20561643511 has 4 divisors (see below), whose sum is σ = 20743605072. Its totient is φ = 20379681952.

The previous prime is 20561643499. The next prime is 20561643517. The reversal of 20561643511 is 11534616502.

It is a semiprime because it is the product of two primes, and also an emirpimes, since its reverse is a distinct semiprime: 11534616502 = 2 ⋅5767308251.

It is a cyclic number.

It is not a de Polignac number, because 20561643511 - 2⁹ = 20561642999 is a prime.

It is a Duffinian number.

It is a congruent number.

It is not an unprimeable number, because it can be changed into a prime (20561643517) 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, 90980611 + ... + 90980836.

It is an arithmetic number, because the mean of its divisors is an integer number (5185901268).

Almost surely, 2^20561643511 is an apocalyptic number.

20561643511 is a deficient number, since it is larger than the sum of its proper divisors (181961561).

20561643511 is a wasteful number, since it uses less digits than its factorization.

20561643511 is an evil number, because the sum of its binary digits is even.

The sum of its prime factors is 181961560.

The product of its (nonzero) digits is 21600, while the sum is 34.

The spelling of 20561643511 in words is "twenty billion, five hundred sixty-one million, six hundred forty-three thousand, five hundred eleven".