Equal partial derivatives


by sunrah
Tags: derivatives, equal, partial
sunrah
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#1
Nov3-11, 03:11 PM
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Hi, in general can we tell anything about the partial derivatives of a differentiable function if they are equal?

for example I would like them to have to equal some constant. Would this be true?
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HallsofIvy
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#2
Nov3-11, 07:29 PM
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Do you mean [itex]\partial f/\partial x= C[/itex] and [itex]\partial f/\partial y= C[/itex]? The same constant or different constants? From [itex]\partial f/\partial x= C[/itex], we get [itex]f(x,y)= Cx+ g(y)[/itex] where g can be any function of y. Differentiating that with respect to y, [itex]\partial f/\partial y= g'(y)= C[/itex] which tells us that g(y)= Cy+ C' where C' is an arbitrary constant of integration. That is, f(x,y)= Cx+ Cy+ C'.


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