165581991 has 24 divisors (see below), whose sum is σ = 253319040. Its totient is φ = 104003856.

The previous prime is 165581971. The next prime is 165582019. The reversal of 165581991 is 199185561.

165581991 is a `hidden beast` number, since 1 + 6 + 558 + 1 + 9 + 91 = 666.

It is not a de Polignac number, because 165581991 - 2¹¹ = 165579943 is a prime.

It is a congruent number.

It is not an unprimeable number, because it can be changed into a prime (165581971) by changing a digit.

It is a polite number, since it can be written in 23 ways as a sum of consecutive naturals, for example, 7900 + ... + 19838.

It is an arithmetic number, because the mean of its divisors is an integer number (10554960).

Almost surely, 2^165581991 is an apocalyptic number.

165581991 is a deficient number, since it is larger than the sum of its proper divisors (87737049).

165581991 is a wasteful number, since it uses less digits than its factorization.

165581991 is an evil number, because the sum of its binary digits is even.

The sum of its prime factors is 12035 (or 12032 counting only the distinct ones).

The product of its digits is 97200, while the sum is 45.

The square root of 165581991 is about 12867.8666063959. The cubic root of 165581991 is about 549.1247688393.

The spelling of 165581991 in words is "one hundred sixty-five million, five hundred eighty-one thousand, nine hundred ninety-one".