21115286721 has 9 divisors (see below), whose sum is σ = 30500488291. Its totient is φ = 14076567192.

The previous prime is 21115286671. The next prime is 21115286741. The reversal of 21115286721 is 12768251112.

It is a happy number.

The square root of 21115286721 is 145311.

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., 8518367025 + 12596919696 = 92295^2 + 112236^2 .

It is a tau number, because it is divible by the number of its divisors (9).

It is not a de Polignac number, because 21115286721 - 2⁷ = 21115286593 is a prime.

It is a Duffinian number.

It is not an unprimeable number, because it can be changed into a prime (21115286741) 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, 411715 + ... + 460151.

Almost surely, 2^21115286721 is an apocalyptic number.

21115286721 is the 145311-st square number.

21115286721 is the 72656-th centered octagonal number.

It is an amenable number.

21115286721 is a deficient number, since it is larger than the sum of its proper divisors (9385201570).

21115286721 is an frugal number, since it uses more digits than its factorization.

21115286721 is an evil number, because the sum of its binary digits is even.

The sum of its prime factors is 96880 (or 48440 counting only the distinct ones).

The product of its digits is 13440, while the sum is 36.

Multiplying 21115286721 by its sum of digits (36), we get a square (760150321956 = 871866²).

The spelling of 21115286721 in words is "twenty-one billion, one hundred fifteen million, two hundred eighty-six thousand, seven hundred twenty-one".