where H x denotes the Hamiltonian written in terms of the x coordinates, δ (x) denotes the Dirac delta-function, Θ(t) is the Heaviside step function and K (x, t ; x′, t′) is the kernel of the above Schrödinger differential operator in the big parentheses. The term propagator is sometimes used in this context to refer to G, and sometimes to K. This article will use the term to refer to K (see Duhamel's principle).