20561866565 has 12 divisors (see below), whose sum is σ = 25016905764. Its totient is φ = 16224136704.

The previous prime is 20561866543. The next prime is 20561866579. The reversal of 20561866565 is 56566816502.

It can be written as a sum of positive squares in 6 ways, for example, as 1831754401 + 18730112164 = 42799^2 + 136858^2 .

It is not a de Polignac number, because 20561866565 - 2¹² = 20561862469 is a prime.

It is a Duffinian number.

It is a congruent number.

It is an unprimeable number.

It is a pernicious number, because its binary representation contains a prime number (17) of ones.

It is a polite number, since it can be written in 11 ways as a sum of consecutive naturals, for example, 359204 + ... + 412493.

It is an arithmetic number, because the mean of its divisors is an integer number (2084742147).

Almost surely, 2^20561866565 is an apocalyptic number.

It is an amenable number.

20561866565 is a deficient number, since it is larger than the sum of its proper divisors (4455039199).

20561866565 is an frugal number, since it uses more digits than its factorization.

20561866565 is an odious number, because the sum of its binary digits is odd.

The sum of its prime factors is 771848 (or 771775 counting only the distinct ones).

The product of its (nonzero) digits is 2592000, while the sum is 50.

The spelling of 20561866565 in words is "twenty billion, five hundred sixty-one million, eight hundred sixty-six thousand, five hundred sixty-five".