21691663519761 has 27 divisors (see below), whose sum is σ = 32754395999689. Its totient is φ = 13832160216072.

The previous prime is 21691663519759. The next prime is 21691663519781. The reversal of 21691663519761 is 16791536619612.

It is a happy number.

The square root of 21691663519761 is 4657431.

It is a perfect power (a square), and thus also a powerful number.

21691663519761 is a `hidden beast` number, since 2 + 1 + 69 + 166 + 351 + 9 + 7 + 61 = 666.

It is not a de Polignac number, because 21691663519761 - 2¹ = 21691663519759 is a prime.

It is a Duffinian number.

It is a Curzon number.

It is not an unprimeable number, because it can be changed into a prime (21691663519781) by changing a digit.

It is a polite number, since it can be written in 26 ways as a sum of consecutive naturals, for example, 321328990 + ... + 321396488.

Almost surely, 2^{21691663519761} is an apocalyptic number.

21691663519761 is the 4657431-st square number.

21691663519761 is the 2328716-th centered octagonal number.

It is an amenable number.

21691663519761 is a deficient number, since it is larger than the sum of its proper divisors (11062732479928).

21691663519761 is an frugal number, since it uses more digits than its factorization.

21691663519761 is an evil number, because the sum of its binary digits is even.

The sum of its prime factors is 135050 (or 67525 counting only the distinct ones).

The product of its digits is 22044960, while the sum is 63.

The spelling of 21691663519761 in words is "twenty-one trillion, six hundred ninety-one billion, six hundred sixty-three million, five hundred nineteen thousand, seven hundred sixty-one".