• 646 can be written using four 4's:

See also 113.

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646 has 8 divisors (see below), whose sum is σ = 1080. Its totient is φ = 288.

The previous prime is 643. The next prime is 647.

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 a fibodiv number, since the Fibonacci-like sequence with seeds 6 and 46 contains 646 itself.

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.

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

It is a polite number, since it can be written in 3 ways as a sum of consecutive naturals, for example, 25 + ... + 43.

It is an arithmetic number, because the mean of its divisors is an integer number (135).

2⁶⁴⁶ is an apocalyptic number.

646 is a deficient number, since it is larger than the sum of its proper divisors (434).

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 product of its digits is 144, while the sum is 16.

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).

It can be divided in two parts, 6 and 46, that multiplied together give a triangular number (276 = T₂₃).

The spelling of 646 in words is "six hundred forty-six", and thus it is an aban number.