21615844781 has 2 divisors, whose sum is σ = 21615844782. Its totient is φ = 21615844780.

The previous prime is 21615844739. The next prime is 21615844841. The reversal of 21615844781 is 18744851612.

It is a weak prime.

It can be written as a sum of positive squares in only one way, i.e., 15917859556 + 5697985225 = 126166^2 + 75485^2 .

It is a cyclic number.

It is not a de Polignac number, because 21615844781 - 2⁶ = 21615844717 is a prime.

It is a super-2 number, since 2×21615844781² (a number of 21 digits) contains 22 as substring.

It is a congruent number.

It is not a weakly prime, because it can be changed into another prime (21615844721) by changing a digit.

It is a polite number, since it can be written as a sum of consecutive naturals, namely, 10807922390 + 10807922391.

It is an arithmetic number, because the mean of its divisors is an integer number (10807922391).

Almost surely, 2^21615844781 is an apocalyptic number.

It is an amenable number.

21615844781 is a deficient number, since it is larger than the sum of its proper divisors (1).

21615844781 is an equidigital number, since it uses as much as digits as its factorization.

21615844781 is an evil number, because the sum of its binary digits is even.

The product of its digits is 430080, while the sum is 47.

The spelling of 21615844781 in words is "twenty-one billion, six hundred fifteen million, eight hundred forty-four thousand, seven hundred eighty-one".