Belevitch's theorem is a theorem in electrical network analysis due to the Russo-Belgian mathematician Vitold Belevitch (1921–1999).
The theorem provides a test for a given S-matrix to determine whether or not it can be constructed as a lossless rational two-port network.
Lossless implies that the network contains only inductances and capacitances – no resistances.
Rational (meaning the driving point impedance Z(p) is a rational function of p) implies that the network consists solely of discrete elements (inductors and capacitors only – no distributed elements).
represents a lossless rational network if and only if,[1]