Recent Developments in Filter Theory

This paper is a review of the most important developments in filter theory during the postwar years. The 57 items contained in the bibliography appended to this paper have been selected either for their intrinsic novelty or because they contain reviews of earlier work with historical references. The integration of the various contributions has been attempted in this paper and several original ideas and results are included. No attempt has been made to comment on the practical value of insertion loss design vs image parameter design. As appears from the review, recent extensions of both methods have brought them closer to each other, and their cross fertilization has contributed to progress towards a unified filter theory.