81637920 has 432 divisors, whose sum is σ = 352923480. Its totient is φ = 17031168.

The previous prime is 81637909. The next prime is 81637921. The reversal of 81637920 is 2973618.

It can be written as a sum of positive squares in 4 ways, for example, as 37601424 + 44036496 = 6132^2 + 6636^2 .

It is a Harshad number since it is a multiple of its sum of digits (36).

It is not an unprimeable number, because it can be changed into a prime (81637921) by changing a digit.

It is a polite number, since it can be written in 71 ways as a sum of consecutive naturals, for example, 917236 + ... + 917324.

Almost surely, 2^81637920 is an apocalyptic number.

81637920 is a gapful number since it is divisible by the number (80) formed by its first and last digit.

It is an amenable number.

It is a practical number, because each smaller number is the sum of distinct divisors of 81637920, and also a Zumkeller number, because its divisors can be partitioned in two sets with the same sum (176461740).

81637920 is an abundant number, since it is smaller than the sum of its proper divisors (271285560).

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

81637920 is a wasteful number, since it uses less digits than its factorization.

81637920 is an evil number, because the sum of its binary digits is even.

The sum of its prime factors is 137 (or 119 counting only the distinct ones).

The product of its (nonzero) digits is 18144, while the sum is 36.

The square root of 81637920 is about 9035.3704960007. The cubic root of 81637920 is about 433.8077539111.

The spelling of 81637920 in words is "eighty-one million, six hundred thirty-seven thousand, nine hundred twenty".