215991 has 12 divisors (see below), whose sum is σ = 316368. Its totient is φ = 141984.

The previous prime is 215983. The next prime is 216023. The reversal of 215991 is 199512.

It is a happy number.

It is not a de Polignac number, because 215991 - 2³ = 215983 is a prime.

It is a super-2 number, since 2×215991² = 93304224162, which contains 22 as substring.

It is a plaindrome in base 14.

It is a congruent number.

It is not an unprimeable number, because it can be changed into a prime (215921) by changing a digit.

It is a polite number, since it can be written in 11 ways as a sum of consecutive naturals, for example, 811 + ... + 1043.

It is an arithmetic number, because the mean of its divisors is an integer number (26364).

215991 is a Friedman number, since it can be written as (51+9)^(2+1)-9, using all its digits and the basic arithmetic operations.

2²¹⁵⁹⁹¹ is an apocalyptic number.

215991 is a deficient number, since it is larger than the sum of its proper divisors (100377).

215991 is a wasteful number, since it uses less digits than its factorization.

215991 is an evil number, because the sum of its binary digits is even.

The sum of its prime factors is 342 (or 339 counting only the distinct ones).

The product of its digits is 810, while the sum is 27.

The square root of 215991 is about 464.7483189857. The cubic root of 215991 is about 59.9991666551. Note that the first 3 decimals are identical.

The spelling of 215991 in words is "two hundred fifteen thousand, nine hundred ninety-one".