The sophomore dream

Johann Bernoulli's identity, known as the sophomore dream (1697).

References.

[1] Johannis Bernoulli, Opera omnia, vol. 3, pp. 376–381

[2] William Dunham "3: The Bernoullis (Johann and $x^x$)", The Calculus Gallery, Masterpieces from Newton to Lebesgue, Princeton, NJ: Princeton University Press, pp. 46–51 (2005)