We can use 'field operators' as above, or creation and annihilation operators associated with other single-particle states, perhaps eigenstates of the (noninteracting) kinetic energy. We then use ψ ψ ( x , τ τ ) = φ φ α α ( x ) ψ ψ α α ( τ τ ) , where ψ ψ α α is the annihilation operator for the single-particle state α α and φ φ α α ( x ) is that state's wavefunction in the position basis. This gives G α α 1 … … α α n | β β 1 … … β β n ( n ) ( τ τ 1 … … τ τ n | τ τ 1 ′ … … τ τ n ′ ) = ⟨ ⟨ T ψ ψ α α 1 ( τ τ 1 ) … … ψ ψ α α n ( τ τ n ) ψ ψ ¯ ¯ β β n ( τ τ n ′ ) … … ψ ψ ¯ ¯ β β 1 ( τ τ 1 ′ ) ⟩ ⟩ with a similar expression for G ( n ) .