The right context turns out to be the minor-closed graph classes, which are
closed under contractions and have bounded density.
-\defn
-A~graph~$H$ is a \df{minor} of a~graph~$G$ iff it can be obtained
-from a subgraph of~$G$ by a sequence of simple graph contractions (see \ref{simpcont}).
+\defn\id{minordef}%
+A~graph~$H$ is a \df{minor} of a~graph~$G$ (written as $H\minorof G$) iff it can be obtained
+from a~subgraph of~$G$ by a sequence of simple graph contractions (see \ref{simpcont}).
\defn
A~class~$\cal C$ of graphs is \df{minor-closed}, when for every $G\in\cal C$ and
\df{non-trivial} if at least one graph lies in~$\cal C$ and at least one lies outside~$\cal C$.
\example
-Non-trivial minor-closed classes include planar graphs and more generally graphs
-embeddable in any fixed surface. Many nice properties of planar graphs extend
-to these classes, too, most notably the linearity of the number of edges.
+Non-trivial minor-closed classes include:
+\itemize\ibull
+\:planar graphs,
+\:graphs embeddable in any fixed surface (i.e., graphs of bounded genus),
+\:graphs embeddable in~${\bb R}^3$ without knots or without interlocking cycles,
+\:graphs of bounded tree-width or path-width.
+\endlist
+
+\para
+Many of the nice structural properties of planar graphs extend to
+minor-closed classes, too (see \cite{diestel:gt} for a~nice overview
+of this theory). The most important property is probably the characterization
+of such classes by forbidden minors.
+
+\defn
+For a~class~$\cal H$ of graphs we define $\Forb({\cal H})$ as the class
+of graphs which do not contain any of the graphs in~$\cal H$ as a~minor.
+We will call $\cal H$ the set of \df{forbidden (or excluded) minors} for this class.
+
+\obs
+For every~${\cal H}\ne\emptyset$, the class $\Forb({\cal H})$ is non-trivial
+and closed on minors. This works in the opposite direction as well: for every
+minor-closed class~$\cal C$ there is a~class $\cal H$ such that ${\cal C}=\Forb({\cal H})$.
+One such~$\cal H$ is the complement of~$\cal C$, but smaller sets can be found, too.
+For example, the planar graphs exclude exactly $K_5$ and~$K_{3,3}$ --- this follows
+from the Kuratowski's theorem (the theorem uses forbidden subdivisions, but while
+in general this is not the same as forbidden minors, it is for $K_5$ and $K_{3,3}$).
+The celebrated theorem by Robertson and Seymour guarantees that we can always find
+a~finite set of forbidden minors.
+
+\thmn{Excluded minors, Robertson \& Seymour \cite{rs:wagner}}
+For every non-trivial minor-closed graph class~$\cal C$ there exists
+a~finite set~$\cal H$ of graphs such that ${\cal C}=\Forb({\cal H})$.
+
+\proof
+This theorem has been proven in the series of papers on graph minors
+culminating with~\cite{rs:wagner}. See this paper and follow the references
+to the previous articles in the series.
+\qed
+
+\para
+We will make use of another important property --- the bounded density of minor-closed
+classes. The connection between minors and density dates back to Mader in the 1960's
+and it can be proven without use of the Robertson-Seymour theorem.
\defn\id{density}%
Let $\cal C$ be a class of graphs. We define its \df{edge density} $\varrho(\cal C)$
to be the infimum of all~$\varrho$'s such that $m(G) \le \varrho\cdot n(G)$
holds for every $G\in\cal C$.
+\thmn{Mader \cite{mader:dens}}
+For every $k\in{\bb N}$ there exists $h(k)\in{\bb R}$ such that every graph
+with average degree at least~$h(k)$ contains a~subdivision of~$K_{k}$ as a~subgraph.
+
+\proofsketch
+(See Lemma 3.5.1 in \cite{diestel:gt} for a~complete proof in English.)
+
+Let us fix~$k$ and prove by induction on~$m$ that every graph with average
+degree $a\ge 2^m$ contains a~subdivision of some graph with $k$~vertices
+and ${k\choose 2}\ge m\ge k$~edges. For $m={k\choose 2}$ the theorem follows
+as the only graph with~$k$ vertices and~$k\choose 2$ edges is~$K_k$.
+
+The base case $m=k$: Let us observe that when the average degree
+is~$a$, removing any vertex of degree less than~$a/2$ does not decrease the
+average degree. A~graph with $a\ge 2^k$ therefore has a~subgraph
+with minimum degree $\delta\ge a/2=2^{k-1}$. Such subgraph contains
+a~cycle on more than~$\delta$ vertices, in other words a~subdivision of
+the cycle~$C_k$.
+
+Induction step: Let~$G$ be a~graph with average degree at least~$2^m$ and
+assume that the theorem already holds for $m-1$. Without loss of generality,
+$G$~is connected. Consider a~maximal set $U\subseteq V$ such that $G[U]$ is connected
+and the graph $G.U$ ($G$~with $U$~contracted to a~single vertex) has average
+degree at least~$2^m$ (such~$U$ exists, because $G=G.U$ whenever $\vert U\vert=1$).
