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EGG BOX
Homemade name. 
Cartesian equation: .
Translation surface. 
The egg box is the surface obtained by translating a sinusoid along another isometric sinusoid, perpendicularly.
This surface can be interesting to introduce the notions of extrema and saddle points.
Remark: weirdly, if the + is replaced by a is the above equation, the surface obtained is also an egg box; indeed: .
Projections of contour lines: sin x/b sin y/b = cte. Projections of slope lines:

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© Robert FERRÉOL, Jacques MANDONNET 2017