81721600 has 81 divisors (see below), whose sum is σ = 204079603. Its totient is φ = 32399360.

The previous prime is 81721597. The next prime is 81721609. The reversal of 81721600 is 612718.

The square root of 81721600 is 9040.

It is a perfect power (a square), and thus also a powerful number.

It can be written as a sum of positive squares in 4 ways, for example, as 41576704 + 40144896 = 6448^2 + 6336^2 .

It is a Harshad number since it is a multiple of its sum of digits (25).

It is a Duffinian number.

It is not an unprimeable number, because it can be changed into a prime (81721609) by changing a digit.

It is a pernicious number, because its binary representation contains a prime number (13) of ones.

It is a polite number, since it can be written in 8 ways as a sum of consecutive naturals, for example, 723144 + ... + 723256.

Almost surely, 2^81721600 is an apocalyptic number.

81721600 is a gapful number since it is divisible by the number (80) formed by its first and last digit.

81721600 is the 9040-th square number.

It is an amenable number.

It is a practical number, because each smaller number is the sum of distinct divisors of 81721600

81721600 is an abundant number, since it is smaller than the sum of its proper divisors (122358003).

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

81721600 is an equidigital number, since it uses as much as digits as its factorization.

81721600 is an odious number, because the sum of its binary digits is odd.

The sum of its prime factors is 252 (or 120 counting only the distinct ones).

The product of its (nonzero) digits is 672, while the sum is 25.

The cubic root of 81721600 is about 433.9559229519.

Multiplying 81721600 by its sum of digits (25), we get a square (2043040000 = 45200²).

81721600 divided by its sum of digits (25) gives a square (3268864 = 1808²).

The spelling of 81721600 in words is "eighty-one million, seven hundred twenty-one thousand, six hundred".