But that would sum everything, not just the parts which need it. You'd actually have to write this: \sum_a(A_ia x_a) + v_i + \sum_a(x_a y_a) ( w_i + \sum_b(B_i

But that would sum everything, not just the parts which need it. You'd actually have to write this: \sum_a(A_ia x_a) + v_i + \sum_a(x_a y_a) ( w_i + \sum_b(B_ib z_b) ) My example is actually still easier without indices. (Although you lose some information, like the fact that x & y are vectors in the indexed by a,b.) Here it's clear that Ax and x⋅y each involve a sum, which never includes v: Ax + v + (x⋅y)(w + Bz)