A distinguished 1-cell in a logic functions Karnaugh map is a cell (and a corresponding input combination) that is covered by only one prime implicant. Such an essential prime implicant must be present in any minimal sum for the function. Thus, an efficient minimization algorithm looks first for essential prime implicants, and then selects additional prime implicants only as needed for 1-cells that are still uncovered, if any. The following logic function have one or more essential prime implicants; find its minimal sum:

F = W , X , Y , Z ( 1 , 4 , 5 , 6 , 7 , 9 , 4 , 15 )