1111111111111111111

This number is a prime.

                                                        111111111 1111111111

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The smallest prime Kaprekar number. E.g., 1111111111111111111^2 = 1234567901234567900987654320987654321 and 123456790123456790 + 0987654320987654321 = 1111111111111111111. [Rivera]

11*11*1111111111111111111 = R(2)*R(2)*R(19) = 134444444444444444431, four palindromes. The first three, 11, 11 and 1111111111111111111, are palindromic primes. Note that the concatenation of these three palprimes is R(2).R(2).R(19) = R(23) = 11111111111111111111111. Found by Giovanni Resta. [Rivera]