31

31 (thirty-one) is a positive integer following 30 and preceding 32. Its ordinal form is written "thirty-first" or "31st".
== Properties ==
*31 is an . It is a Mersenne primeOEIS, Sequence A000668, 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 .
== In googology ==
In Greek-based number-naming systems, 31 is associated with prefix "hentriaconta-", and with prefix "untriginti-" in Latin systems.
==Sources==
Category:Numbers
Category:Class 1
Category:Numbers 0-99
Category:Numbers with 1 to 2 digits
Category:Pi-related numbers
Category:Prime numbers