31991981 has 4 divisors (see below), whose sum is σ = 36562272. Its totient is φ = 27421692.

The previous prime is 31991941. The next prime is 31991989. The reversal of 31991981 is 18919913.

It is a happy number.

It is a semiprime because it is the product of two primes, and also a Blum integer, because the two primes are equal to 3 mod 4.

It is a cyclic number.

It is not a de Polignac number, because 31991981 - 2¹⁴ = 31975597 is a prime.

It is a super-2 number, since 2×31991981² = 2046973696608722, which contains 22 as substring.

It is a Duffinian number.

It is a congruent number.

It is not an unprimeable number, because it can be changed into a prime (31991989) by changing a digit.

It is a polite number, since it can be written in 3 ways as a sum of consecutive naturals, for example, 2285135 + ... + 2285148.

It is an arithmetic number, because the mean of its divisors is an integer number (9140568).

Almost surely, 2^31991981 is an apocalyptic number.

It is an amenable number.

31991981 is a deficient number, since it is larger than the sum of its proper divisors (4570291).

31991981 is an equidigital number, since it uses as much as digits as its factorization.

31991981 is an evil number, because the sum of its binary digits is even.

The sum of its prime factors is 4570290.

The product of its digits is 17496, while the sum is 41.

The square root of 31991981 is about 5656.1454189227. The cubic root of 31991981 is about 317.4536886593.

The spelling of 31991981 in words is "thirty-one million, nine hundred ninety-one thousand, nine hundred eighty-one".