21565561 has 4 divisors (see below), whose sum is σ = 22148452. Its totient is φ = 20982672.

The previous prime is 21565547. The next prime is 21565567. The reversal of 21565561 is 16556512.

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 20475625 + 1089936 = 4525^2 + 1044^2 .

It is a cyclic number.

It is not a de Polignac number, because 21565561 - 2¹¹ = 21563513 is a prime.

It is a Duffinian number.

It is not an unprimeable number, because it can be changed into a prime (21565567) 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, 291390 + ... + 291463.

It is an arithmetic number, because the mean of its divisors is an integer number (5537113).

Almost surely, 2^21565561 is an apocalyptic number.

It is an amenable number.

21565561 is a deficient number, since it is larger than the sum of its proper divisors (582891).

21565561 is an equidigital number, since it uses as much as digits as its factorization.

21565561 is an evil number, because the sum of its binary digits is even.

The sum of its prime factors is 582890.

The product of its digits is 9000, while the sum is 31.

The square root of 21565561 is about 4643.8734909556. The cubic root of 21565561 is about 278.3472404715.

The spelling of 21565561 in words is "twenty-one million, five hundred sixty-five thousand, five hundred sixty-one".