31

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Summary

Factorization

The number is a prime number.

Properties and families

Property or family	Parameter values	First few numbers	Proof of satisfaction/membership/containment
prime number	11th prime number	2, 3, 5, 7, 11, 13, 17, 19, 23, 29, 31, 37, 41, 43, 47, 53, 59, 61, 67, 71, 73, 79, 83, 89, 97, [SHOW MORE] 101, 103, 107, 109, 113, 127, 131, 137, 139, 149, 151, 157, 163, 167, 173, 179, 181, 191, 193, 197, 199, 211, 223, 227, 229, 233, 239, 241, 251, 257, 263, 269, 271 View list on OEIS	divide and check
Mersenne number $M_{n}=2^{n}-1$	$n=5$, i.e., $M_{5}=2^{5}-1$		plug and check
Mersenne prime (both a Mersenne number and a prime number)
regular prime	10th regular prime (note that 2 is neither a regular nor an irregular prime)	3, 5, 7, 11, 13, 17, 19, 23, 29, 31, 41, 43, 47, 53, 61, [SHOW MORE] 71, 73, 79, 83, 89, 97, 107, 109, 113, 127, 137, 139, 151, 163, 167, 173, 179, 181, 191, 193, 197, 199, 211, 223, 227, 229, 239, 241, 251, 269, 277, 281, 313, 317, 331, 337, 349, 359, 367, 373, 383, 397, 419, 431 View list on OEIS

Prime-generating polynomials

Below are some polynomials that give prime numbers for small input values, which give the value 31 for suitable input choice.

Polynomial	Degree	Some values for which it generates primes	Input value $n$ at which it generates 31
$n^{2}-n+11$	2	all numbers 1-10, because 11 is one of the lucky numbers of Euler.	5

Multiples

Interesting multiples

Number	Prime factorization	What's interesting about it
341	11 times 31	smallest Poulet number (also called Sarrus number), i.e., smallest Fermat pseudoprime to base 2
2821	7 times 13 times 31	one of the Carmichael numbers, i.e., absolute pseudoprimes