81661440 has 320 divisors, whose sum is σ = 314265600. Its totient is φ = 18063360.

The previous prime is 81661439. The next prime is 81661451. The reversal of 81661440 is 4416618.

It is a tau number, because it is divible by the number of its divisors (320).

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 = 81661398 and 81661407.

It is a congruent number.

It is an unprimeable number.

It is a polite number, since it can be written in 31 ways as a sum of consecutive naturals, for example, 2634225 + ... + 2634255.

It is an arithmetic number, because the mean of its divisors is an integer number (982080).

Almost surely, 2^81661440 is an apocalyptic number.

81661440 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 81661440, and also a Zumkeller number, because its divisors can be partitioned in two sets with the same sum (157132800).

81661440 is an abundant number, since it is smaller than the sum of its proper divisors (232604160).

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

81661440 is an equidigital number, since it uses as much as digits as its factorization.

81661440 is an evil number, because the sum of its binary digits is even.

The sum of its prime factors is 78 (or 48 counting only the distinct ones).

The product of its (nonzero) digits is 4608, while the sum is 30.

The square root of 81661440 is about 9036.6719537671. The cubic root of 81661440 is about 433.8494101198.

The spelling of 81661440 in words is "eighty-one million, six hundred sixty-one thousand, four hundred forty".