63991 has 4 divisors (see below), whose sum is σ = 64800. Its totient is φ = 63184.

The previous prime is 63977. The next prime is 63997. The reversal of 63991 is 19936.

It is a happy number.

It is a semiprime because it is the product of two primes.

It is a cyclic number.

It is not a de Polignac number, because 63991 - 2⁷ = 63863 is a prime.

It is a Duffinian number.

It is a congruent number.

It is not an unprimeable number, because it can be changed into a prime (63997) 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 in 3 ways as a sum of consecutive naturals, for example, 271 + ... + 448.

It is an arithmetic number, because the mean of its divisors is an integer number (16200).

2⁶³⁹⁹¹ is an apocalyptic number.

63991 is a deficient number, since it is larger than the sum of its proper divisors (809).

63991 is an equidigital number, since it uses as much as digits as its factorization.

63991 is an odious number, because the sum of its binary digits is odd.

The sum of its prime factors is 808.

The product of its digits is 1458, while the sum is 28.

The square root of 63991 is about 252.9644243762. The cubic root of 63991 is about 39.9981249121.

It can be divided in two parts, 6399 and 1, that added together give a square (6400 = 80²).

The spelling of 63991 in words is "sixty-three thousand, nine hundred ninety-one".