81981691 has 8 divisors (see below), whose sum is σ = 89664624. Its totient is φ = 74337120.

The previous prime is 81981671. The next prime is 81981703. The reversal of 81981691 is 19618918.

It is a happy number.

It is a sphenic number, since it is the product of 3 distinct primes.

It is not a de Polignac number, because 81981691 - 2⁵ = 81981659 is a prime.

It is a Duffinian number.

It is not an unprimeable number, because it can be changed into a prime (81981611) by changing a digit.

It is a polite number, since it can be written in 7 ways as a sum of consecutive naturals, for example, 5020 + ... + 13753.

It is an arithmetic number, because the mean of its divisors is an integer number (11208078).

Almost surely, 2^81981691 is an apocalyptic number.

81981691 is a deficient number, since it is larger than the sum of its proper divisors (7682933).

81981691 is a wasteful number, since it uses less digits than its factorization.

81981691 is an evil number, because the sum of its binary digits is even.

The sum of its prime factors is 19181.

The product of its digits is 31104, while the sum is 43.

The square root of 81981691 is about 9054.3741362946. The cubic root of 81981691 is about 434.4158115718.

It can be divided in two parts, 8198 and 1691, that added together give a palindrome (9889).

The spelling of 81981691 in words is "eighty-one million, nine hundred eighty-one thousand, six hundred ninety-one".