This starts with a proof of what Euler calls Waring's theorem, now known as Wilson's theorem, that (n-1)! is congruent to 1 modulo n if n is prime. The next problem is to find four numbers whose pairwise products, increased by 1, give squares. He also cites a problem posed by Leibniz: find two numbers, p and q, so that their sum is a square and the sum of their squares is a fourth power.
Original Source Citation
Opuscula analytica, Volume 1, pp. 329-344.
Opera Omnia Citation
Series 1, Volume 4, pp.91-104.