643691 has 2 divisors, whose sum is σ = 643692. Its totient is φ = 643690.

The previous prime is 643681. The next prime is 643693. The reversal of 643691 is 196346.

It is a strong prime.

It is a cyclic number.

It is a de Polignac number, because none of the positive numbers 2^k-643691 is a prime.

Together with 643693, it forms a pair of twin primes.

It is a Chen prime.

It is not a weakly prime, because it can be changed into another prime (643693) by changing a digit.

It is a pernicious number, because its binary representation contains a prime number (11) of ones.

It is a polite number, since it can be written as a sum of consecutive naturals, namely, 321845 + 321846.

It is an arithmetic number, because the mean of its divisors is an integer number (321846).

2⁶⁴³⁶⁹¹ is an apocalyptic number.

643691 is a deficient number, since it is larger than the sum of its proper divisors (1).

643691 is an equidigital number, since it uses as much as digits as its factorization.

643691 is an odious number, because the sum of its binary digits is odd.

The product of its digits is 3888, while the sum is 29.

The square root of 643691 is about 802.3035585114. The cubic root of 643691 is about 86.3427371910.

The spelling of 643691 in words is "six hundred forty-three thousand, six hundred ninety-one".