11111...31111 (55-digits)

This number is a prime.

11111 1111113333 3331135555 3113575311 3553113531 1331131111

1 1 1 1 1 1 1 1 1 1 1 3 3 3 3 3 3 3 1 1 3 5 5 5 5 3 1 1 3 5 7 5 3 1 1 3 5 5 3 1 1 3 5 3 1 1 3 3 1 1 3 1 1 1 1

A triangle-congruent prime consisting of only odd digits. [Patterson]