215731991 has 2 divisors, whose sum is σ = 215731992. Its totient is φ = 215731990.

The previous prime is 215731987. The next prime is 215731993. The reversal of 215731991 is 199137512.

It is a strong prime.

It is a cyclic number.

It is not a de Polignac number, because 215731991 - 2² = 215731987 is a prime.

Together with 215731993, it forms a pair of twin primes.

It is a Chen prime.

It is a congruent number.

It is not a weakly prime, because it can be changed into another prime (215731993) by changing a digit.

It is a polite number, since it can be written as a sum of consecutive naturals, namely, 107865995 + 107865996.

It is an arithmetic number, because the mean of its divisors is an integer number (107865996).

Almost surely, 2^215731991 is an apocalyptic number.

215731991 is a deficient number, since it is larger than the sum of its proper divisors (1).

215731991 is an equidigital number, since it uses as much as digits as its factorization.

215731991 is an evil number, because the sum of its binary digits is even.

The product of its digits is 17010, while the sum is 38.

The square root of 215731991 is about 14687.8177752857. The cubic root of 215731991 is about 599.7517408116.

The spelling of 215731991 in words is "two hundred fifteen million, seven hundred thirty-one thousand, nine hundred ninety-one".