31 (thirty-one) is the natural number following 30 and preceding 32.
Mathematics 31 is a prime number, a twin prime (with 29), a super-prime, a member of an emirp prime pair, a sexy prime, and a Mersenne prime. Since it is a Mersenne prime, it can be expressed as 25 - 1.
31 is a centered triangular number, a centered pentagonal number, and a centered decagonal number.
In other fields 31 equal temperament is a popular microtonal tuning for musical instruments because it provides a good approximation of harmonic intervals.
January, March, May, July, August, October and December have 31 days.
References
External links
Prime Curios! 31 from the Prime Pages