Definition:Tree (Graph Theory)

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This page is about Tree in the context of Graph Theory. For other uses, see Tree.


Definition 1

A tree is a simple connected graph with no circuits.

Definition 2

A tree is a simple connected graph with no cycles.


The vertices of a tree are called its nodes.

Leaf Node

Let $v$ be a node of a tree $T$.

Then $v$ is a leaf node of a $T$ if and only if $v$ is of degree $1$.

If $T$ is a rooted tree, this is equivalent to saying that $v$ has no child nodes.

Finite or Infinite

Finite Tree

A tree is finite if and only if it contains a finite number of nodes.

Infinite Tree

A tree is infinite if it contains a (countably) infinite number of nodes.

Also defined as

In some contexts, the term tree is used to mean rooted tree.

Also see

  • Results about trees can be found here.