81981991 has 16 divisors (see below), whose sum is σ = 101538304. Its totient is φ = 64455264.

The previous prime is 81981989. The next prime is 81981997. The reversal of 81981991 is 19918918.

It is a cyclic number.

It is not a de Polignac number, because 81981991 - 2¹ = 81981989 is a prime.

It is a congruent number.

It is not an unprimeable number, because it can be changed into a prime (81981997) by changing a digit.

It is a polite number, since it can be written in 15 ways as a sum of consecutive naturals, for example, 12070 + ... + 17596.

It is an arithmetic number, because the mean of its divisors is an integer number (6346144).

Almost surely, 2^81981991 is an apocalyptic number.

81981991 is a deficient number, since it is larger than the sum of its proper divisors (19556313).

81981991 is a wasteful number, since it uses less digits than its factorization.

81981991 is an evil number, because the sum of its binary digits is even.

The sum of its prime factors is 5710.

The product of its digits is 46656, while the sum is 46.

The square root of 81981991 is about 9054.3907028579. The cubic root of 81981991 is about 434.4163414649.

The spelling of 81981991 in words is "eighty-one million, nine hundred eighty-one thousand, nine hundred ninety-one".