§
Harmonic function
Solve Poisson's equation (wave equation) :
Δ f = 0
.
is a vector field
V
=
∇
f
V = ∇f
V
=
∇
f
arose from a potential, then it
satisfies
∇
x
(
∇
f
)
=
0
∇ x(∇f) = 0
∇
x
(
∇
f
)
=
0
(by the exact sequence).