664821 has 8 divisors (see below), whose sum is σ = 984960. Its totient is φ = 443196.

The previous prime is 664799. The next prime is 664843. The reversal of 664821 is 128466.

664821 is digitally balanced in base 2, because in such base it contains all the possibile digits an equal number of times.

It is an interprime number because it is at equal distance from previous prime (664799) and next prime (664843).

It is not a de Polignac number, because 664821 - 2⁵ = 664789 is a prime.

It is a Harshad number since it is a multiple of its sum of digits (27), and also a Moran number because the ratio is a prime number: 24623 = 664821 / (6 + 6 + 4 + 8 + 2 + 1).

It is a congruent number.

It is not an unprimeable number, because it can be changed into a prime (664891) 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, 12285 + ... + 12338.

It is an arithmetic number, because the mean of its divisors is an integer number (123120).

2⁶⁶⁴⁸²¹ is an apocalyptic number.

It is an amenable number.

664821 is a deficient number, since it is larger than the sum of its proper divisors (320139).

664821 is a wasteful number, since it uses less digits than its factorization.

664821 is an evil number, because the sum of its binary digits is even.

The sum of its prime factors is 24632 (or 24626 counting only the distinct ones).

The product of its digits is 2304, while the sum is 27.

The square root of 664821 is about 815.3655621867. The cubic root of 664821 is about 87.2773550458.

It can be divided in two parts, 664 and 821, that added together give a triangular number (1485 = T₅₄).

The spelling of 664821 in words is "six hundred sixty-four thousand, eight hundred twenty-one".