21766561 has 4 divisors (see below), whose sum is σ = 21786624. Its totient is φ = 21746500.

The previous prime is 21766553. The next prime is 21766573. The reversal of 21766561 is 16566712.

It is a happy number.

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 a cyclic number.

It is not a de Polignac number, because 21766561 - 2³ = 21766553 is a prime.

It is a Duffinian number.

It is not an unprimeable number, because it can be changed into a prime (21766961) 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, 8305 + ... + 10606.

It is an arithmetic number, because the mean of its divisors is an integer number (5446656).

Almost surely, 2^21766561 is an apocalyptic number.

It is an amenable number.

21766561 is a deficient number, since it is larger than the sum of its proper divisors (20063).

21766561 is a wasteful number, since it uses less digits than its factorization.

21766561 is an odious number, because the sum of its binary digits is odd.

The sum of its prime factors is 20062.

The product of its digits is 15120, while the sum is 34.

The square root of 21766561 is about 4665.4647142594. The cubic root of 21766561 is about 279.2093383151.

The spelling of 21766561 in words is "twenty-one million, seven hundred sixty-six thousand, five hundred sixty-one".