31 (thirty-one) is a positive integer following 30 and preceding 32. Its ordinal form is written "thirty-first" or "31st".


  • 31 is an odd prime number. It is a Mersenne prime[1], since it is equal to 25-1.
  • 31 is a primorial prime.
  • 31 is the largest number whose square contains all digits in descending order.

Relation to pi

  • The integral parts of π3, 10π, 3π and 100/π are all equal to 31.
  • The number π3 is almost equal to 31. The emphasis is on "almost", since π is a transcendental number.

In googology

In Greek-based number-naming systems, 31 is associated with prefix "hentriaconta-", and with prefix "untriginti-" in Latin systems.



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