#OTD in 1675. Leibniz makes the first use of the long s (∫) as a symbol of the integral in calculus representing an elongated S, from the Latin word summa, and the d used for differentials ({\frac {dy}{dx}}), from the Latin word differentia.
Leibniz expressed the inverse relation of integration and differentiation, later called the fundamental theorem of calculus, by means of a figure in his 1693 paper Supplementum geometriae dimensoriae.... via @wikipedia
