21664307521 has 2 divisors, whose sum is σ = 21664307522. Its totient is φ = 21664307520.

The previous prime is 21664307501. The next prime is 21664307531. The reversal of 21664307521 is 12570346612.

It is a strong prime.

It can be written as a sum of positive squares in only one way, i.e., 20374707600 + 1289599921 = 142740^2 + 35911^2 .

It is a cyclic number.

It is not a de Polignac number, because 21664307521 - 2¹⁹ = 21663783233 is a prime.

It is not a weakly prime, because it can be changed into another prime (21664307501) by changing a digit.

It is a polite number, since it can be written as a sum of consecutive naturals, namely, 10832153760 + 10832153761.

It is an arithmetic number, because the mean of its divisors is an integer number (10832153761).

Almost surely, 2^21664307521 is an apocalyptic number.

It is an amenable number.

21664307521 is a deficient number, since it is larger than the sum of its proper divisors (1).

21664307521 is an equidigital number, since it uses as much as digits as its factorization.

21664307521 is an evil number, because the sum of its binary digits is even.

The product of its (nonzero) digits is 60480, while the sum is 37.

The spelling of 21664307521 in words is "twenty-one billion, six hundred sixty-four million, three hundred seven thousand, five hundred twenty-one".