161991 has 12 divisors (see below), whose sum is σ = 240240. Its totient is φ = 105120.

The previous prime is 161983. The next prime is 161999. The reversal of 161991 is 199161.

It is an interprime number because it is at equal distance from previous prime (161983) and next prime (161999).

It is not a de Polignac number, because 161991 - 2³ = 161983 is a prime.

It is a Smith number, since the sum of its digits (27) coincides with the sum of the digits of its prime factors.

Its product of digits (486) is a multiple of the sum of its prime factors (486).

It is a congruent number.

It is not an unprimeable number, because it can be changed into a prime (161999) 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, 150 + ... + 588.

It is an arithmetic number, because the mean of its divisors is an integer number (20020).

2¹⁶¹⁹⁹¹ is an apocalyptic number.

161991 is a deficient number, since it is larger than the sum of its proper divisors (78249).

161991 is a wasteful number, since it uses less digits than its factorization.

161991 is an evil number, because the sum of its binary digits is even.

The sum of its prime factors is 486 (or 483 counting only the distinct ones).

The product of its digits is 486, while the sum is 27.

The square root of 161991 is about 402.4810554548. The cubic root of 161991 is about 54.5126082548.

The spelling of 161991 in words is "one hundred sixty-one thousand, nine hundred ninety-one".