21571517521 has 4 divisors (see below), whose sum is σ = 21577760932. Its totient is φ = 21565274112.

The previous prime is 21571517491. The next prime is 21571517567. The reversal of 21571517521 is 12571517512.

It is a semiprime because it is the product of two primes.

It can be written as a sum of positive squares in 2 ways, for example, as 5570532496 + 16000985025 = 74636^2 + 126495^2 .

It is a cyclic number.

It is not a de Polignac number, because 21571517521 - 2⁷ = 21571517393 is a prime.

It is a Duffinian number.

It is not an unprimeable number, because it can be changed into a prime (21571517021) by changing a digit.

It is a polite number, since it can be written in 3 ways as a sum of consecutive naturals, for example, 3116520 + ... + 3123433.

It is an arithmetic number, because the mean of its divisors is an integer number (5394440233).

Almost surely, 2^21571517521 is an apocalyptic number.

It is an amenable number.

21571517521 is a deficient number, since it is larger than the sum of its proper divisors (6243411).

21571517521 is an equidigital number, since it uses as much as digits as its factorization.

21571517521 is an odious number, because the sum of its binary digits is odd.

The sum of its prime factors is 6243410.

The product of its digits is 24500, while the sum is 37.

Subtracting from 21571517521 its reverse (12571517512), we obtain a palindrome (9000000009).

The spelling of 21571517521 in words is "twenty-one billion, five hundred seventy-one million, five hundred seventeen thousand, five hundred twenty-one".