77777...37777 (55-digits)

This number is a prime.

77777 7777773333 3337735555 3773525377 3553773537 7337737777

7 7 7 7 7 7 7 7 7 7 7 3 3 3 3 3 3 3 7 7 3 5 5 5 5 3 7 7 3 5 2 5 3 7 7 3 5 5 3 7 7 3 5 3 7 7 3 3 7 7 3 7 7 7 7

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