666921821 has 2 divisors, whose sum is σ = 666921822. Its totient is φ = 666921820.

The previous prime is 666921803. The next prime is 666921833. The reversal of 666921821 is 128129666.

It is a happy number.

It is a strong prime.

It can be written as a sum of positive squares in only one way, i.e., 459244900 + 207676921 = 21430^2 + 14411^2 .

It is a cyclic number.

It is a de Polignac number, because none of the positive numbers 2^k-666921821 is a prime.

It is a Chen prime.

It is equal to p_34626983 and since 666921821 and 34626983 have the same sum of digits, it is a Honaker prime.

It is a congruent number.

It is not a weakly prime, because it can be changed into another prime (666921721) by changing a digit.

It is a polite number, since it can be written as a sum of consecutive naturals, namely, 333460910 + 333460911.

It is an arithmetic number, because the mean of its divisors is an integer number (333460911).

Almost surely, 2^666921821 is an apocalyptic number.

It is an amenable number.

666921821 is a deficient number, since it is larger than the sum of its proper divisors (1).

666921821 is an equidigital number, since it uses as much as digits as its factorization.

666921821 is an evil number, because the sum of its binary digits is even.

The product of its digits is 62208, while the sum is 41.

The square root of 666921821 is about 25824.8295444520. The cubic root of 666921821 is about 873.6918994415.

The spelling of 666921821 in words is "six hundred sixty-six million, nine hundred twenty-one thousand, eight hundred twenty-one".