21563561 has 4 divisors (see below), whose sum is σ = 21917124. Its totient is φ = 21210000.

The previous prime is 21563557. The next prime is 21563587. The reversal of 21563561 is 16536512.

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 7123561 + 14440000 = 2669^2 + 3800^2 .

It is a cyclic number.

It is not a de Polignac number, because 21563561 - 2² = 21563557 is a prime.

It is a Duffinian number.

It is not an unprimeable number, because it can be changed into a prime (21563161) 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, 176690 + ... + 176811.

It is an arithmetic number, because the mean of its divisors is an integer number (5479281).

Almost surely, 2^21563561 is an apocalyptic number.

It is an amenable number.

21563561 is a deficient number, since it is larger than the sum of its proper divisors (353563).

21563561 is an equidigital number, since it uses as much as digits as its factorization.

21563561 is an odious number, because the sum of its binary digits is odd.

The sum of its prime factors is 353562.

The product of its digits is 5400, while the sum is 29.

The square root of 21563561 is about 4643.6581484860. The cubic root of 21563561 is about 278.3386355220.

The spelling of 21563561 in words is "twenty-one million, five hundred sixty-three thousand, five hundred sixty-one".