21705016521 has 4 divisors (see below), whose sum is σ = 28940022032. Its totient is φ = 14470011012.

The previous prime is 21705016517. The next prime is 21705016537. The reversal of 21705016521 is 12561050712.

It is a semiprime because it is the product of two primes, and also a Blum integer, because the two primes are equal to 3 mod 4.

It is not a de Polignac number, because 21705016521 - 2² = 21705016517 is a prime.

It is a super-2 number, since 2×21705016521² (a number of 21 digits) contains 22 as substring.

It is a Duffinian number.

It is not an unprimeable number, because it can be changed into a prime (21705016321) 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, 3617502751 + ... + 3617502756.

It is an arithmetic number, because the mean of its divisors is an integer number (7235005508).

Almost surely, 2^21705016521 is an apocalyptic number.

It is an amenable number.

21705016521 is a deficient number, since it is larger than the sum of its proper divisors (7235005511).

21705016521 is an equidigital number, since it uses as much as digits as its factorization.

21705016521 is an odious number, because the sum of its binary digits is odd.

The sum of its prime factors is 7235005510.

The product of its (nonzero) digits is 4200, while the sum is 30.

The spelling of 21705016521 in words is "twenty-one billion, seven hundred five million, sixteen thousand, five hundred twenty-one".