21511565021 has 2 divisors, whose sum is σ = 21511565022. Its totient is φ = 21511565020.

The previous prime is 21511565011. The next prime is 21511565057. The reversal of 21511565021 is 12056511512.

It is a weak prime.

It can be written as a sum of positive squares in only one way, i.e., 20991952996 + 519612025 = 144886^2 + 22795^2 .

It is a cyclic number.

It is a de Polignac number, because none of the positive numbers 2^k-21511565021 is a prime.

It is a congruent number.

It is not a weakly prime, because it can be changed into another prime (21511565011) by changing a digit.

It is a polite number, since it can be written as a sum of consecutive naturals, namely, 10755782510 + 10755782511.

It is an arithmetic number, because the mean of its divisors is an integer number (10755782511).

Almost surely, 2^21511565021 is an apocalyptic number.

It is an amenable number.

21511565021 is a deficient number, since it is larger than the sum of its proper divisors (1).

21511565021 is an equidigital number, since it uses as much as digits as its factorization.

21511565021 is an evil number, because the sum of its binary digits is even.

The product of its (nonzero) digits is 3000, while the sum is 29.

The spelling of 21511565021 in words is "twenty-one billion, five hundred eleven million, five hundred sixty-five thousand, twenty-one".