31991 has 2 divisors, whose sum is σ = 31992. Its totient is φ = 31990.

The previous prime is 31981. The next prime is 32003. The reversal of 31991 is 19913.

31991 is nontrivially palindromic in base 8.

It is a weak prime.

It is an emirp because it is prime and its reverse (19913) is a distict prime.

It is a cyclic number.

It is not a de Polignac number, because 31991 - 2¹⁴ = 15607 is a prime.

It is an Ulam number.

It is a nialpdrome in base 14.

It is a congruent number.

It is an inconsummate number, since it does not exist a number n which divided by its sum of digits gives 31991.

It is not a weakly prime, because it can be changed into another prime (31981) by changing a digit.

It is a polite number, since it can be written as a sum of consecutive naturals, namely, 15995 + 15996.

It is an arithmetic number, because the mean of its divisors is an integer number (15996).

2³¹⁹⁹¹ is an apocalyptic number.

31991 is a deficient number, since it is larger than the sum of its proper divisors (1).

31991 is an equidigital number, since it uses as much as digits as its factorization.

31991 is an evil number, because the sum of its binary digits is even.

The product of its digits is 243, while the sum is 23.

The square root of 31991 is about 178.8602806662. The cubic root of 31991 is about 31.7450443833.

The spelling of 31991 in words is "thirty-one thousand, nine hundred ninety-one".