656721 has 16 divisors (see below), whose sum is σ = 1048320. Its totient is φ = 403920.

The previous prime is 656707. The next prime is 656737. The reversal of 656721 is 127656.

656721 is a `hidden beast` number, since 656 + 7 + 2 + 1 = 666.

It is not a de Polignac number, because 656721 - 2⁶ = 656657 is a prime.

It is a Harshad number since it is a multiple of its sum of digits (27).

It is an alternating number because its digits alternate between even and odd.

It is a Curzon number.

It is a congruent number.

It is not an unprimeable number, because it can be changed into a prime (656701) by changing a digit.

It is a pernicious number, because its binary representation contains a prime number (7) of ones.

It is a polite number, since it can be written in 15 ways as a sum of consecutive naturals, for example, 585 + ... + 1286.

It is an arithmetic number, because the mean of its divisors is an integer number (65520).

2⁶⁵⁶⁷²¹ is an apocalyptic number.

It is an amenable number.

656721 is a deficient number, since it is larger than the sum of its proper divisors (391599).

656721 is a wasteful number, since it uses less digits than its factorization.

656721 is an odious number, because the sum of its binary digits is odd.

The sum of its prime factors is 1893 (or 1887 counting only the distinct ones).

The product of its digits is 2520, while the sum is 27.

The square root of 656721 is about 810.3832426698. The cubic root of 656721 is about 86.9214511002.

It can be divided in two parts, 65 and 6721, that added together give a triangular number (6786 = T₁₁₆).

The spelling of 656721 in words is "six hundred fifty-six thousand, seven hundred twenty-one".