In imaginary time, the corresponding definition is G ( n ) ( 1 … … n ∣ ∣ 1 ′ … … n ′ ) = ⟨ ⟨ T ψ ψ ( 1 ) … … ψ ψ ( n ) ψ ψ ¯ ¯ ( n ′ ) … … ψ ψ ¯ ¯ ( 1 ′ ) ⟩ ⟩ , where j signifies x j , τ τ j . (The imaginary-time variables τ τ j are restricted to the range from 0 to the inverse temperature β β = 1 k B T .)