7

This number is a prime.

The last digit of Graham's number, and the rarest digit among the 500 rightmost decimal digits of Graham's number (these 500 digits include 56 0s, 56 2s, 56 9s, 55 5s, 54 8s, 49 1s, 47 3s, 46 4s, 46 6s, but only 35 7s). [Olry]