Criterion of absolute stability of two-ports

The criterion of absolute stability of linear two-ports is well known, and derivations may be found in various monographs on linear active network theory. It generally appears that such derivations are founded on several pages of calculation, admittedly of an elementary kind, but lacking in perspicuity. It is the aim of this correspondence to present a simple and direct derivation of the main part of this criterion, i.e., the part pretaining to frequencies.