122-year-old triangle-to-square puzzle solution confirmed as best possible

A team of mathematicians from US and Japan proved that the 122-year-old solution to the Dudeney’s dissection or haberdasher’s problem is optimal. The problem, which involves dissecting an equilateral triangle into smallest number of pieces to form square, was solved in four pieces by English clerk Charles McElroy. The mathematicians now confirmed that a two or three-piece solution is impossible.