An O(1) Solution to the Prefix Sum Problem on a Specialized Memory   Architecture
  In this paper we study the Prefix Sum problem introduced by Fredman.   We show that it is possible to perform both update and retrieval in O(1) time simultaneously under a memory model in which individual bits may be shared by several words.   We also show that two variants (generalizations) of the problem can be solved optimally in $\Theta(\lg N)$ time under the comparison based model of computation. 
Andrej Brodnik, Johan Karlsson, J. Ian Munro, Andreas Nilsson