20815865521 has 4 divisors (see below), whose sum is σ = 22708216944. Its totient is φ = 18923514100.

The previous prime is 20815865519. The next prime is 20815865537. The reversal of 20815865521 is 12556851802.

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 a cyclic number.

It is not a de Polignac number, because 20815865521 - 2¹ = 20815865519 is a prime.

It is a Duffinian number.

It is not an unprimeable number, because it can be changed into a prime (20815865591) 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, 946175695 + ... + 946175716.

It is an arithmetic number, because the mean of its divisors is an integer number (5677054236).

Almost surely, 2^20815865521 is an apocalyptic number.

It is an amenable number.

20815865521 is a deficient number, since it is larger than the sum of its proper divisors (1892351423).

20815865521 is a wasteful number, since it uses less digits than its factorization.

20815865521 is an evil number, because the sum of its binary digits is even.

The sum of its prime factors is 1892351422.

The product of its (nonzero) digits is 192000, while the sum is 43.

The spelling of 20815865521 in words is "twenty billion, eight hundred fifteen million, eight hundred sixty-five thousand, five hundred twenty-one".