• 640 can be written using four 4's:
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 is a tau number, because it is divible by the number of its divisors (16).
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.
In principle, a polygon with 640 sides can be constructed with ruler and compass.
It is an amenable number.
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 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.