MathDB
Subgraphs with even cross-edges

Source: 2024 Israel TST Test 6 P1

March 20, 2024
combinatoricsTSTgraph theoryColoring

Problem Statement

Let GG be a connected (simple) graph with nn vertices and at least nn edges. Prove that it is possible to color the vertices of GG red and blue, so that the following conditions hold:
i. There is at least one vertex of each color, ii. There is an even number of edges connecting a red vertex to a blue vertex, and iii. If all such edges are deleted, one is left with two connected graphs.