21881921 has 4 divisors (see below), whose sum is σ = 22636500. Its totient is φ = 21127344.

The previous prime is 21881917. The next prime is 21881927. The reversal of 21881921 is 12918812.

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 7924225 + 13957696 = 2815^2 + 3736^2 .

It is a cyclic number.

It is not a de Polignac number, because 21881921 - 2² = 21881917 is a prime.

It is a Duffinian number.

It is a Curzon number.

It is not an unprimeable number, because it can be changed into a prime (21881927) 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, 377246 + ... + 377303.

It is an arithmetic number, because the mean of its divisors is an integer number (5659125).

Almost surely, 2^21881921 is an apocalyptic number.

It is an amenable number.

21881921 is a deficient number, since it is larger than the sum of its proper divisors (754579).

21881921 is an equidigital number, since it uses as much as digits as its factorization.

21881921 is an odious number, because the sum of its binary digits is odd.

The sum of its prime factors is 754578.

The product of its digits is 2304, while the sum is 32.

The square root of 21881921 is about 4677.8115609759. The cubic root of 21881921 is about 279.7017274433.

The spelling of 21881921 in words is "twenty-one million, eight hundred eighty-one thousand, nine hundred twenty-one".