21805561 has 2 divisors, whose sum is σ = 21805562. Its totient is φ = 21805560.

The previous prime is 21805549. The next prime is 21805571. The reversal of 21805561 is 16550812.

It is a strong prime.

It can be written as a sum of positive squares in only one way, i.e., 19184400 + 2621161 = 4380^2 + 1619^2 .

It is a cyclic number.

It is not a de Polignac number, because 21805561 - 2⁵ = 21805529 is a prime.

It is not a weakly prime, because it can be changed into another prime (21805571) by changing a digit.

It is a polite number, since it can be written as a sum of consecutive naturals, namely, 10902780 + 10902781.

It is an arithmetic number, because the mean of its divisors is an integer number (10902781).

Almost surely, 2^21805561 is an apocalyptic number.

It is an amenable number.

21805561 is a deficient number, since it is larger than the sum of its proper divisors (1).

21805561 is an equidigital number, since it uses as much as digits as its factorization.

21805561 is an odious number, because the sum of its binary digits is odd.

The product of its (nonzero) digits is 2400, while the sum is 28.

The square root of 21805561 is about 4669.6424916689. The cubic root of 21805561 is about 279.3759955889.

It can be divided in two parts, 21805 and 561, that added together give a triangular number (22366 = T₂₁₁).

The spelling of 21805561 in words is "twenty-one million, eight hundred five thousand, five hundred sixty-one".