20556561 has 4 divisors (see below), whose sum is σ = 27408752. Its totient is φ = 13704372.

The previous prime is 20556551. The next prime is 20556607. The reversal of 20556561 is 16565502.

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 not a de Polignac number, because 20556561 - 2⁷ = 20556433 is a prime.

It is a Duffinian number.

It is not an unprimeable number, because it can be changed into a prime (20556521) 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, 3426091 + ... + 3426096.

It is an arithmetic number, because the mean of its divisors is an integer number (6852188).

Almost surely, 2^20556561 is an apocalyptic number.

It is an amenable number.

20556561 is a deficient number, since it is larger than the sum of its proper divisors (6852191).

20556561 is an equidigital number, since it uses as much as digits as its factorization.

20556561 is an evil number, because the sum of its binary digits is even.

The sum of its prime factors is 6852190.

The product of its (nonzero) digits is 9000, while the sum is 30.

The square root of 20556561 is about 4533.9343841745. The cubic root of 20556561 is about 273.9366584184.

The spelling of 20556561 in words is "twenty million, five hundred fifty-six thousand, five hundred sixty-one".