81991215 has 12 divisors (see below), whose sum is σ = 142118184. Its totient is φ = 43728624.

The previous prime is 81991201. The next prime is 81991229. The reversal of 81991215 is 51219918.

It is an interprime number because it is at equal distance from previous prime (81991201) and next prime (81991229).

It is not a de Polignac number, because 81991215 - 2⁴ = 81991199 is a prime.

It is a self number, because there is not a number n which added to its sum of digits gives 81991215.

It is a congruent number.

It is an unprimeable number.

It is a polite number, since it can be written in 11 ways as a sum of consecutive naturals, for example, 910969 + ... + 911058.

It is an arithmetic number, because the mean of its divisors is an integer number (11843182).

Almost surely, 2^81991215 is an apocalyptic number.

81991215 is a deficient number, since it is larger than the sum of its proper divisors (60126969).

81991215 is a wasteful number, since it uses less digits than its factorization.

81991215 is an evil number, because the sum of its binary digits is even.

The sum of its prime factors is 1822038 (or 1822035 counting only the distinct ones).

The product of its digits is 6480, while the sum is 36.

The square root of 81991215 is about 9054.9000546665. The cubic root of 81991215 is about 434.4326332789.

The spelling of 81991215 in words is "eighty-one million, nine hundred ninety-one thousand, two hundred fifteen".