+Now consider the subgraph~$H$ induced in~$G$ by the
+neighbors of~$U$. Every $v\in V(H)$ must have degree at least~$2^{m-1}$
+(otherwise we can add this vertex to~$U$, contradicting its maximality), so by
+the induction hypothesis $H$ contains a~subdivision of some graph~$R$ with
+$r$~vertices and $m-1$ edges. Any two non-adjacent vertices of~$R$ can be
+connected in the subdivision by a~path lying entirely in~$G[U]$, which reveals
+a~subdivision of a~graph on $m$~vertices. \qed
+
\thmn{Density of minor-closed classes, Mader~\cite{mader:dens}}
-A~minor-closed class of graphs has finite edge density if and only if it is
-a non-trivial class.
+Every non-trivial minor-closed class of graphs has finite edge density.
\proof
-See Theorem 6.1 in \cite{nesetril:minors}, which also lists some other equivalent conditions.
+Let~$\cal C$ be any such class, $X$~its smallest excluded minor and $x=n(X)$.
+As $H\minorof K_x$, the class $\cal C$ entirely lies in ${\cal C}'=\Forb(\{K_x\})$, so
+$\varrho({\cal C}) \le \varrho({\cal C}')$ and therefore it suffices to prove the
+theorem for classes excluding a~single complete graph~$K_x$.
+
+We will show that $\varrho({\cal C})\le 2h(x)$, where $h$~is the function
+from the previous theorem. If any $G\in{\cal C}$ had more than $2h(x)\cdot n(G)$
+edges, its average degree would be at least~$h(x)$, so by the previous theorem
+$G$~would contain a~subdivision of~$K_x$ and therefore $K_x$ as a~minor.
\qed
+\rem
+See also Theorem 6.1 in \cite{nesetril:minors}, which also lists some other equivalent conditions.
+
\thmn{MST on minor-closed classes \cite{mm:mst}}\id{mstmcc}%
For any fixed non-trivial minor-closed class~$\cal C$ of graphs, Algorithm \ref{contbor} finds
the MST of any graph in this class in time $\O(n)$. (The constant hidden in the~$\O$
\n{$n(G)$}{number of vertices of a graph~$G$, that is $\vert V(G)\vert$}
\n{$m(G)$}{number of edges of a graph~$G$, that is $\vert E(G)\vert$}
\n{$V,E,n,m$}{when used without $(G)$, they refer to the input of the current algorithm}
+\n{$G[U]$}{subgraph induced by a~set $U\subset V(G)$}
\n{$\delta_G(U)$}{all edges connecting $U\subset V(G)$ with $V(G)\setminus U$; we usually omit the~$G$}
\n{$\delta_G(v)$}{the edges of a one-vertex cut, i.e., $\delta_G(\{v\})$}
\n{MST}{minimum spanning tree \[mstdef]}
\n{$X \choose k$}{a set of all $k$-element subsets of a set~$X$}
\n{$G/e$}{multigraph contraction \[contract]}
\n{$G.e$}{simple graph contraction \[simpcont]}
+\n{$G/X$, $G.X$}{contraction by a~set $X$ of vertices or edges \[setcont]}
\n{$\alpha(n)$}{the inverse Ackermann's function}
\n{$f[X]$}{function applied to a set: $f[X]:=\{ f(x) ; x\in X \}$}
\n{$f[e]$}{as edges are two-element sets, $f[e]$ maps both endpoints of an edge~$e$}
\n{$L(\pi,A)$}{lexicographic ranking function for permutations on a~set~$A\subseteq{\bb N}$ \[brackets]}
\n{$L^{-1}(i,A)$}{lexicographic unranking function, the inverse of~$L$ \[brackets]}
\n{$n^{\underline k}$}{the $k$-th falling factorial power: $n\cdot(n-1)\cdot\ldots\cdot(n-k+1)$ \[kpranksect]}
+\n{$H\minorof G$}{$H$ is a~minor of~$G$ \[minordef]}
+\n{$K_k$}{the complete graph on~$k$ vertices}
+\n{$C_k$}{the cycle on~$k$ vertices}
}
%--------------------------------------------------------------------------------
m(x) &= \cases{v_e & \hbox{for $v=x,y,$}\cr v & \hbox{otherwise.}} \cr
}$$
+\defn\id{setcont}%
+We can also extend the above definitions to contractions by a~set of vertices or edges.
+For $F\subseteq E(G)$, the graph $G/F$ is defined as $(G/f_1)/f_2/\ldots/f_k$ where
+$f_1,\ldots,f_k$ are the elements of~$F$ (you can observe that the result
+does not depend on the order of edges). For $U\subseteq V(G)$, we define $G/U$
+as the graph with all vertices of~$U$ merged to a~single vertex, that is $(G\cup U^*)/U^*$,
+where $U^*$ is the complete graph on~$U$. Similarly for $G.F$ and $G.U$.
+
\endpart
projects / saga.git / commitdiff