20561 has 4 divisors (see below), whose sum is σ = 21300. Its totient is φ = 19824.

The previous prime is 20551. The next prime is 20563. The reversal of 20561 is 16502.

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 6400 + 14161 = 80^2 + 119^2 .

It is a cyclic number.

It is not a de Polignac number, because 20561 - 2¹⁴ = 4177 is a prime.

It is a Duffinian number.

It is a nialpdrome in base 12.

It is not an unprimeable number, because it can be changed into a prime (20563) by changing a digit.

It is a pernicious number, because its binary representation contains a prime number (5) of ones.

It is a polite number, since it can be written in 3 ways as a sum of consecutive naturals, for example, 326 + ... + 383.

It is an arithmetic number, because the mean of its divisors is an integer number (5325).

2²⁰⁵⁶¹ is an apocalyptic number.

It is an amenable number.

20561 is a deficient number, since it is larger than the sum of its proper divisors (739).

20561 is an equidigital number, since it uses as much as digits as its factorization.

20561 is an odious number, because the sum of its binary digits is odd.

The sum of its prime factors is 738.

The product of its (nonzero) digits is 60, while the sum is 14.

The square root of 20561 is about 143.3910736413. The cubic root of 20561 is about 27.3956375032.

The spelling of 20561 in words is "twenty thousand, five hundred sixty-one".