56

This number is a composite.

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. (56, 58) is the only pair of successive double-digit even composite numbers m, n with composite reversals, R(m), R(n), such that m^4 + [R(m)]^4 and n^4 + [R(n)]^4, i.e., 56^4 + 65^4 and 58^4 + 85^4, are also primes. [Loungrides]