77777...77771 (100-digits)

This number is a prime.

                                            7777777777 7777777777 7777777777
7777777777 7777777777 7777777777 7777777777 7777777777 7777777777 7777777771

The one hundredth term in the almost repunit sequence 1, 71, 771, 7771, etc. The closed formula for this sequence is given by f (n) = (7 * 10 ^ n - 61) / 9 and a recursive formula can be described as follows: Define u (1) = 1 and u (n) = 10 * u (n - 1) + 61 for n > 1. [Schiffman]