13 and 31...........like in P's triangle........or as my dad wrote in The Music of the Primes......."different parts of the mathematical cannon, which seem to have nothing to do with each other"...

Pascal's Triangle

One of the most interesting Number Patterns is Pascal's Triangle (named after Blaise Pascal, a famous French Mathematician and Philosopher).
To build the triangle, start with "1" at the top, then continue placing numbers below it in a triangular pattern.

Each number is the numbers directly above it added together.
(Here I have highlighted that 1+3 = 4)

Patterns Within the Triangle