11111...99999 (38-digits)

This number is a prime.

                                   11111119 9999999999 9999999999 9999999999

+ The fifth (the fourth being 1111111999999, an emirp) of an infinite sequence of primes formed by right-concatenating further instances of an identical terminating digit 9 to 11111119 the near-rep-digit twin prime of 11111117. In this case the prime differs by only a SINGLE additional identical digit from the sixth prime so formed, i.e. 111111199999999999999999999999999999999. [Harrison]

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