81643840 has 28 divisors (see below), whose sum is σ = 194415156. Its totient is φ = 32657408.

The previous prime is 81643831. The next prime is 81643847. The reversal of 81643840 is 4834618.

It can be written as a sum of positive squares in 2 ways, for example, as 39237696 + 42406144 = 6264^2 + 6512^2 .

It is a junction number, because it is equal to n+sod(n) for n = 81643796 and 81643805.

It is a congruent number.

It is not an unprimeable number, because it can be changed into a prime (81643847) by changing a digit.

It is a polite number, since it can be written in 3 ways as a sum of consecutive naturals, for example, 127249 + ... + 127888.

Almost surely, 2^81643840 is an apocalyptic number.

81643840 is a gapful number since it is divisible by the number (80) formed by its first and last digit.

It is an amenable number.

81643840 is an abundant number, since it is smaller than the sum of its proper divisors (112771316).

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

81643840 is a wasteful number, since it uses less digits than its factorization.

81643840 is an evil number, because the sum of its binary digits is even.

The sum of its prime factors is 255154 (or 255144 counting only the distinct ones).

The product of its (nonzero) digits is 18432, while the sum is 34.

The square root of 81643840 is about 9035.6980914592. The cubic root of 81643840 is about 433.8182395606.

The spelling of 81643840 in words is "eighty-one million, six hundred forty-three thousand, eight hundred forty".