20866561 has 2 divisors, whose sum is σ = 20866562. Its totient is φ = 20866560.

The previous prime is 20866529. The next prime is 20866591. The reversal of 20866561 is 16566802.

It is a strong prime.

It can be written as a sum of positive squares in only one way, i.e., 20295025 + 571536 = 4505^2 + 756^2 .

It is a cyclic number.

It is not a de Polignac number, because 20866561 - 2⁵ = 20866529 is a prime.

It is not a weakly prime, because it can be changed into another prime (20866501) by changing a digit.

It is a pernicious number, because its binary representation contains a prime number (11) of ones.

It is a polite number, since it can be written as a sum of consecutive naturals, namely, 10433280 + 10433281.

It is an arithmetic number, because the mean of its divisors is an integer number (10433281).

Almost surely, 2^20866561 is an apocalyptic number.

It is an amenable number.

20866561 is a deficient number, since it is larger than the sum of its proper divisors (1).

20866561 is an equidigital number, since it uses as much as digits as its factorization.

20866561 is an odious number, because the sum of its binary digits is odd.

The product of its (nonzero) digits is 17280, while the sum is 34.

The square root of 20866561 is about 4567.9931041979. The cubic root of 20866561 is about 275.3068134882.

The spelling of 20866561 in words is "twenty million, eight hundred sixty-six thousand, five hundred sixty-one".