101

This number is a prime.

Combine two digits of a number in order (from right to left) alternating them with + then -. If the result is 0 or a multiple of 101, the prime number 101 divides it. E.g., Is 575519513 divisible by 101? 13 - 95 + 51 - 75 + 05 = -101. Yes! [Luhn]