In real time, the 2 n -point Green function is defined by G ( n ) ( 1 … … n ∣ ∣ 1 ′ … … n ′ ) = i n ⟨ ⟨ T ψ ψ ( 1 ) … … ψ ψ ( n ) ψ ψ ¯ ¯ ( n ′ ) … … ψ ψ ¯ ¯ ( 1 ′ ) ⟩ ⟩ , where we have used a condensed notation in which j signifies ( x j , t j ) and j ′ signifies ( x j ′ , t j ′ ) . The operator T denotes time ordering, and indicates that the field operators that follow it are to be ordered so that their time arguments increase from right to left.