Then we can introduce notation of path ordering on this contour, by defining T c ( X ( 1 ) ( c 1 ) X ( 2 ) ( c 2 ) … … X ( n ) ( c n ) ) = ( ± ± 1 ) σ σ X ( σ σ ( 1 ) ) ( c σ σ ( 1 ) ) X ( σ σ ( 2 ) ) ( c σ σ ( 2 ) ) … … X ( σ σ ( n ) ) ( c σ σ ( n ) ) , where σ σ is a permutation such that c σ σ ( 1 ) < c σ σ ( 2 ) < … … c σ σ ( n ) , and the plus and minus signs are for bosonic and fermionic operators respectively. Note that this is a generalization of time ordering.