78640721 has 8 divisors (see below), whose sum is σ = 82161000. Its totient is φ = 75174400.

The previous prime is 78640717. The next prime is 78640733. The reversal of 78640721 is 12704687.

It is a happy number.

It can be written as a sum of positive squares in 4 ways, for example, as 23658496 + 54982225 = 4864^2 + 7415^2 .

It is a sphenic number, since it is the product of 3 distinct primes.

It is a cyclic number.

It is not a de Polignac number, because 78640721 - 2² = 78640717 is a prime.

It is a Duffinian number.

It is not an unprimeable number, because it can be changed into a prime (78640021) by changing a digit.

It is a polite number, since it can be written in 7 ways as a sum of consecutive naturals, for example, 10496 + ... + 16353.

It is an arithmetic number, because the mean of its divisors is an integer number (10270125).

Almost surely, 2^78640721 is an apocalyptic number.

It is an amenable number.

78640721 is a deficient number, since it is larger than the sum of its proper divisors (3520279).

78640721 is a wasteful number, since it uses less digits than its factorization.

78640721 is an evil number, because the sum of its binary digits is even.

The sum of its prime factors is 26979.

The product of its (nonzero) digits is 18816, while the sum is 35.

The square root of 78640721 is about 8867.9603630147. The cubic root of 78640721 is about 428.4325862576.

The spelling of 78640721 in words is "seventy-eight million, six hundred forty thousand, seven hundred twenty-one".