646991 has 2 divisors, whose sum is σ = 646992. Its totient is φ = 646990.

The previous prime is 646981. The next prime is 646993. The reversal of 646991 is 199646.

It is a strong prime.

It is a cyclic number.

It is not a de Polignac number, because 646991 - 2⁶ = 646927 is a prime.

It is a Sophie Germain prime.

Together with 646993, 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 (646993) by changing a digit.

It is a polite number, since it can be written as a sum of consecutive naturals, namely, 323495 + 323496.

It is an arithmetic number, because the mean of its divisors is an integer number (323496).

2⁶⁴⁶⁹⁹¹ is an apocalyptic number.

646991 is a deficient number, since it is larger than the sum of its proper divisors (1).

646991 is an equidigital number, since it uses as much as digits as its factorization.

646991 is an evil number, because the sum of its binary digits is even.

The product of its digits is 11664, while the sum is 35.

The square root of 646991 is about 804.3575075798. The cubic root of 646991 is about 86.4900363856.

The spelling of 646991 in words is "six hundred forty-six thousand, nine hundred ninety-one".