81991910 has 64 divisors (see below), whose sum is σ = 198180864. Its totient is φ = 23587200.

The previous prime is 81991883. The next prime is 81991913. The reversal of 81991910 is 1919918.

It is a happy number.

It is a congruent number.

It is not an unprimeable number, because it can be changed into a prime (81991913) by changing a digit.

It is a pernicious number, because its binary representation contains a prime number (13) of ones.

It is a polite number, since it can be written in 31 ways as a sum of consecutive naturals, for example, 5915 + ... + 14105.

It is an arithmetic number, because the mean of its divisors is an integer number (3096576).

Almost surely, 2^81991910 is an apocalyptic number.

81991910 is an abundant number, since it is smaller than the sum of its proper divisors (116188954).

It is a pseudoperfect number, because it is the sum of a subset of its proper divisors.

81991910 is a wasteful number, since it uses less digits than its factorization.

81991910 is an odious number, because the sum of its binary digits is odd.

The sum of its prime factors is 8229.

The product of its (nonzero) digits is 5832, while the sum is 38.

The square root of 81991910 is about 9054.9384315963. The cubic root of 81991910 is about 434.4338607675.

It can be divided in two parts, 819 and 91910, that added together give a palindrome (92729).

The spelling of 81991910 in words is "eighty-one million, nine hundred ninety-one thousand, nine hundred ten".