4. For each leaf node, accumulate the S α ′ from each remote node, as determined by the appropriate scale for each interaction. At each scale there are O(1) remote boxes due to the box size increasing with decreasing scale, and the prefactor can be optimized by drawing data from the optimal level of the tree. This step requires O(log 8 (N )) memory imports and FLOPs per leaf node, totaling O(N log 8 (N )).