21765561 has 4 divisors (see below), whose sum is σ = 29020752. Its totient is φ = 14510372.

The previous prime is 21765551. The next prime is 21765571. The reversal of 21765561 is 16556712.

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 an interprime number because it is at equal distance from previous prime (21765551) and next prime (21765571).

It is a cyclic number.

It is not a de Polignac number, because 21765561 - 2⁵ = 21765529 is a prime.

It is not an unprimeable number, because it can be changed into a prime (21765511) by changing a digit.

It is a pernicious number, because its binary representation contains a prime number (13) of ones.

It is a polite number, since it can be written in 3 ways as a sum of consecutive naturals, for example, 3627591 + ... + 3627596.

It is an arithmetic number, because the mean of its divisors is an integer number (7255188).

Almost surely, 2^21765561 is an apocalyptic number.

It is an amenable number.

21765561 is a deficient number, since it is larger than the sum of its proper divisors (7255191).

21765561 is an equidigital number, since it uses as much as digits as its factorization.

21765561 is an odious number, because the sum of its binary digits is odd.

The sum of its prime factors is 7255190.

The product of its digits is 12600, while the sum is 33.

The square root of 21765561 is about 4665.3575425684. The cubic root of 21765561 is about 279.2050624351.

The spelling of 21765561 in words is "twenty-one million, seven hundred sixty-five thousand, five hundred sixty-one".