• 640 can be written using four 4's:

See also 113.

---

640 has 16 divisors (see below), whose sum is σ = 1530. Its totient is φ = 256.

The previous prime is 631. The next prime is 641. The reversal of 640 is 46.

640 is nontrivially palindromic in base 12 and base 13.

640 is an esthetic number in base 12, because in such base its adjacent digits differ by 1.

It can be written as a sum of positive squares in only one way, i.e., 576 + 64 = 24^2 + 8^2 .

It is a tau number, because it is divible by the number of its divisors (16).

It is a Harshad number since it is a multiple of its sum of digits (10).

640 is an undulating number in base 12 and base 13.

It is a plaindrome in base 14.

It is a nialpdrome in base 4, base 10 and base 11.

It is a zygodrome in base 4.

It is not an unprimeable number, because it can be changed into a prime (641) by changing a digit.

It is a pernicious number, because its binary representation contains a prime number (2) of ones.

In principle, a polygon with 640 sides can be constructed with ruler and compass.

It is a polite number, since it can be written as a sum of consecutive naturals, namely, 126 + ... + 130.

It is an amenable number.

It is a practical number, because each smaller number is the sum of distinct divisors of 640, and also a Zumkeller number, because its divisors can be partitioned in two sets with the same sum (765).

640 is an abundant number, since it is smaller than the sum of its proper divisors (890).

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

640 is an equidigital number, since it uses as much as digits as its factorization.

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

The sum of its prime factors is 19 (or 7 counting only the distinct ones).

The product of its (nonzero) digits is 24, while the sum is 10.

The square root of 640 is about 25.2982212813. The cubic root of 640 is about 8.6177387601.

Subtracting from 640 its sum of digits (10), we obtain a triangular number (630 = T₃₅).

Multiplying 640 by its sum of digits (10), we get a square (6400 = 80²).

640 divided by its sum of digits (10) gives a 6-th power (64 = 2⁶).

Subtracting from 640 its product of nonzero digits (24), we obtain a palindrome (616).

Adding to 640 its reverse (46), we get a palindrome (686).

The spelling of 640 in words is "six hundred forty", and thus it is an aban number.