31 (thirty-one) is the natural number following 30 and preceding 32. It is a prime number.

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.

135px|left|thumb|31 is a [[centered pentagonal 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.

31 is the international calling code for the Netherlands.

Notes

References

  • Prime Curios! 31 from the Prime Pages