21878561 has 4 divisors (see below), whose sum is σ = 21888900. Its totient is φ = 21868224.

The previous prime is 21878531. The next prime is 21878567. The reversal of 21878561 is 16587812.

It is a semiprime because it is the product of two primes, and also a brilliant number, because the two primes have the same length.

It can be written as a sum of positive squares in 2 ways, for example, as 8737936 + 13140625 = 2956^2 + 3625^2 .

It is a cyclic number.

It is not a de Polignac number, because 21878561 - 2¹⁸ = 21616417 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 (21878567) 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 3 ways as a sum of consecutive naturals, for example, 716 + ... + 6653.

It is an arithmetic number, because the mean of its divisors is an integer number (5472225).

Almost surely, 2^21878561 is an apocalyptic number.

It is an amenable number.

21878561 is a deficient number, since it is larger than the sum of its proper divisors (10339).

21878561 is an equidigital number, since it uses as much as digits as its factorization.

21878561 is an odious number, because the sum of its binary digits is odd.

The sum of its prime factors is 10338.

The product of its digits is 26880, while the sum is 38.

The square root of 21878561 is about 4677.4524048888. The cubic root of 21878561 is about 279.6874105115.

The spelling of 21878561 in words is "twenty-one million, eight hundred seventy-eight thousand, five hundred sixty-one".