1231

This number is a prime.

The smallest prime that can be represented as the sum of a prime and its reversal in three different ways. [Gupta]

The last "prime day" of the year is 12/31. [Hallyburton]

The smallest mountain emirp. [Loungrides]

The last two prime days of the year, 12/29 and the New Year's Eve prime, 12/31, form a 'twimirp' (a twin prime pair whose members are emirps). Submitted on 12/31. [Green]

There are exactly 1231 isomers of undecene (C₁₁H₂₂). [Gudipati]

The smallest prime (emirp) of form x+R(x) where x is a prime and R(x) its reversal, in three ways, i.e., 1231 = 269+962 = 467+764 = 863+368. [Loungrides]