A note on [A, B, C, D]

For arbitrary linear passive networks it is possible to suggest bounds of the type f(A, B, C, D) ≥ 0. A novel inequality is presented for linear reciprocal resistive two-ports: A²+ B²+ C²+ D²+ AB + CD ≥ √3 It is common practice in electrical engineering to consider a two-port as an operator represented by a 2 × 2 matrice acting on the space of the input signals.