21816561 has 32 divisors (see below), whose sum is σ = 32488960. Its totient is φ = 12939264.

The previous prime is 21816547. The next prime is 21816577. The reversal of 21816561 is 16561812.

It is not a de Polignac number, because 21816561 - 2²² = 17622257 is a prime.

It is an alternating number because its digits alternate between even and odd.

It is a Duffinian number.

It is a Curzon number.

It is not an unprimeable number, because it can be changed into a prime (21818561) 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 31 ways as a sum of consecutive naturals, for example, 224865 + ... + 224961.

It is an arithmetic number, because the mean of its divisors is an integer number (1015280).

Almost surely, 2^21816561 is an apocalyptic number.

It is an amenable number.

21816561 is a deficient number, since it is larger than the sum of its proper divisors (10672399).

21816561 is a wasteful number, since it uses less digits than its factorization.

21816561 is an odious number, because the sum of its binary digits is odd.

The sum of its prime factors is 265.

The product of its digits is 2880, while the sum is 30.

The square root of 21816561 is about 4670.8201635259. The cubic root of 21816561 is about 279.4229655544.

The spelling of 21816561 in words is "twenty-one million, eight hundred sixteen thousand, five hundred sixty-one".