523

This number is a prime.

The long-abiding abstruseness behind Ramanujan's mysterious partition congruences such as p(5n + 4) = 0 (mod 5), i.e., that 5 (= 2 + 3) divides p(n) for any n ending in prime square digits 2² and 3², got cracked down finally thanks to a 2011 major breakthrough by Ken Ono's team revealing the underlying fractal nature of p(n). [Beedassy]