20211221 has 2 divisors, whose sum is σ = 20211222. Its totient is φ = 20211220.

The previous prime is 20211209. The next prime is 20211241. The reversal of 20211221 is 12211202.

It is a happy number.

It is a weak prime.

It can be written as a sum of positive squares in only one way, i.e., 20124196 + 87025 = 4486^2 + 295^2 .

It is a cyclic number.

It is not a de Polignac number, because 20211221 - 2⁶ = 20211157 is a prime.

It is a Chen prime.

It is a congruent number.

It is not a weakly prime, because it can be changed into another prime (20211241) 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, 10105610 + 10105611.

It is an arithmetic number, because the mean of its divisors is an integer number (10105611).

Almost surely, 2^20211221 is an apocalyptic number.

It is an amenable number.

20211221 is a deficient number, since it is larger than the sum of its proper divisors (1).

20211221 is an equidigital number, since it uses as much as digits as its factorization.

20211221 is an odious number, because the sum of its binary digits is odd.

The product of its (nonzero) digits is 16, while the sum is 11.

The square root of 20211221 is about 4495.6891574040. The cubic root of 20211221 is about 272.3939873235.

Adding to 20211221 its reverse (12211202), we get a palindrome (32422423).

The spelling of 20211221 in words is "twenty million, two hundred eleven thousand, two hundred twenty-one".