subgraphSubgraph2002-03-06 23:50:502007-10-08 17:31:44Definitioninducedspanned byspanningspanning subgraphinduced subgraphgraph% this is the default PlanetMath preamble. as your knowledge
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% define commands hereWe say that $G' = (V',E')$ is a \emph{subgraph} of $G = (V,E)$ if $V' \subseteq V$ and $E' \subseteq E$. In this case we write $G' \subseteq G$.
If $G'$ contains \emph{all edges} of $G$ that join two vertices in $V'$ then $G'$ is said to be the subgraph \emph{induced} or \emph{spanned by} $V'$ and is denoted by $G[V']$. Thus, a subgraph $G'$ of $G$ is an induced subgraph if $G' = G[V(G')]$. If $V' = V$, then $G'$ is said to be a \emph{spanning} subgraph of $G$.
Often, new graphs are constructed from old ones by deleting or adding some vertices and edges. If $W \subset V(G)$, then $G - W = G[V\setminus W]$ is the subgraph of $G$ obtained by deleting the vertices in $W$ \emph{and all edges incident with them}. Similarly, if $E' \subseteq E(G)$, then $G - E' = (V(G),E(G) \setminus E')$. If $W = {w}$ and $E' = {xy}$, then this notation is simplified to $G - w$ and $G - xy$. Similarly, if $x$ and $y$ are nonadjacent vertices of $G$, then $G + xy$ is obtained from $G$ by joining $x$ to $y$.
\footnotesize{Adapted with permission of the author from \emph{\PMlinkescapetext{Modern Graph Theory}} by B\'{e}la Bollob\'{a}s, published by Springer-Verlag New York, Inc., 1998.}