This paper presents a method of analysis associated with the specification of optimal energy-group and space-interval structures in neutron diffusion calculations. Initially, an extremal algorithm is formulated to minimize the integrated error between two arbitrary piecewise-constant functions of two variables. The minimization is attained by steepest descent in piecewise-constant, non-convex, multidimensional phase-space. It is found that given an initial reference neutron diffusion calculation, the extremal algorithm may be effectively used to specify a reduced energy-group structure and/or a reduced space-interval structure such that the error in the effective multiplication constant is minimized. The extremalnodal analysis discussed herein appears to be particularly useful for repetitious nuclear reactor calculations which seek to maximize numerical accuracy and minimize computer execution time.