$ C = AB $を想定し、答えは
$ \開始{整列} \ notag ANS＆= \和\ limits_ {i = 0} ^ {N-1} \和\ limits_ {J = 0} ^ {N-1} C [I] [J]、P ^ {（NI）N-1-J} \\ \ notag＆= \和\ limits_ {i = 0} ^ {N-1} \和\ limits_ {J = 0} ^ {N-1} \和\ limits_ {K = 0} ^ {N-1} A [i]が[K] B [K] [j]はP ^ {（NI）N-1-J} \\＆= \和\ limits_ {K = 0} ^ {N-1} \ビッグ（\和\ limits_ {i = 0} ^ {N-1} A [i]が[K] P ^ {（NI）は、n-1} \ビッグ）\ビッグ（\和\ limits_ {J = 0} ^ {N-1} B [K] [j]はP ^ { - } J \ビッグ）\ notag \端{整列} $