21818921 has 2 divisors, whose sum is σ = 21818922. Its totient is φ = 21818920.

The previous prime is 21818899. The next prime is 21818941. The reversal of 21818921 is 12981812.

It is a strong prime.

It can be written as a sum of positive squares in only one way, i.e., 16362025 + 5456896 = 4045^2 + 2336^2 .

It is a cyclic number.

It is not a de Polignac number, because 21818921 - 2¹⁰ = 21817897 is a prime.

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

It is not a weakly prime, because it can be changed into another prime (21818941) 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 as a sum of consecutive naturals, namely, 10909460 + 10909461.

It is an arithmetic number, because the mean of its divisors is an integer number (10909461).

Almost surely, 2^21818921 is an apocalyptic number.

It is an amenable number.

21818921 is a deficient number, since it is larger than the sum of its proper divisors (1).

21818921 is an equidigital number, since it uses as much as digits as its factorization.

21818921 is an odious number, because the sum of its binary digits is odd.

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

The square root of 21818921 is about 4671.0727889854. The cubic root of 21818921 is about 279.4330406898.

The spelling of 21818921 in words is "twenty-one million, eight hundred eighteen thousand, nine hundred twenty-one".