21003656521 has 2 divisors, whose sum is σ = 21003656522. Its totient is φ = 21003656520.

The previous prime is 21003656489. The next prime is 21003656543. The reversal of 21003656521 is 12565630012.

It is a strong prime.

It can be written as a sum of positive squares in only one way, i.e., 17977446400 + 3026210121 = 134080^2 + 55011^2 .

It is a cyclic number.

It is not a de Polignac number, because 21003656521 - 2⁵ = 21003656489 is a prime.

It is not a weakly prime, because it can be changed into another prime (21003656321) by changing a digit.

It is a pernicious number, because its binary representation contains a prime number (19) of ones.

It is a polite number, since it can be written as a sum of consecutive naturals, namely, 10501828260 + 10501828261.

It is an arithmetic number, because the mean of its divisors is an integer number (10501828261).

Almost surely, 2^21003656521 is an apocalyptic number.

It is an amenable number.

21003656521 is a deficient number, since it is larger than the sum of its proper divisors (1).

21003656521 is an equidigital number, since it uses as much as digits as its factorization.

21003656521 is an odious number, because the sum of its binary digits is odd.

The product of its (nonzero) digits is 10800, while the sum is 31.

The spelling of 21003656521 in words is "twenty-one billion, three million, six hundred fifty-six thousand, five hundred twenty-one".