418. What sequence is formed by the products of the corresponding $a_{k}, b_{k}, c_{k} \ldots$ terms of several given geometric progressions? What more general function could be taken instead of the product without changing the answer to the question?
418. a) Geometric progression; b) the product of different (but for each of the given progressions the same) powers of the corresponding terms $a_{k}^{p} b_{k}^{q} c_{k}^{r} \ldots$