131333131

This number is a prime.

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The smallest "plus sign prime."

        1 3 1
        3 3 3
        1 3 1

A plus sign prime is a palindromic prime of n^2 digits (n, odd > 2) arranged left-right, top-down, in an n-by-n array, where the central row and column make a plus sign (+) and consist of identical digits b. The other digits of the array are identical digits different from b. Thus, an order 3 plus sign prime is of the form ababbbaba, order 5 of the form aabaaaabaabbbbbaabaaaabaa, etc. [Loungrides]