667991 has 2 divisors, whose sum is σ = 667992. Its totient is φ = 667990.

The previous prime is 667987. The next prime is 667999. The reversal of 667991 is 199766.

667991 is digitally balanced in base 2, because in such base it contains all the possibile digits an equal number of times.

It is a weak prime.

It is a cyclic number.

It is not a de Polignac number, because 667991 - 2² = 667987 is a prime.

It is a Chen prime.

It is a congruent number.

It is not a weakly prime, because it can be changed into another prime (667999) by changing a digit.

It is a polite number, since it can be written as a sum of consecutive naturals, namely, 333995 + 333996.

It is an arithmetic number, because the mean of its divisors is an integer number (333996).

2⁶⁶⁷⁹⁹¹ is an apocalyptic number.

667991 is a deficient number, since it is larger than the sum of its proper divisors (1).

667991 is an equidigital number, since it uses as much as digits as its factorization.

667991 is an evil number, because the sum of its binary digits is even.

The product of its digits is 20412, while the sum is 38.

The square root of 667991 is about 817.3071638007. The cubic root of 667991 is about 87.4158537989.

The spelling of 667991 in words is "six hundred sixty-seven thousand, nine hundred ninety-one".