666666686666621 has 2 divisors, whose sum is σ = 666666686666622. Its totient is φ = 666666686666620.

The previous prime is 666666686666599. The next prime is 666666686666699. The reversal of 666666686666621 is 126666686666666.

It is a weak prime.

It can be written as a sum of positive squares in only one way, i.e., 624044865192100 + 42621821474521 = 24980890^2 + 6528539^2 .

It is a cyclic number.

It is not a de Polignac number, because 666666686666621 - 2⁶ = 666666686666557 is a prime.

It is a super-2 number, since 2×666666686666621² (a number of 30 digits) contains 22 as substring.

It is a Sophie Germain prime.

It is a Curzon number.

It is a self number, because there is not a number n which added to its sum of digits gives 666666686666621.

It is a congruent number.

It is not a weakly prime, because it can be changed into another prime (566666686666621) by changing a digit.

It is a pernicious number, because its binary representation contains a prime number (29) of ones.

It is a polite number, since it can be written as a sum of consecutive naturals, namely, 333333343333310 + 333333343333311.

It is an arithmetic number, because the mean of its divisors is an integer number (333333343333311).

Almost surely, 2^{666666686666621} is an apocalyptic number.

It is an amenable number.

666666686666621 is a deficient number, since it is larger than the sum of its proper divisors (1).

666666686666621 is an equidigital number, since it uses as much as digits as its factorization.

666666686666621 is an odious number, because the sum of its binary digits is odd.

The product of its digits is 34828517376, while the sum is 83.

The spelling of 666666686666621 in words is "six hundred sixty-six trillion, six hundred sixty-six billion, six hundred eighty-six million, six hundred sixty-six thousand, six hundred twenty-one".