656641021 has 4 divisors (see below), whose sum is σ = 664019100. Its totient is φ = 649262944.

The previous prime is 656641019. The next prime is 656641033. The reversal of 656641021 is 120146656.

It is a semiprime because it is the product of two primes.

It can be written as a sum of positive squares in 2 ways, for example, as 100200100 + 556440921 = 10010^2 + 23589^2 .

It is a cyclic number.

It is not a de Polignac number, because 656641021 - 2¹ = 656641019 is a prime.

It is a Duffinian number.

It is a congruent number.

It is not an unprimeable number, because it can be changed into a prime (656671021) 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, 3688906 + ... + 3689083.

It is an arithmetic number, because the mean of its divisors is an integer number (166004775).

Almost surely, 2^656641021 is an apocalyptic number.

It is an amenable number.

656641021 is a deficient number, since it is larger than the sum of its proper divisors (7378079).

656641021 is an equidigital number, since it uses as much as digits as its factorization.

656641021 is an evil number, because the sum of its binary digits is even.

The sum of its prime factors is 7378078.

The product of its (nonzero) digits is 8640, while the sum is 31.

The square root of 656641021 is about 25625.0077268281. The cubic root of 656641021 is about 869.1792237039.

Adding to 656641021 its reverse (120146656), we get a palindrome (776787677).

The spelling of 656641021 in words is "six hundred fifty-six million, six hundred forty-one thousand, twenty-one".