Thus, Feynman diagrams for correlation functions can be drawn and their values computed the same way as in ground state theory, except with the following modifications to the Feynman rules: Each internal vertex of the diagram is labeled with either + or − − , while external vertices are labelled with − − . Then each (unrenormalized) edge directed from a vertex a (with position x a , time t a and sign s a ) to a vertex b (with position x b , time t b and sign s b ) corresponds to the propagator G 0 s a s b ( x a , t a , x b , t b ) . Then the diagram values for each choice of ± ± signs (there are 2 v such choices, where v is the number of internal vertices) are all added up to find the total value of the diagram.