668867821 has 2 divisors, whose sum is σ = 668867822. Its totient is φ = 668867820.

The previous prime is 668867819. The next prime is 668867851. The reversal of 668867821 is 128768866.

It is a weak prime.

It can be written as a sum of positive squares in only one way, i.e., 520296100 + 148571721 = 22810^2 + 12189^2 .

It is a cyclic number.

It is not a de Polignac number, because 668867821 - 2¹ = 668867819 is a prime.

Together with 668867819, it forms a pair of twin primes.

It is a congruent number.

It is not a weakly prime, because it can be changed into another prime (668867851) by changing a digit.

It is a pernicious number, because its binary representation contains a prime number (19) of ones.

It is a polite number, since it can be written as a sum of consecutive naturals, namely, 334433910 + 334433911.

It is an arithmetic number, because the mean of its divisors is an integer number (334433911).

Almost surely, 2^668867821 is an apocalyptic number.

It is an amenable number.

668867821 is a deficient number, since it is larger than the sum of its proper divisors (1).

668867821 is an equidigital number, since it uses as much as digits as its factorization.

668867821 is an odious number, because the sum of its binary digits is odd.

The product of its digits is 1548288, while the sum is 52.

The square root of 668867821 is about 25862.4790188412. The cubic root of 668867821 is about 874.5408512438.

The spelling of 668867821 in words is "six hundred sixty-eight million, eight hundred sixty-seven thousand, eight hundred twenty-one".