81661920 has 96 divisors (see below), whose sum is σ = 261545760. Its totient is φ = 21411840.

The previous prime is 81661919. The next prime is 81661927. The reversal of 81661920 is 2916618.

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

It is a congruent number.

It is not an unprimeable number, because it can be changed into a prime (81661927) 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, 27886 + ... + 30674.

It is an arithmetic number, because the mean of its divisors is an integer number (2724435).

Almost surely, 2^81661920 is an apocalyptic number.

81661920 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 81661920, and also a Zumkeller number, because its divisors can be partitioned in two sets with the same sum (130772880).

81661920 is an abundant number, since it is smaller than the sum of its proper divisors (179883840).

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

81661920 is a wasteful number, since it uses less digits than its factorization.

81661920 is an evil number, because the sum of its binary digits is even.

The sum of its prime factors is 2868 (or 2860 counting only the distinct ones).

The product of its (nonzero) digits is 5184, while the sum is 33.

The square root of 81661920 is about 9036.6985121780. The cubic root of 81661920 is about 433.8502601632.

The spelling of 81661920 in words is "eighty-one million, six hundred sixty-one thousand, nine hundred twenty".