21563881 has 4 divisors (see below), whose sum is σ = 21598164. Its totient is φ = 21529600.

The previous prime is 21563831. The next prime is 21563887. The reversal of 21563881 is 18836512.

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 18619225 + 2944656 = 4315^2 + 1716^2 .

It is a cyclic number.

It is not a de Polignac number, because 21563881 - 2⁷ = 21563753 is a prime.

It is a super-2 number, since 2×21563881² = 930001927564322, which contains 22 as substring.

It is a Duffinian number.

It is not an unprimeable number, because it can be changed into a prime (21563887) by changing a digit.

It is a pernicious number, because its binary representation contains a prime number (11) of ones.

It is a polite number, since it can be written in 3 ways as a sum of consecutive naturals, for example, 16180 + ... + 17461.

It is an arithmetic number, because the mean of its divisors is an integer number (5399541).

Almost surely, 2^21563881 is an apocalyptic number.

It is an amenable number.

21563881 is a deficient number, since it is larger than the sum of its proper divisors (34283).

21563881 is an equidigital number, since it uses as much as digits as its factorization.

21563881 is an odious number, because the sum of its binary digits is odd.

The sum of its prime factors is 34282.

The product of its digits is 11520, while the sum is 34.

The square root of 21563881 is about 4643.6926039522. The cubic root of 21563881 is about 278.3400123496.

The spelling of 21563881 in words is "twenty-one million, five hundred sixty-three thousand, eight hundred eighty-one".