13

This number is a prime.

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sod(M14 exponent) = sod(M17 exponent) = sod(M18 exponent) = sod(M20 exponent) = 13. [Harrison]

The Gregorian Calendar is named after Pope Gregory XIII. [Harrison]

There are 13 alphabetic characters in the text of both "twelve plus one" and it's anagram, "eleven plus two". [Harrison]