Question

Find the minimum sum-of-products expression f(a, b, c, d) = (maxterm numbers)(1, 2, 3, 4, 9, 15) When I made a Karnaugh map for the maxterms, I came up with (B+C+D')(A+B +C')(A+B'+C+D)(A'+B'+C'+D') a) I think that's incorrect, perhaps due to my groupings of the 0's. b) If my answer is in fact correct, I can't figure out how to put it into a product of sums form. I just applied DeMorgan laws on it, but I'm not sure that's correct either. Please help? This is basically a maxterm expansion from a Karnaugh map (which I may or may not have grouped incorrectly) that I need in SOP form.