20661266561 has 4 divisors (see below), whose sum is σ = 22250594772. Its totient is φ = 19071938352.

The previous prime is 20661266513. The next prime is 20661266567. The reversal of 20661266561 is 16566216602.

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 3085135936 + 17576130625 = 55544^2 + 132575^2 .

It is a cyclic number.

It is not a de Polignac number, because 20661266561 - 2²² = 20657072257 is a prime.

It is a Duffinian number.

It is a Curzon number.

It is not an unprimeable number, because it can be changed into a prime (20661266567) 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, 794664086 + ... + 794664111.

It is an arithmetic number, because the mean of its divisors is an integer number (5562648693).

Almost surely, 2^20661266561 is an apocalyptic number.

It is an amenable number.

20661266561 is a deficient number, since it is larger than the sum of its proper divisors (1589328211).

20661266561 is a wasteful number, since it uses less digits than its factorization.

20661266561 is an odious number, because the sum of its binary digits is odd.

The sum of its prime factors is 1589328210.

The product of its (nonzero) digits is 155520, while the sum is 41.

The spelling of 20661266561 in words is "twenty billion, six hundred sixty-one million, two hundred sixty-six thousand, five hundred sixty-one".