815991 has 16 divisors (see below), whose sum is σ = 1205760. Its totient is φ = 486720.

The previous prime is 815989. The next prime is 816019. The reversal of 815991 is 199518.

It is not a de Polignac number, because 815991 - 2¹ = 815989 is a prime.

It is a Harshad number since it is a multiple of its sum of digits (33).

815991 is a lucky number.

It is a congruent number.

It is not an unprimeable number, because it can be changed into a prime (815291) 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 15 ways as a sum of consecutive naturals, for example, 2451 + ... + 2763.

It is an arithmetic number, because the mean of its divisors is an integer number (75360).

2⁸¹⁵⁹⁹¹ is an apocalyptic number.

815991 is a deficient number, since it is larger than the sum of its proper divisors (389769).

815991 is a wasteful number, since it uses less digits than its factorization.

815991 is an odious number, because the sum of its binary digits is odd.

The sum of its prime factors is 406.

The product of its digits is 3240, while the sum is 33.

The square root of 815991 is about 903.3222016534. The cubic root of 815991 is about 93.4462310123.

The spelling of 815991 in words is "eight hundred fifteen thousand, nine hundred ninety-one".