541

This number is a prime.

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The only non-titanic prime of form 7*77*777*...*777...777 + 2, i.e., 541 = 7*77 + 2. [Loungrides]

The larger prime in the only known case of consecutive primes, (523, 541), that have the same sum of digits. Note that their concatenations in order and reverse order, i.e., 523541 and 541523, are both primes. [Loungrides]