21606561 has 8 divisors (see below), whose sum is σ = 32009760. Its totient is φ = 14404356.

The previous prime is 21606547. The next prime is 21606569. The reversal of 21606561 is 16560612.

It is a happy number.

21606561 is a `hidden beast` number, since 2 + 1 + 6 + 0 + 656 + 1 = 666.

It is not a de Polignac number, because 21606561 - 2⁹ = 21606049 is a prime.

It is a Harshad number since it is a multiple of its sum of digits (27), and also a Moran number because the ratio is a prime number: 800243 = 21606561 / (2 + 1 + 6 + 0 + 6 + 5 + 6 + 1).

It is not an unprimeable number, because it can be changed into a prime (21606569) 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, 400095 + ... + 400148.

It is an arithmetic number, because the mean of its divisors is an integer number (4001220).

Almost surely, 2^21606561 is an apocalyptic number.

It is an amenable number.

21606561 is a deficient number, since it is larger than the sum of its proper divisors (10403199).

21606561 is an equidigital number, since it uses as much as digits as its factorization.

21606561 is an evil number, because the sum of its binary digits is even.

The sum of its prime factors is 800252 (or 800246 counting only the distinct ones).

The product of its (nonzero) digits is 2160, while the sum is 27.

The square root of 21606561 is about 4648.2858130713. The cubic root of 21606561 is about 278.5235248144.

The spelling of 21606561 in words is "twenty-one million, six hundred six thousand, five hundred sixty-one".