21115516445581 has 2 divisors, whose sum is σ = 21115516445582. Its totient is φ = 21115516445580.

The previous prime is 21115516445549. The next prime is 21115516445659. The reversal of 21115516445581 is 18554461551112.

It is a weak prime.

It can be written as a sum of positive squares in only one way, i.e., 14652435622500 + 6463080823081 = 3827850^2 + 2542259^2 .

It is a cyclic number.

It is not a de Polignac number, because 21115516445581 - 2⁵ = 21115516445549 is a prime.

It is a super-2 number, since 2×21115516445581² (a number of 27 digits) contains 22 as substring.

It is a congruent number.

It is not a weakly prime, because it can be changed into another prime (21115516445381) by changing a digit.

It is a polite number, since it can be written as a sum of consecutive naturals, namely, 10557758222790 + 10557758222791.

It is an arithmetic number, because the mean of its divisors is an integer number (10557758222791).

Almost surely, 2^{21115516445581} is an apocalyptic number.

It is an amenable number.

21115516445581 is a deficient number, since it is larger than the sum of its proper divisors (1).

21115516445581 is an equidigital number, since it uses as much as digits as its factorization.

21115516445581 is an evil number, because the sum of its binary digits is even.

The product of its digits is 960000, while the sum is 49.

Adding to 21115516445581 its reverse (18554461551112), we get a palindrome (39669977996693).

The spelling of 21115516445581 in words is "twenty-one trillion, one hundred fifteen billion, five hundred sixteen million, four hundred forty-five thousand, five hundred eighty-one".