Let's say we start from j is equal to 1-- j's going to be the column, so we're going to take the sum of each of these terms from j is equal to 1 to n. And then remember our checkerboard pattern, so we don't know if this is a positive or negative. We can figure it out by taking negative 1 to the i plus j-- remember, this is the ith row that we're talking about-- times xj-- xj is the coefficient, xsubj, times the sub matrix for xsubj.