Letter to the Editor

In a private communication, Professor R. Aris of the Chemical Engineering Department at the University of Minnesota has raised a significant question concerning our article Application of Dynamic Programming to Countercurrent Flow Processes [Opns Res9 388–401 (1961)]. His inquiry requests that we define the class of problems for which the application of the “dynamic programming principle of optimality” will yield total returns as high as those achieved by more conventional maximization procedures.