I just bought the nice book “Problems with a POINT – Exploring Math and Computer Science” by William Gasarch and Clyde Kruskal. The book explains many nice mathematical and theoretical computer science problems that are “easy-to-understand-but-not-so-easy-to-solve”, and most of them solicit the reader’s curiosity and invite him to futher explore the related topics (plenty of references are given at the end of each chapter).

Chapter 23 explains a sane reduction between COL4 (the problem of deciding whether a graph is 4-colorable) to COL3 (the problem of deciding whether a graph is 3-colorable). Both are well known NP-complete problems. I started reading the chapter, but suddenly stopped and tried to figure out a reduction by myself.

The idea presented in the book is to replace each node of the graph $G$ of the COL4 problem with four nodes, force them to be colorable with colors F and T (the third color R is not allowed), force exactly one of them to be T, and finally adding a gadget that prevents two adjacent quadruples from having two corresponding nodes colored with T.

I came up with a very similar reduction, but the gadgets involved are slightly smaller: indeed a 4 colorable node can be represented “in binary” with two nodes each one colorable with T or F (two bits).