• 646 can be written using four 4's:
646 is nontrivially palindromic in base 9 and base 10.
646 is an esthetic number in base 9, because in such base its adjacent digits differ by 1.
It is a sphenic number, since it is the product of 3 distinct primes.
It is an Ulam number.
646 is an undulating number in base 9 and base 10.
It is a plaindrome in base 12.
It is a congruent number.
2646 is an apocalyptic number.
646 is a wasteful number, since it uses less digits than its factorization.
646 is an evil number, because the sum of its binary digits is even.
The sum of its prime factors is 38.
The square root of 646 is about 25.4165300543. The cubic root of 646 is about 8.6445854717.
Subtracting from 646 its sum of digits (16), we obtain a triangular number (630 = T35).
The spelling of 646 in words is "six hundred forty-six", and thus it is an aban number.