666821 has 2 divisors, whose sum is σ = 666822. Its totient is φ = 666820.

The previous prime is 666811. The next prime is 666823. The reversal of 666821 is 128666.

It is a strong prime.

It can be written as a sum of positive squares in only one way, i.e., 662596 + 4225 = 814^2 + 65^2 .

It is a cyclic number.

It is a de Polignac number, because none of the positive numbers 2^k-666821 is a prime.

Together with 666823, it forms a pair of twin primes.

It is a Chen prime.

It is a congruent number.

It is not a weakly prime, because it can be changed into another prime (666823) by changing a digit.

It is a polite number, since it can be written as a sum of consecutive naturals, namely, 333410 + 333411.

It is an arithmetic number, because the mean of its divisors is an integer number (333411).

2⁶⁶⁶⁸²¹ is an apocalyptic number.

It is an amenable number.

666821 is a deficient number, since it is larger than the sum of its proper divisors (1).

666821 is an equidigital number, since it uses as much as digits as its factorization.

666821 is an odious number, because the sum of its binary digits is odd.

The product of its digits is 3456, while the sum is 29.

The square root of 666821 is about 816.5910849379. The cubic root of 666821 is about 87.3647870828.

The spelling of 666821 in words is "six hundred sixty-six thousand, eight hundred twenty-one".