Euler's Pentagonal Numbers Theorem/Historical Note

Historical Note on Generalized Pentagonal Numbers form Exponents of Infinite Product of $1 - x^n$
This result was first noticed by, who noticed the pattern on starting to multiply out the given infinite product, but he was initially unable to prove it except by a process of informal induction.

Despite this inability, he was so certain of the pattern that he used it as the basis of the development of further theorems.