721644021 has 36 divisors (see below), whose sum is σ = 1265296032. Its totient is φ = 394433424.

The previous prime is 721644019. The next prime is 721644029. The reversal of 721644021 is 120446127.

721644021 is a `hidden beast` number, since 7 + 216 + 440 + 2 + 1 = 666.

It is not a de Polignac number, because 721644021 - 2¹ = 721644019 is a prime.

It is a Curzon number.

It is a congruent number.

It is not an unprimeable number, because it can be changed into a prime (721644029) by changing a digit.

It is a polite number, since it can be written in 35 ways as a sum of consecutive naturals, for example, 25431 + ... + 45716.

It is an arithmetic number, because the mean of its divisors is an integer number (35147112).

Almost surely, 2^721644021 is an apocalyptic number.

It is an amenable number.

721644021 is a deficient number, since it is larger than the sum of its proper divisors (543652011).

721644021 is a wasteful number, since it uses less digits than its factorization.

721644021 is an evil number, because the sum of its binary digits is even.

The sum of its prime factors is 71190 (or 71180 counting only the distinct ones).

The product of its (nonzero) digits is 2688, while the sum is 27.

The square root of 721644021 is about 26863.4327851077. The cubic root of 721644021 is about 896.9626088522.

The spelling of 721644021 in words is "seven hundred twenty-one million, six hundred forty-four thousand, twenty-one".