In Coordinate Space The Green’s function of a fermion can be written as the sum of all connected diagrams

# Month: September 2017

## Wick’s Theorem

Wick’s theorem is a way to reduce a time-ordered operator into the sum of normal-ordered operator and various contractions. Then, where is the time-ordered operator from? Why we need this reduction?

## Quantum Pictures

This note gives a comparison on the three quantum pictures. Also, the Dyson series are introduced with the help of time-ordering operators.