摘要：Comparison theorems may be used to prove the existence and
uniqueness of solutions to certain types of partial differential
equations. They provide bounds for solutions and can be used as the
basis of numerical techniques for the computation of solutions. In
1961 Alex McNabb published one of the first papers extending their
use to multi-component systems. Developments in the theory and
applications of such results, through citations of this original
paper, are reviewed.
