421

This number is a prime.

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The only non-titanic prime concatenated in reverse order from consecutive powers of 2. [Loungrides]

The largest prime formed by concatenating all distinct divisors of a composite digit. [Loungrides]

The product of first 421 positive even numbers minus 1 (i.e., 2^421*421!-1) is the smallest titanic prime of form 2^p*p!-1, where p is a prime. [Loungrides]