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I am using Mathematica 9 and get these strange spikes when I plot the following simple function. I have seen a previous post about this but cannot figure out what is wrong with the following code. Does anyone know how to get rid of them? Any help would be much appreciated.

f[q_, L_] := If[q < 1/(1 + L^0.5), 10*q]

Plot3D[f[q, L], {q, 0, 1}, {L, 0, 5}, BoxRatios -> 1, Mesh -> None, 
 AxesLabel -> {"q", "L", "Price"}, 
 PlotStyle -> Directive[Opacity[0.6], Blue]]

spikes

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I don't know why it looks like that. However:

It looks like you are trying to restrict the plot region and you do this by defining the function to return Null in other areas. This is not the usual way to do it.

Use RegionFunction instead.

Plot3D[10*q, {q, 0, 1}, {L, 0, 5}, BoxRatios -> 1, Mesh -> None, 
 AxesLabel -> {"q", "L", "Price"}, 
 PlotStyle -> Directive[Opacity[0.6], Blue], 
 RegionFunction -> Function[{q, L}, q < 1/(1 + L^0.5)]]

Mathematica graphics

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