81670721 has 32 divisors (see below), whose sum is σ = 97822080. Its totient is φ = 67392000.

The previous prime is 81670711. The next prime is 81670727. The reversal of 81670721 is 12707618.

It is a cyclic number.

It is not a de Polignac number, because 81670721 - 2¹⁰ = 81669697 is a prime.

It is a Smith number, since the sum of its digits (32) coincides with the sum of the digits of its prime factors. Since it is squarefree, it is also a hoax number.

It is an alternating number because its digits alternate between even and odd.

It is a Duffinian number.

It is not an unprimeable number, because it can be changed into a prime (81670727) by changing a digit.

It is a polite number, since it can be written in 31 ways as a sum of consecutive naturals, for example, 808571 + ... + 808671.

It is an arithmetic number, because the mean of its divisors is an integer number (3056940).

Almost surely, 2^81670721 is an apocalyptic number.

It is an amenable number.

81670721 is a deficient number, since it is larger than the sum of its proper divisors (16151359).

81670721 is a wasteful number, since it uses less digits than its factorization.

81670721 is an evil number, because the sum of its binary digits is even.

The sum of its prime factors is 257.

The product of its (nonzero) digits is 4704, while the sum is 32.

The square root of 81670721 is about 9037.1854578735. The cubic root of 81670721 is about 433.8658454730.

The spelling of 81670721 in words is "eighty-one million, six hundred seventy thousand, seven hundred twenty-one".