Källén–Lehmann spectral representation
EXPRESSION FOR TWO-POINT CORRELATION FUNCTIONS
Kallen-Lehmann spectral representation; Källén-Lehmann spectral representation; Kallen–Lehmann spectral representation; Lehmann-Källén spectral representation
The Källén–Lehmann spectral representation gives a general expression for the (time ordered) two-point function of an interacting quantum field theory as a sum of free propagators. It was discovered by Gunnar Källén and Harry Lehmann independently.