644820 has 48 divisors (see below), whose sum is σ = 1971648. Its totient is φ = 156160.

The previous prime is 644801. The next prime is 644837. The reversal of 644820 is 28446.

644820 = T₆₁ + T₆₂ + ... + T₁₅₉.

It is a congruent number.

It is an inconsummate number, since it does not exist a number n which divided by its sum of digits gives 644820.

It is an unprimeable number.

It is a pernicious number, because its binary representation contains a prime number (11) of ones.

It is a polite number, since it can be written in 15 ways as a sum of consecutive naturals, for example, 172 + ... + 1148.

It is an arithmetic number, because the mean of its divisors is an integer number (41076).

2⁶⁴⁴⁸²⁰ is an apocalyptic number.

644820 is a gapful number since it is divisible by the number (60) formed by its first and last digit.

It is an amenable number.

It is a practical number, because each smaller number is the sum of distinct divisors of 644820, and also a Zumkeller number, because its divisors can be partitioned in two sets with the same sum (985824).

644820 is an abundant number, since it is smaller than the sum of its proper divisors (1326828).

It is a pseudoperfect number, because it is the sum of a subset of its proper divisors.

644820 is a wasteful number, since it uses less digits than its factorization.

644820 is an odious number, because the sum of its binary digits is odd.

The sum of its prime factors is 1000 (or 998 counting only the distinct ones).

The product of its (nonzero) digits is 1536, while the sum is 24.

The square root of 644820 is about 803.0068492859. The cubic root of 644820 is about 86.3931879060.

It can be divided in two parts, 64 and 4820, that added together give a palindrome (4884).

The spelling of 644820 in words is "six hundred forty-four thousand, eight hundred twenty".