Prefix Sums ...
Sum the prefixes of a sequence of numbers, x1, x2 , x3 , ..., xn, such that yi = ?i<n xi
Each yi result seems to depend on computing the previous item
One solution is to apply the binary tree summation to compute each yi in parallel ... this would take 1+1+2+2+3+3+...+n/2+n/2
and a lot of data communication