15991 has 2 divisors, whose sum is σ = 15992. Its totient is φ = 15990.

The previous prime is 15973. The next prime is 16001. The reversal of 15991 is 19951.

It is a strong prime.

It is a cyclic number.

It is not a de Polignac number, because 15991 - 2⁵ = 15959 is a prime.

It is a congruent number.

It is not a weakly prime, because it can be changed into another prime (15901) 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, 7995 + 7996.

It is an arithmetic number, because the mean of its divisors is an integer number (7996).

2¹⁵⁹⁹¹ is an apocalyptic number.

15991 is a deficient number, since it is larger than the sum of its proper divisors (1).

15991 is an equidigital number, since it uses as much as digits as its factorization.

15991 is an odious number, because the sum of its binary digits is odd.

The product of its digits is 405, while the sum is 25.

The square root of 15991 is about 126.4555257788. The cubic root of 15991 is about 25.1936954078.

It can be divided in two parts, 1599 and 1, that added together give a square (1600 = 40²).

The spelling of 15991 in words is "fifteen thousand, nine hundred ninety-one".