Differential equation Green's function
GreenFunction[{ℒ[u[x]], ℬ[u[x]]}, u, {x, xmin, xmax}, y]
GreenFunction[{ℒ[u[x1, x2, …]], ℬ[u[x1, x2, …]]}, u, {x1, x2, …}∈ Ω, {y1, y2, …}]
GreenFunction[{ℒ[u[x, t]], ℬ[u[x, t]]}, u, {x, xmin, xmax}, t, {y, τ}]
GreenFunction[{ℒ[u[x1, …, t]], ℬ[u[x1, …, t]]}, u, {x1, …}∈ Ω, t, {y1, …, τ}]