81800721 has 12 divisors (see below), whose sum is σ = 118442740. Its totient is φ = 54401760.

The previous prime is 81800711. The next prime is 81800743. The reversal of 81800721 is 12700818.

It can be written as a sum of positive squares in 2 ways, for example, as 13242321 + 68558400 = 3639^2 + 8280^2 .

It is not a de Polignac number, because 81800721 - 2⁵ = 81800689 is a prime.

It is a super-2 number, since 2×81800721² = 13382715912239682, which contains 22 as substring.

It is a Duffinian number.

It is not an unprimeable number, because it can be changed into a prime (81800711) by changing a digit.

It is a polite number, since it can be written in 11 ways as a sum of consecutive naturals, for example, 7006 + ... + 14583.

Almost surely, 2^81800721 is an apocalyptic number.

It is an amenable number.

81800721 is a deficient number, since it is larger than the sum of its proper divisors (36642019).

81800721 is a wasteful number, since it uses less digits than its factorization.

81800721 is an evil number, because the sum of its binary digits is even.

The sum of its prime factors is 22016 (or 22013 counting only the distinct ones).

The product of its (nonzero) digits is 896, while the sum is 27.

The square root of 81800721 is about 9044.3751027918. The cubic root of 81800721 is about 434.0959265402.

The spelling of 81800721 in words is "eighty-one million, eight hundred thousand, seven hundred twenty-one".