991931 has 2 divisors, whose sum is σ = 991932. Its totient is φ = 991930.

The previous prime is 991927. The next prime is 991943. The reversal of 991931 is 139199.

It is a weak prime.

It is an emirp because it is prime and its reverse (139199) is a distict prime.

It is a cyclic number.

It is not a de Polignac number, because 991931 - 2² = 991927 is a prime.

It is a super-2 number, since 2×991931² = 1967854217522, which contains 22 as substring.

It is equal to p₇₇₉₁₈ and since 991931 and 77918 have the same sum of digits, it is a Honaker prime.

It is not a weakly prime, because it can be changed into another prime (991901) by changing a digit.

It is a polite number, since it can be written as a sum of consecutive naturals, namely, 495965 + 495966.

It is an arithmetic number, because the mean of its divisors is an integer number (495966).

2⁹⁹¹⁹³¹ is an apocalyptic number.

991931 is a deficient number, since it is larger than the sum of its proper divisors (1).

991931 is an equidigital number, since it uses as much as digits as its factorization.

991931 is an evil number, because the sum of its binary digits is even.

The product of its digits is 2187, while the sum is 32.

The square root of 991931 is about 995.9573284032. The cubic root of 991931 is about 99.7303066421.

The spelling of 991931 in words is "nine hundred ninety-one thousand, nine hundred thirty-one".