$ a~heap of quadruples $(\delta,r,e,f)$ ordered on~$\delta$, initially empty.
+ \hfil\break
+ \cmt{Each quadruple describes an~exchange of~$e$ for~$f$ in a~leaf~$r$ of~$R$ and $\delta=w(f)-w(e)$
+ is the weight gain of this exchange.}
+\:Find the best edge exchange in~$(G,T_1)$ and insert it to~$H$.
+\:$i\= 1$.
+\:While $i<K$:
+\::Delete the minimum quadruple $(\delta,r,e,f)$ from~$H$.
+\::$(G',T',F') \=$ the triple carried by the leaf~$r$.
+\::$i\=i+1$.
+\::$T_i\=(T'-e+f) \cup F'$. \cmt{The next spanning tree}
+\::$r_1\=$ a~new leaf carrying $(G'/e,T'/e,F'+e)$.
+\::$r_2\=$ a~new leaf carrying $(G'-e,T_i,F')$.
+\::Attach~$r_1$ and~$r_2$ as sons of~$r$.
+\::Find the best edge exchanges in~$r_1$ and~$r_2$ and insert them to~$H$.
+\algout The spanning trees $T_2,\ldots,T_K$.
+\endalgo
+
+\lemma\id{kbestl}%
+Given~$G$ and~$T_1$, we can find $T_2,\ldots,T_K$ in time $\O(Km + K\log K)$.
+
+\proof
+Generating each~$T_i$ requires finding the best exchange for two graphs and also $\O(1)$
+operations on the heap. The former takes $\O(m)$ according to Corollary \ref{rampeaks},
+and each heap operation takes $\O(\log K)$.
+\qed
+
+\rem
+The meta-tree is not needed for the actual operation of the algorithm --- it suffices
+to keep its leaves in the heap.
+
+\paran{Arbitrary weights}%
+While the assumption that the weights of all spanning trees are distinct has helped us
+in thinking about the problem, we should not forget that it is somewhat unrealistic.
+We could refine the proof of our algorithm and demonstrate that the algorithm indeed works
+without this assumption, but we will rather show that the ties can be broken easily.
+
+Let~$\delta$ be the minimum positive difference among the weights of all spanning trees
+of~$G$ and $e_1,\ldots,e_m$ be the edges of~$G$. We observe that it suffices to
+increase $w(e_i)$ by~$\delta_i = \delta/2^{i+1}$. The cost of every spanning tree
+has increased by at most $\sum_i\delta_i < \delta/2$, so if $T$~was lighter
+than~$T'$, it still is. On the other hand, no two trees share the same
+weight adjustment, so all tree weights are now distinct.
+
+The exact value of~$\delta$ is not easy to calculate, but closer inspection of the algorithm
+reveals that it is not needed at all. The only place where the edge weights are examined
+is when we search for the best exchange. In this case, we compare the differences of
+pairs of edge weights with each other. Each such difference is therefore adjusted
+by $\delta\cdot(2^{-i}-2^{-j})$ for some $i,j>1$, which again does not influence comparison
+of originally distinct differences. If two differences were identical, it is sufficient
+to look at their values of~$i$ and~$j$, i.e., at the identifiers of the edges.
+
+\paran{Invariant edges}%
+Our algorithm can be further improved for small values of~$K$ (which seems to be the common
+case in most applications) by the reduction of Eppstein \cite{eppstein:ksmallest}.
+We will observe that there are many edges of~$T_1$
+which are guaranteed to be contained in $T_2,\ldots,T_K$ as well, and likewise there are
+many edges of $G\setminus T_1$ which are excluded from all those spanning trees.
+The idea is the following (again assuming that the tree weights are distinct):
+
+\defn
+For an~edge $e\in T_1$, we define its \df{gain} $g(e)$ as the minimum weight gained by exchanging~$e$
+for another edge. Similarly, we define the gain $G(f)$ for $f\not\in T_1$. Put formally:
+$$\eqalign{
+g(e) &:= \min\{ w(f)-w(e) \mid f\in E, e\in T[f] \}, \cr
+G(f) &:= \min\{ w(f)-w(e) \mid e\in T[f] \}.\cr
+}$$
+
+\lemma\id{gaina}%
+When $t_1,\ldots,t_{n-1}$ are the edges of~$T_1$ in order of increasing gain,
+the edges $t_K,\ldots,t_{n-1}$ are present in all trees $T_2,\ldots,T_K$.
