81731640 has 64 divisors (see below), whose sum is σ = 246535200. Its totient is φ = 21676032.

The previous prime is 81731621. The next prime is 81731641. The reversal of 81731640 is 4613718.

It is a happy number.

It is a Harshad number since it is a multiple of its sum of digits (30).

It is a junction number, because it is equal to n+sod(n) for n = 81731598 and 81731607.

It is a congruent number.

It is not an unprimeable number, because it can be changed into a prime (81731641) 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, 21396 + ... + 24924.

Almost surely, 2^81731640 is an apocalyptic number.

It is an amenable number.

It is a practical number, because each smaller number is the sum of distinct divisors of 81731640, and also a Zumkeller number, because its divisors can be partitioned in two sets with the same sum (123267600).

81731640 is an abundant number, since it is smaller than the sum of its proper divisors (164803560).

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

81731640 is a wasteful number, since it uses less digits than its factorization.

81731640 is an evil number, because the sum of its binary digits is even.

The sum of its prime factors is 3736 (or 3732 counting only the distinct ones).

The product of its (nonzero) digits is 4032, while the sum is 30.

The square root of 81731640 is about 9040.5552926798. Note that the first 3 decimals coincide. The cubic root of 81731640 is about 433.9736936068.

The spelling of 81731640 in words is "eighty-one million, seven hundred thirty-one thousand, six hundred forty".