21111...11113 (25-digits)

This number is a prime.

21111 1111111111 1111111113

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The only prime formed by inserting a prime number p of 1's between the digits of a double-digit prime p, (p=23). [Loungrides]

Submitted: 2020-08-01 06:55:35; Last Modified: 2020-08-01 13:52:32.