21017997842521 has 9 divisors (see below), whose sum is σ = 21065246427999. Its totient is φ = 20970855027984.

The previous prime is 21017997842477. The next prime is 21017997842623. The reversal of 21017997842521 is 12524879971012.

The square root of 21017997842521 is 4584539.

It is a perfect power (a square), and thus also a powerful number.

It can be written as a sum of positive squares in only one way, i.e., 20881198464025 + 136799378496 = 4569595^2 + 369864^2 .

It is not a de Polignac number, because 21017997842521 - 2⁷ = 21017997842393 is a prime.

It is a Duffinian number.

It is not an unprimeable number, because it can be changed into a prime (21017997842921) by changing a digit.

It is a polite number, since it can be written in 8 ways as a sum of consecutive naturals, for example, 2140974805 + ... + 2140984621.

Almost surely, 2^{21017997842521} is an apocalyptic number.

21017997842521 is the 4584539-th square number.

21017997842521 is the 2292270-th centered octagonal number.

It is an amenable number.

21017997842521 is a deficient number, since it is larger than the sum of its proper divisors (47248585478).

21017997842521 is an frugal number, since it uses more digits than its factorization.

21017997842521 is an odious number, because the sum of its binary digits is odd.

The sum of its prime factors is 20568 (or 10284 counting only the distinct ones).

The product of its (nonzero) digits is 5080320, while the sum is 58.

The spelling of 21017997842521 in words is "twenty-one trillion, seventeen billion, nine hundred ninety-seven million, eight hundred forty-two thousand, five hundred twenty-one".