161691981 has 32 divisors (see below), whose sum is σ = 250293888. Its totient is φ = 91760640.

The previous prime is 161691973. The next prime is 161691983. The reversal of 161691981 is 189196161.

It is a happy number.

161691981 is digitally balanced in base 2, because in such base it contains all the possibile digits an equal number of times.

It is not a de Polignac number, because 161691981 - 2³ = 161691973 is a prime.

It is a super-2 number, since 2×161691981² = 52288593439408722, which contains 22 as substring.

It is a congruent number.

It is not an unprimeable number, because it can be changed into a prime (161691983) by changing a digit.

It is a polite number, since it can be written in 31 ways as a sum of consecutive naturals, for example, 130095 + ... + 131331.

It is an arithmetic number, because the mean of its divisors is an integer number (7821684).

Almost surely, 2^161691981 is an apocalyptic number.

161691981 is a gapful number since it is divisible by the number (11) formed by its first and last digit.

It is an amenable number.

161691981 is a deficient number, since it is larger than the sum of its proper divisors (88601907).

161691981 is a wasteful number, since it uses less digits than its factorization.

161691981 is an evil number, because the sum of its binary digits is even.

The sum of its prime factors is 1501.

The product of its digits is 23328, while the sum is 42.

The square root of 161691981 is about 12715.8161751419. The cubic root of 161691981 is about 544.7904600862.

The spelling of 161691981 in words is "one hundred sixty-one million, six hundred ninety-one thousand, nine hundred eighty-one".