21971221 has 2 divisors, whose sum is σ = 21971222. Its totient is φ = 21971220.

The previous prime is 21971197. The next prime is 21971227. The reversal of 21971221 is 12217912.

It is a strong prime.

It can be written as a sum of positive squares in only one way, i.e., 13060996 + 8910225 = 3614^2 + 2985^2 .

It is a cyclic number.

It is not a de Polignac number, because 21971221 - 2¹⁵ = 21938453 is a prime.

It is a congruent number.

It is not a weakly prime, because it can be changed into another prime (21971227) 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 as a sum of consecutive naturals, namely, 10985610 + 10985611.

It is an arithmetic number, because the mean of its divisors is an integer number (10985611).

Almost surely, 2^21971221 is an apocalyptic number.

It is an amenable number.

21971221 is a deficient number, since it is larger than the sum of its proper divisors (1).

21971221 is an equidigital number, since it uses as much as digits as its factorization.

21971221 is an odious number, because the sum of its binary digits is odd.

The product of its digits is 504, while the sum is 25.

The square root of 21971221 is about 4687.3469041666. The cubic root of 21971221 is about 280.0816980987.

The spelling of 21971221 in words is "twenty-one million, nine hundred seventy-one thousand, two hundred twenty-one".