↑ Mathematics and its History, John Stillwell, Springer 1989, p. 110
↑ Early Mathematical Manuscripts of Leibniz, J. M. Child, Open Court Publishing Co., 1920, pp. 73–74, 80.
Etymology
Gottfried Leibniz based the symbol on the Latin word summa(“sum”), which he wrote ſumma (with a long s 'ſ' ). This use first appeared publicly in his paper De Geometria, published in Acta Eruditorum of June, 1686,[1] but he had been using it in private manuscripts since at least 1675.[2]