666666

This number is a composite.

The only six-digit repdigit such that the sum of itself and all truncations of itself plus one is an emirp. [Bajpai]

The larger of only two six-digit repdigit such that the product of itself and all truncations of itself plus one, i.e., 666666 * 66666 * 6666 * 666 * 66 * 6 + 1 = 78135326102739761857 is a prime. The other number is 444444. [Bajpai]