The variables a, b, c, and z must have a stated correlation

They do! a is the pronumeral in the 1st factor, b is the pronumeral in the 2nd factor, c is the pronumeral in the 3rd factor - i.e. the first 3 terms in a sequence - and the nth factor has the pronumeral z, and you think that ISN'T stating a relationship between term t of the sequence and the t-th pronumeral? 😂

"the series can only be inferred using the rules of the English language" - well, they haven't used Greek letters for it, have they?? 😂