31891981 has 4 divisors (see below), whose sum is σ = 34791264. Its totient is φ = 28992700.

The previous prime is 31891943. The next prime is 31891997. The reversal of 31891981 is 18919813.

It is a happy number.

It is the 2306-th star 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, and also an emirpimes, since its reverse is a distinct semiprime: 18919813 = 11 ⋅1719983.

It is a de Polignac number, because none of the positive numbers 2^k-31891981 is a prime.

It is a super-2 number, since 2×31891981² = 2034196904208722, which contains 22 as substring.

It is a Smith number, since the sum of its digits (40) coincides with the sum of the digits of its prime factors. Since it is squarefree, it is also a hoax number.

It is a Duffinian number.

It is a congruent number.

It is not an unprimeable number, because it can be changed into a prime (31891381) 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, 1449625 + ... + 1449646.

It is an arithmetic number, because the mean of its divisors is an integer number (8697816).

Almost surely, 2^31891981 is an apocalyptic number.

It is an amenable number.

31891981 is a deficient number, since it is larger than the sum of its proper divisors (2899283).

31891981 is a wasteful number, since it uses less digits than its factorization.

31891981 is an evil number, because the sum of its binary digits is even.

The sum of its prime factors is 2899282.

The product of its digits is 15552, while the sum is 40.

The square root of 31891981 is about 5647.2985577177. The cubic root of 31891981 is about 317.1225796145.

The spelling of 31891981 in words is "thirty-one million, eight hundred ninety-one thousand, nine hundred eighty-one".