N
the differential operator (∂/∂x) + (∂/∂y) + (∂/∂z), where , , and are unit vectors in the x, y, and z directions 倒三角算子 ( symbol: ∇Also called nabla)
[maths]
Burnett, GrahamD. Masters ofAll They Surveyed: Exploration, Geography, anda BritishEl Dorado. Chicago: University of Chicago Press, 2000. ISBN: 0226081206.