Divergence of a vector field.
Div[{f1, …, fn}, {x1, …, xn}]
Div[{f1, …, fn}, {x1, …, xn}, chart]
Div[{x y, y z, z x}, {x, y, z}]
→ x + y + zDiv[{{x, y}, {z, w}}, {x, y}]
→ {2, 0}Div[{r, 0}, {r, t}, "Polar"]
→ 2Div[{r, 0, 0}, {r, t, p}, "Spherical"]
→ 3