31991581 has 2 divisors, whose sum is σ = 31991582. Its totient is φ = 31991580.

The previous prime is 31991579. The next prime is 31991591. The reversal of 31991581 is 18519913.

It is a happy number.

It is a weak prime.

It can be written as a sum of positive squares in only one way, i.e., 29539225 + 2452356 = 5435^2 + 1566^2 .

It is an emirp because it is prime and its reverse (18519913) is a distict prime.

It is a cyclic number.

It is not a de Polignac number, because 31991581 - 2¹ = 31991579 is a prime.

It is a super-2 number, since 2×31991581² = 2046922509759122, which contains 22 as substring.

Together with 31991579, it forms a pair of twin primes.

It is a self number, because there is not a number n which added to its sum of digits gives 31991581.

It is a congruent number.

It is not a weakly prime, because it can be changed into another prime (31991591) by changing a digit.

It is a pernicious number, because its binary representation contains a prime number (13) of ones.

It is a polite number, since it can be written as a sum of consecutive naturals, namely, 15995790 + 15995791.

It is an arithmetic number, because the mean of its divisors is an integer number (15995791).

Almost surely, 2^31991581 is an apocalyptic number.

It is an amenable number.

31991581 is a deficient number, since it is larger than the sum of its proper divisors (1).

31991581 is an equidigital number, since it uses as much as digits as its factorization.

31991581 is an odious number, because the sum of its binary digits is odd.

The product of its digits is 9720, while the sum is 37.

The square root of 31991581 is about 5656.1100590423. The cubic root of 31991581 is about 317.4523655985.

The spelling of 31991581 in words is "thirty-one million, nine hundred ninety-one thousand, five hundred eighty-one".