31

31 < 30 | 32 >
Numbers 0 - 99
0	1	2	3	4	5	6	7	8	9
10	11	12	13	14	15	16	17	18	19
20	21	22	23	24	25	26	27	28	29
30	31	32	33	34	35	36	37	38	39
40	41	42	43	44	45	46	47	48	49
50	51	52	53	54	55	56	57	58	59
60	61	62	63	64	65	66	67	68	69
70	71	72	73	74	75	76	77	78	79
80	81	82	83	84	85	86	87	88	89
90	91	92	93	94	95	96	97	98	99

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 odd prime number. It is a Mersenne prime, 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, 3^π 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.