There is a mistake on Page 335 of Computation with Finitely Presented Groups, by Charles S. Sims. The example starts with this matrix and computes its Smith normal form: M := [ [2, 2, 6, 4, 0], [6, 4, 16, 10, -14], [4, 3, 11, 7, 0], [8, 5, 21, 13, -4] ];; Sims works it out to say the answer has 1, 2, 4, 0 on the diagonal. But it should actually be 1, 2, 2, 0 according to GAP: S := SmithNormalFormIntegerMat( M ); [ [ 1, 0, 0, 0, 0 ], [ 0, 2, 0, 0, 0 ], [ 0, 0, 2, 0, 0 ], [ 0, 0, 0, 0, 0 ] ] Feel free to mail the reward check to the address shown below!