+**31** is a [natural number](/wiki/Natural_Number) and a [prime number](/wiki/Prime_Number). It is the eleventh prime, and also a Mersenne prime exponent, holding a notable place in number theory and various calendar systems.
+## See also
+- [Mersenne Prime](/wiki/Mersenne_Prime)
+- [Calendar](/wiki/Calendar)
+- [Prime Number](/wiki/Prime_Number)
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