21531201221 has 2 divisors, whose sum is σ = 21531201222. Its totient is φ = 21531201220.

The previous prime is 21531201191. The next prime is 21531201227. The reversal of 21531201221 is 12210213512.

It is a strong prime.

It can be written as a sum of positive squares in only one way, i.e., 20323638721 + 1207562500 = 142561^2 + 34750^2 .

It is a cyclic number.

It is a de Polignac number, because none of the positive numbers 2^k-21531201221 is a prime.

It is a junction number, because it is equal to n+sod(n) for n = 21531201193 and 21531201202.

It is a congruent number.

It is not a weakly prime, because it can be changed into another prime (21531201227) by changing a digit.

It is a polite number, since it can be written as a sum of consecutive naturals, namely, 10765600610 + 10765600611.

It is an arithmetic number, because the mean of its divisors is an integer number (10765600611).

Almost surely, 2^21531201221 is an apocalyptic number.

It is an amenable number.

21531201221 is a deficient number, since it is larger than the sum of its proper divisors (1).

21531201221 is an equidigital number, since it uses as much as digits as its factorization.

21531201221 is an odious number, because the sum of its binary digits is odd.

The product of its (nonzero) digits is 240, while the sum is 20.

Adding to 21531201221 its reverse (12210213512), we get a palindrome (33741414733).

The spelling of 21531201221 in words is "twenty-one billion, five hundred thirty-one million, two hundred one thousand, two hundred twenty-one".