631991 has 2 divisors, whose sum is σ = 631992. Its totient is φ = 631990.

The previous prime is 631987. The next prime is 631993. The reversal of 631991 is 199136.

It is a strong prime.

It is a cyclic number.

It is not a de Polignac number, because 631991 - 2² = 631987 is a prime.

Together with 631993, it forms a pair of twin primes.

It is a Chen prime.

It is a congruent number.

It is not a weakly prime, because it can be changed into another prime (631993) by changing a digit.

It is a pernicious number, because its binary representation contains a prime number (11) of ones.

It is a polite number, since it can be written as a sum of consecutive naturals, namely, 315995 + 315996.

It is an arithmetic number, because the mean of its divisors is an integer number (315996).

2⁶³¹⁹⁹¹ is an apocalyptic number.

631991 is a deficient number, since it is larger than the sum of its proper divisors (1).

631991 is an equidigital number, since it uses as much as digits as its factorization.

631991 is an odious number, because the sum of its binary digits is odd.

The product of its digits is 1458, while the sum is 29.

The square root of 631991 is about 794.9786160646. The cubic root of 631991 is about 85.8164011803.

The spelling of 631991 in words is "six hundred thirty-one thousand, nine hundred ninety-one".