1000000021
This number is a prime.
The largest prime that can be represented as the sum of two
repdigit numbers each consisting of d times each digit d,
i.e., 22+999999999. [Loungrides]
A prime that remains prime after inserting the digit X in all internal positions at the same time:
1X0X0X0X0X0X0X0X2X1. This is valid for X=5 and for X=8, producing the following two primes: 1505050505050505251 and
1808080808080808281. [Rivera]
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