644071521 has 4 divisors (see below), whose sum is σ = 858762032. Its totient is φ = 429381012.

The previous prime is 644071513. The next prime is 644071541. The reversal of 644071521 is 125170446.

It is a semiprime because it is the product of two primes, and also a Blum integer, because the two primes are equal to 3 mod 4.

It is not a de Polignac number, because 644071521 - 2³ = 644071513 is a prime.

It is a Duffinian number.

It is not an unprimeable number, because it can be changed into a prime (644071511) by changing a digit.

It is a polite number, since it can be written in 3 ways as a sum of consecutive naturals, for example, 107345251 + ... + 107345256.

It is an arithmetic number, because the mean of its divisors is an integer number (214690508).

Almost surely, 2^644071521 is an apocalyptic number.

It is an amenable number.

644071521 is a deficient number, since it is larger than the sum of its proper divisors (214690511).

644071521 is a wasteful number, since it uses less digits than its factorization.

644071521 is an evil number, because the sum of its binary digits is even.

The sum of its prime factors is 214690510.

The product of its (nonzero) digits is 6720, while the sum is 30.

The square root of 644071521 is about 25378.5642028859. The cubic root of 644071521 is about 863.5974781287.

The spelling of 644071521 in words is "six hundred forty-four million, seventy-one thousand, five hundred twenty-one".