21166221202561 has 2 divisors, whose sum is σ = 21166221202562. Its totient is φ = 21166221202560.

The previous prime is 21166221202519. The next prime is 21166221202571. The reversal of 21166221202561 is 16520212266112.

It is a strong prime.

It can be written as a sum of positive squares in only one way, i.e., 13285040868225 + 7881180334336 = 3644865^2 + 2807344^2 .

It is a cyclic number.

It is not a de Polignac number, because 21166221202561 - 2¹¹ = 21166221200513 is a prime.

It is a self number, because there is not a number n which added to its sum of digits gives 21166221202561.

It is not a weakly prime, because it can be changed into another prime (21166221202571) by changing a digit.

It is a polite number, since it can be written as a sum of consecutive naturals, namely, 10583110601280 + 10583110601281.

It is an arithmetic number, because the mean of its divisors is an integer number (10583110601281).

Almost surely, 2^{21166221202561} is an apocalyptic number.

It is an amenable number.

21166221202561 is a deficient number, since it is larger than the sum of its proper divisors (1).

21166221202561 is an equidigital number, since it uses as much as digits as its factorization.

21166221202561 is an evil number, because the sum of its binary digits is even.

The product of its (nonzero) digits is 34560, while the sum is 37.

Adding to 21166221202561 its reverse (16520212266112), we get a palindrome (37686433468673).

The spelling of 21166221202561 in words is "twenty-one trillion, one hundred sixty-six billion, two hundred twenty-one million, two hundred two thousand, five hundred sixty-one".