21551221 has 2 divisors, whose sum is σ = 21551222. Its totient is φ = 21551220.

The previous prime is 21551177. The next prime is 21551251. The reversal of 21551221 is 12215512.

It is a strong prime.

It can be written as a sum of positive squares in only one way, i.e., 15602500 + 5948721 = 3950^2 + 2439^2 .

It is a cyclic number.

It is not a de Polignac number, because 21551221 - 2¹⁹ = 21026933 is a prime.

It is a congruent number.

It is not a weakly prime, because it can be changed into another prime (21551251) by changing a digit.

It is a polite number, since it can be written as a sum of consecutive naturals, namely, 10775610 + 10775611.

It is an arithmetic number, because the mean of its divisors is an integer number (10775611).

Almost surely, 2^21551221 is an apocalyptic number.

It is an amenable number.

21551221 is a deficient number, since it is larger than the sum of its proper divisors (1).

21551221 is an equidigital number, since it uses as much as digits as its factorization.

21551221 is an evil number, because the sum of its binary digits is even.

The product of its digits is 200, while the sum is 19.

The square root of 21551221 is about 4642.3292644964. The cubic root of 21551221 is about 278.2855312109.

Adding to 21551221 its reverse (12215512), we get a palindrome (33766733).

The spelling of 21551221 in words is "twenty-one million, five hundred fifty-one thousand, two hundred twenty-one".