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Apr 15, 2010 at 20:27 comment added Garabed Gulbenkian Let U be the non-empty open connected subset of the Cartesian x-y plane at all points of which F(x,y) is defined and continuous. What makes this problem difficult is that once having specified F(x,y), one then has to prove that no arc in R^3 with an equation of the form "x=g(t),y=h(t),z=F(g(t),h(t))-where t lies in the closure of (0,1)" has a tangent at any of its points, whenever x=g(t),y=h(t)is the equation of an arc lying in U.
Mar 24, 2010 at 11:23 comment added Petya Your update is wrong. If $g$ is odd function then the surface $z=g(x)+g(y)$ contains a line.
Mar 23, 2010 at 20:14 history edited Yuri Bakhtin CC BY-SA 2.5
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Mar 23, 2010 at 19:59 history edited Yuri Bakhtin CC BY-SA 2.5
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Mar 23, 2010 at 19:42 history answered Yuri Bakhtin CC BY-SA 2.5