21786391661 has 2 divisors, whose sum is σ = 21786391662. Its totient is φ = 21786391660.

The previous prime is 21786391631. The next prime is 21786391733. The reversal of 21786391661 is 16619368712.

It is a weak prime.

It can be written as a sum of positive squares in only one way, i.e., 13761636100 + 8024755561 = 117310^2 + 89581^2 .

It is a cyclic number.

It is not a de Polignac number, because 21786391661 - 2¹⁸ = 21786129517 is a prime.

It is a Sophie Germain prime.

It is a Curzon number.

It is a congruent number.

It is not a weakly prime, because it can be changed into another prime (21786391631) by changing a digit.

It is a pernicious number, because its binary representation contains a prime number (17) of ones.

It is a polite number, since it can be written as a sum of consecutive naturals, namely, 10893195830 + 10893195831.

It is an arithmetic number, because the mean of its divisors is an integer number (10893195831).

Almost surely, 2^21786391661 is an apocalyptic number.

It is an amenable number.

21786391661 is a deficient number, since it is larger than the sum of its proper divisors (1).

21786391661 is an equidigital number, since it uses as much as digits as its factorization.

21786391661 is an odious number, because the sum of its binary digits is odd.

The product of its digits is 653184, while the sum is 50.

The spelling of 21786391661 in words is "twenty-one billion, seven hundred eighty-six million, three hundred ninety-one thousand, six hundred sixty-one".