21806561 has 8 divisors (see below), whose sum is σ = 24969600. Its totient is φ = 18655488.

The previous prime is 21806549. The next prime is 21806573. The reversal of 21806561 is 16560812.

It is a happy number.

It is an interprime number because it is at equal distance from previous prime (21806549) and next prime (21806573).

It is a sphenic number, since it is the product of 3 distinct primes.

It is a cyclic number.

It is a de Polignac number, because none of the positive numbers 2^k-21806561 is a prime.

It is a super-2 number, since 2×21806561² = 951052205293442, which contains 22 as substring.

It is a Duffinian number.

It is a Curzon number.

It is not an unprimeable number, because it can be changed into a prime (21805561) by changing a digit.

It is a polite number, since it can be written in 7 ways as a sum of consecutive naturals, for example, 1340 + ... + 6738.

It is an arithmetic number, because the mean of its divisors is an integer number (3121200).

Almost surely, 2^21806561 is an apocalyptic number.

It is an amenable number.

21806561 is a deficient number, since it is larger than the sum of its proper divisors (3163039).

21806561 is an equidigital number, since it uses as much as digits as its factorization.

21806561 is an evil number, because the sum of its binary digits is even.

The sum of its prime factors is 5983.

The product of its (nonzero) digits is 2880, while the sum is 29.

The square root of 21806561 is about 4669.7495650195. The cubic root of 21806561 is about 279.3802662384.

The spelling of 21806561 in words is "twenty-one million, eight hundred six thousand, five hundred sixty-one".