Butterfly graph

In the mathematical field of graph theory, the butterfly graph (also called the bowtie graph and the hourglass graph) is a planar, undirected graph with 5 vertices and 6 edges. It can be constructed by joining 2 copies of the cycle graph Template:Math with a common vertex and is therefore isomorphic to the friendship graph Template:Math.

The butterfly graph has diameter 2 and girth 3, radius 1, chromatic number 3, chromatic index 4 and is both Eulerian and a penny graph (this implies that it is unit distance and planar). It is also a 1-vertex-connected graph and a 2-edge-connected graph.

There are only three non-graceful simple graphs with five vertices. One of them is the butterfly graph. The two others are cycle graph Template:Math and the complete graph Template:Math.

Bowtie-free graphs

A graph is bowtie-free if it has no butterfly as an induced subgraph. The triangle-free graphs are bowtie-free graphs, since every butterfly contains a triangle.

In a k-vertex-connected graph, an edge is said to be k-contractible if the contraction of the edge results in a k-connected graph. Ando, Kaneko, Kawarabayashi and Yoshimoto proved that every k-vertex-connected bowtie-free graph has a k-contractible edge.

Algebraic properties

The full automorphism group of the butterfly graph is a group of order 8 isomorphic to the dihedral group D₄, the group of symmetries of a square, including both rotations and reflections.

The characteristic polynomial of the butterfly graph is $-(x-1)(x+1{)}^2(x^2-x-4)$.