21566221 has 2 divisors, whose sum is σ = 21566222. Its totient is φ = 21566220.

The previous prime is 21566219. The next prime is 21566239. The reversal of 21566221 is 12266512.

It is a weak prime.

It can be written as a sum of positive squares in only one way, i.e., 18275625 + 3290596 = 4275^2 + 1814^2 .

It is a cyclic number.

It is not a de Polignac number, because 21566221 - 2¹ = 21566219 is a prime.

Together with 21566219, it forms a pair of twin primes.

It is a congruent number.

It is not a weakly prime, because it can be changed into another prime (21566201) by changing a digit.

It is a polite number, since it can be written as a sum of consecutive naturals, namely, 10783110 + 10783111.

It is an arithmetic number, because the mean of its divisors is an integer number (10783111).

Almost surely, 2^21566221 is an apocalyptic number.

It is an amenable number.

21566221 is a deficient number, since it is larger than the sum of its proper divisors (1).

21566221 is an equidigital number, since it uses as much as digits as its factorization.

21566221 is an evil number, because the sum of its binary digits is even.

The product of its digits is 1440, while the sum is 25.

The square root of 21566221 is about 4643.9445517792. The cubic root of 21566221 is about 278.3500799881.

The spelling of 21566221 in words is "twenty-one million, five hundred sixty-six thousand, two hundred twenty-one".