81639840 has 96 divisors (see below), whose sum is σ = 258503616. Its totient is φ = 21657600.

The previous prime is 81639839. The next prime is 81639841. The reversal of 81639840 is 4893618.

It is an interprime number because it is at equal distance from previous prime (81639839) and next prime (81639841).

It is a tau number, because it is divible by the number of its divisors (96).

It is not an unprimeable number, because it can be changed into a prime (81639841) 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, 135540 + ... + 136140.

It is an arithmetic number, because the mean of its divisors is an integer number (2692746).

Almost surely, 2^81639840 is an apocalyptic number.

81639840 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 81639840, and also a Zumkeller number, because its divisors can be partitioned in two sets with the same sum (129251808).

81639840 is an abundant number, since it is smaller than the sum of its proper divisors (176863776).

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

81639840 is a wasteful number, since it uses less digits than its factorization.

81639840 is an evil number, because the sum of its binary digits is even.

The sum of its prime factors is 902 (or 894 counting only the distinct ones).

The product of its (nonzero) digits is 41472, while the sum is 39.

The square root of 81639840 is about 9035.4767444778. The cubic root of 81639840 is about 433.8111547179.

The spelling of 81639840 in words is "eighty-one million, six hundred thirty-nine thousand, eight hundred forty".