215981 has 2 divisors, whose sum is σ = 215982. Its totient is φ = 215980.

The previous prime is 215959. The next prime is 215983. The reversal of 215981 is 189512.

It is a happy number.

It is a strong prime.

It can be written as a sum of positive squares in only one way, i.e., 198025 + 17956 = 445^2 + 134^2 .

It is a cyclic number.

It is a de Polignac number, because none of the positive numbers 2^k-215981 is a prime.

It is a super-2 number, since 2×215981² = 93295584722, which contains 22 as substring.

Together with 215983, it forms a pair of twin primes.

It is a Chen prime.

It is equal to p₁₉₂₈₆ and since 215981 and 19286 have the same sum of digits, it is a Honaker prime.

It is a self number, because there is not a number n which added to its sum of digits gives 215981.

It is a congruent number.

It is not a weakly prime, because it can be changed into another prime (215983) 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, 107990 + 107991.

It is an arithmetic number, because the mean of its divisors is an integer number (107991).

2²¹⁵⁹⁸¹ is an apocalyptic number.

It is an amenable number.

215981 is a deficient number, since it is larger than the sum of its proper divisors (1).

215981 is an equidigital number, since it uses as much as digits as its factorization.

215981 is an odious number, because the sum of its binary digits is odd.

The product of its digits is 720, while the sum is 26.

The square root of 215981 is about 464.7375603499. The cubic root of 215981 is about 59.9982406892.

The spelling of 215981 in words is "two hundred fifteen thousand, nine hundred eighty-one".