644921 has 4 divisors (see below), whose sum is σ = 649344. Its totient is φ = 640500.

The previous prime is 644911. The next prime is 644923. The reversal of 644921 is 129446.

644921 = T₆ + T₇ + ... + T₁₅₆.

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 a cyclic number.

It is not a de Polignac number, because 644921 - 2⁶ = 644857 is a prime.

It is an Ulam number.

It is a Duffinian number.

It is an inconsummate number, since it does not exist a number n which divided by its sum of digits gives 644921.

It is not an unprimeable number, because it can be changed into a prime (644923) 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, 1985 + ... + 2286.

It is an arithmetic number, because the mean of its divisors is an integer number (162336).

2⁶⁴⁴⁹²¹ is an apocalyptic number.

It is an amenable number.

644921 is a deficient number, since it is larger than the sum of its proper divisors (4423).

644921 is a wasteful number, since it uses less digits than its factorization.

644921 is an evil number, because the sum of its binary digits is even.

The sum of its prime factors is 4422.

The product of its digits is 1728, while the sum is 26.

The square root of 644921 is about 803.0697354527. The cubic root of 644921 is about 86.3976983412.

The spelling of 644921 in words is "six hundred forty-four thousand, nine hundred twenty-one".