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According to Diestel (page 4): "If $G' \subseteq G$ and $G'$ contains all the edges $xy \in E$ with $x, y \in V'$, then $G'$ is an induced subgraph of $G$"

According to Wikipedia "induced cycle is a cycle that is an induced subgraph of $G$; induced cycles are also called cordless cycles "

Does the definition by Diestel imply induced cycles are chordless?

In this graph, does induced subgraph $G[\{a,b,c,d\}]$ include edge $ac$?

Both $a$ and $c$ are in $V'$.

Graph $G$

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up vote 4 down vote accepted

Shortly, it does.

It is easier to think of induced subgraphs in terms of vertices: you take any $V' \subseteq V$ and take all the edges from $E$ that make sense, that is, both their ends are in $V'$. In fact, $(a,c) \in E$ and $a,c \in V'$ implies $(a,c) \in E'$.

I guess your confusion might be caused by the term "cycle" which has here two similar meanings:

  1. A closed path in a graph, i.e. a sequence $C = (c_0,\ldots,c_{n-1}) \subseteq V$ such that $(c_{i},c_{i+1}) \in E$ and $(c_{n-1},c_0) \in E$. Note that here, there may be many chords inside, and $C$ would still be a cycle.
  2. A graph that is a cycle. There are no other edges, in fact it is a connected 2-regular graph (i.e. connected and $\forall v.\ \mathrm{deg}(v) = 2$).

So a cycle-1 is chordless if and only if it is an induced cycle-2.

I hope it explained something ;-)

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I read the Diestel book more carefully, and page 8 it says "An edge which joins two vertices of a cycle but is not itself an edge of the cycle is a chord of that cycle. Thus, an induced cycle in $G$, a cycle in $G$ forming an induced subgraph, is one that has no chords (Fig. 1.3.3)." Confusing ... – Ohto Nordberg Feb 24 '13 at 11:20
@OhtoNordberg What is confusing you there? A cycle-1 in $G$ is induced if it is an induced cycle-2, that is, an induced subgraph that is a cycle-2. Such a cycle-1 cannot have chords, because then the induced subgraph would not be a cycle-2. – dtldarek Feb 24 '13 at 20:51

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