2026-07-072026-07-07http://salesiana.dossiersoluciones.com/handle/123456789/223467We construct two difference families on each of the cyclic groups of order 109, 145 and 247, and use them to construct skew-Hadamard matrices of orders 436, 580 and 988. Such difference families and matrices are constructed here for the first time. The matrices are constructed by using the Goethals-Seidel array.12 pages, no figuresCombinatorics05B20, 05B30text