+
+\proof
+The best exchanges in~$T_1$ involving $t_1,\ldots,t_{K-1}$ produce~$K-1$ spanning trees
+of increasing weights. Any exchange involving $t_K,\ldots,t_n$ produces a~tree
+which is heavier or equal than all those trees. (We are ascertained by the Monotone exchange lemma
+that the gain of such exchanges need not be reverted later.)
+\qed
+
+\lemma\id{gainb}%
+When $q_1,\ldots,q_{m-n+1}$ are the edges of $G\setminus T_1$ in order of increasing gain,
+the edges $q_K,\ldots,q_{m-n+1}$ are not present in any of $T_2,\ldots,T_K$.
+
+\proof
+Similar to the previous lemma.
+\qed
+
+\para
+It is therefore sufficient to find $T_2,\ldots,T_K$ in the graph obtained from~$G$ by
+contracting the edges $t_K,\ldots,t_n$ and deleting $q_K,\ldots,q_{m-n+1}$. This graph
+has only $\O(K)$ vertices and $\O(K)$ edges. The only remaining hurdle is how to
+calculate the gains. For edges outside~$T_1$, it again suffices to find the peaks of the
+covered paths. The gains of MST edges require a~different algorithm, but Tarjan \cite{tarjan:applpc}
+has shown how to obtain them in time $\O(m\timesalpha(m,n))$.
+
+When we put the results of this section together, we can conclude:
+
+\thmn{Finding $K$ lightest spanning trees}\id{kbestthm}%
+For a~given graph~$G$ with real edge weights and a~positive integer~$K$, the $K$~best spanning trees can be found
+in time $\O(m\timesalpha(m,n) + \min(K^2,Km + K\log K))$.
+
+\proof
+First we find the MST of~$G$ in time $\O(m\timesalpha(m,n))$ using the Pettie's Optimal
+MST algorithm (Theorem \ref{optthm}). Then we calculate the gains of MST edges by the
+Tarjan's algorithm from \cite{tarjan:applpc}, again in $\O(m\timesalpha(m,n))$, and
+the gains of the other edges using our MST verification algorithm (Corollary \ref{rampeaks})
+in $\O(m)$. We use Lemma \ref{gaina} to identify edges that are required, and Lemma \ref{gainb}
+to find edges that are superfluous. We contract the former edges, remove the latter ones
+and run Algorithm \ref{kbestalg} to find the spanning trees. By Lemma \ref{kbestl}, it runs in
+the desired time.
+
+If~$K\ge m$, this reduction does not pay off, so we run Algorithm \ref{kbestalg}
+directly on the input graph.
+\qed
+
+\paran{Improvements}%
+It is an~interesting open question whether the algorithms of Section \ref{verifysect} can
+be modified to calculate all gains in linear time. The main procedure could be, but it requires having
+the input reduced to a~balance tree beforehand and here the Bor\o{u}vka trees fail. The Buchsbaum's
+Pointer-Machine algorithm (\ref{pmverify}) seems to be more promising.
+
+\paran{Large~$K$}%
+When $K$~is large, re-running the verification algorithm for every change of the graph
+is too costly. Frederickson \cite{frederickson:ambivalent} has shown how to find the best
+swaps dynamically, reducing the overall time complexity of Algorithm \ref{kbestalg}
+to $\O(Km^{1/2})$ and improving the bound in Theorem \ref{kbestthm} to $\O(m\timesalpha(m,n)
++ \min( K^{3/2}, Km^{1/2} ))$. It is open if the dynamic data structures of this
+chapter could be modified to bring the complexity of finding the next tree down
+to polylogarithmic.
+
+\paran{Multiple minimum trees}%
+Another nice application of Theorem \ref{kbestthm} is finding all minimum spanning
+trees in a~graph that does not have distinct edge weights. We find a~single MST using
+any of the algorithms of the previous chapters and then we use the enumeration algorithm
+of this section to find further spanning trees as long as their weights are equal to the minimum.
+
+We can even use the reduction of the number of edges from Lemmata \ref{gaina} and \ref{gainb}:
+we start with some fixed~$K$ and when we exhaust all~$K$ trees, we double~$K$ and restart
+the whole process. The extra time spent on these restarts is dominated by the time of the
+final pass.
+
+This finally settles the question that we have asked ourselves in Section \ref{mstbasics},
+namely whether we lose anything by assuming that all weights are distinct and by searching
+for just the single minimum tree.
\endpart
projects / saga.git / blobdiff