643664 has 30 divisors (see below), whose sum is σ = 1452474. Its totient is φ = 275520.

The previous prime is 643661. The next prime is 643681. The reversal of 643664 is 466346.

It can be written as a sum of positive squares in only one way, i.e., 490000 + 153664 = 700^2 + 392^2 .

It is a congruent number.

It is not an unprimeable number, because it can be changed into a prime (643661) by changing a digit.

It is a polite number, since it can be written in 5 ways as a sum of consecutive naturals, for example, 374 + ... + 1194.

2⁶⁴³⁶⁶⁴ is an apocalyptic number.

It is an amenable number.

It is a practical number, because each smaller number is the sum of distinct divisors of 643664, and also a Zumkeller number, because its divisors can be partitioned in two sets with the same sum (726237).

643664 is an abundant number, since it is smaller than the sum of its proper divisors (808810).

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

643664 is a wasteful number, since it uses less digits than its factorization.

643664 is an evil number, because the sum of its binary digits is even.

The sum of its prime factors is 843 (or 830 counting only the distinct ones).

The product of its digits is 10368, while the sum is 29.

The square root of 643664 is about 802.2867317861. The cubic root of 643664 is about 86.3415299417.

The spelling of 643664 in words is "six hundred forty-three thousand, six hundred sixty-four".