21818221 has 2 divisors, whose sum is σ = 21818222. Its totient is φ = 21818220.

The previous prime is 21818207. The next prime is 21818227. The reversal of 21818221 is 12281812.

It is a strong prime.

It can be written as a sum of positive squares in only one way, i.e., 20520900 + 1297321 = 4530^2 + 1139^2 .

It is a cyclic number.

It is not a de Polignac number, because 21818221 - 2¹³ = 21810029 is a prime.

It is a congruent number.

It is not a weakly prime, because it can be changed into another prime (21818227) by changing a digit.

It is a polite number, since it can be written as a sum of consecutive naturals, namely, 10909110 + 10909111.

It is an arithmetic number, because the mean of its divisors is an integer number (10909111).

Almost surely, 2^21818221 is an apocalyptic number.

It is an amenable number.

21818221 is a deficient number, since it is larger than the sum of its proper divisors (1).

21818221 is an equidigital number, since it uses as much as digits as its factorization.

21818221 is an odious number, because the sum of its binary digits is odd.

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

The square root of 21818221 is about 4670.9978591303. The cubic root of 21818221 is about 279.4300523780.

The spelling of 21818221 in words is "twenty-one million, eight hundred eighteen thousand, two hundred twenty-one".