Check grammar: Please check grammatical error the following sentence with explanation. Thx.?

combination of words and forms of words are correct:-
eg:-For all odd primes exceeding 3,
they can either be
expressed by 6k+1
or
by 6k-1,
where k is the twin-pair factor.

More precisely; (there is only one even prime which is "2")

For all primes exceeding 3, they can either be expressed by 6k-1 or 6k+1 where k is any natural numbers.

For prime number 23, it can be expressed by 6k-1 where k = 4.
However, k=4 is not a factor for prime twin pair, as (23, 25) is not a prime twin pair.

For all odd primes exceeding 3, they can either be expressed by 6k+1 or by 6k–1 where k is the twin-pair factor.

It is grammatically acceptable. However, "whose" can also be used in place of "where".

That is:

For all odd primes exceeding 3, they can either be expressed by 6k+1 or by 6k–1 whose k is the twin-pair factor.