It can be seen that functions f_3_3(a, b, c) = abc and f_4_4(a, b, c, d) = abcd are equal to the logical product AND (it generally holds for f_n_n) and functions f_1_3(a, b, c) = a + b + c and f_1_4(a, b, c, d) = a + b + c + d are the logical sum OR (it generally holds for f_1_n).