Leonhard Euler. Variae observationes circa series infinitas. Commentarii academiae scientiarum Petropolitanae 9, 1744, pp. 160–188, Theorems 7 and 8. In Theorem 7 Euler proves the formula in the special case , and in Theorem 8 he proves it more generally. In the first corollary to his Theorem 7 he notes that , and makes use of this latter result in his Theorem 19, in order to show that the sum of the inverses of the prime numbers is .