21881221 has 2 divisors, whose sum is σ = 21881222. Its totient is φ = 21881220.

The previous prime is 21881219. The next prime is 21881243. The reversal of 21881221 is 12218812.

It is a weak prime.

It can be written as a sum of positive squares in only one way, i.e., 11730625 + 10150596 = 3425^2 + 3186^2 .

It is a cyclic number.

It is not a de Polignac number, because 21881221 - 2¹ = 21881219 is a prime.

Together with 21881219, 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 (21881201) by changing a digit.

It is a polite number, since it can be written as a sum of consecutive naturals, namely, 10940610 + 10940611.

It is an arithmetic number, because the mean of its divisors is an integer number (10940611).

Almost surely, 2^21881221 is an apocalyptic number.

It is an amenable number.

21881221 is a deficient number, since it is larger than the sum of its proper divisors (1).

21881221 is an equidigital number, since it uses as much as digits as its factorization.

21881221 is an evil number, because the sum of its binary digits is even.

The product of its digits is 512, while the sum is 25.

The square root of 21881221 is about 4677.7367390652. The cubic root of 21881221 is about 279.6987448700.

The spelling of 21881221 in words is "twenty-one million, eight hundred eighty-one thousand, two hundred twenty-one".