3999999097

This number is a prime.

+ The largest prime P less than 2^32, 3999999097, that generates two palindromic primes concatenating P.Rev(P) and Rev(P).P, where Rev(P) is reversal of P. In both cases we discard one of the two equal digits in the junction. The primes produced are 999999097909999993 and 7909999993999999097. [Rivera]

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