 17 ; this remains the only nontrivial multicolor Ramsey number whose exact value is known . As part of their proof , they used an algebraic construction to show that a 16 @-@ vertex complete graph can be decomposed into three disjoint copies of a triangle @-@ free 5 @-@ regular graph with 16 vertices and 40 edges ( sometimes called the Greenwood – Gleason graph ) . 
