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Suppose I have a function $p(x+\epsilon,y-\epsilon,t)$, and I want to expand it around $(x,y)$ like $$ p(x+\epsilon,y-\epsilon,t)=p(x,y,t)+\dfrac{\partial p}{\partial x}\epsilon+\dfrac{\partial p}{\partial y}(-\epsilon)+\cdots $$

A naive way to do it is

Series[p[x+e,y-e,t],{x+e,x,1},{y-e,y,1}]

which will result in the following error message:

General::ivar: "e+x is not a valid variable."
....

Right now I do the expansion in the following clumsy way:

Replace[
Normal@Series[p[x, y, t], {x, a, 1}, {y, b, 1}], 
{x - a -> \[Epsilon], y - b -> -\[Epsilon]}, 
Infinity] 
/. {a -> x, b -> y}

Question: Is there an elegant way to do it? In the end, I want to expand $p(x,y,t)$ in $$ -(x+z)p(x,y,t)+(x+\epsilon)p(x+\epsilon,y-\epsilon,t)+(1-x-y+\epsilon)p(x,y-\epsilon,t) $$ and keep only the $\epsilon$ terms.

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  • $\begingroup$ What's wrong with Series[p[x+e,y-e,t],{e,0,1},{e,0,1}] or Series[p[xx,yy,t],{xx,x,1},{yy,y,1}]? $\endgroup$
    – sebhofer
    Commented Aug 12, 2014 at 10:25
  • $\begingroup$ @sebhofer,Actually Series[p[x+e,y-e,t],{e,0,1},{e,0,1}] is what I wanted. $\endgroup$
    – wdg
    Commented Aug 12, 2014 at 10:46
  • $\begingroup$ Ups, I just realized this should have been Series[p[x+ex,y-ey,t],{ex,0,1},{ey,0,1}] or just Series[p[x+e,y-e,t],{e,0,1}] $\endgroup$
    – sebhofer
    Commented Aug 12, 2014 at 10:48
  • $\begingroup$ @sebhofer Don't forget to post this as an answer. $\endgroup$
    – C. E.
    Commented Aug 12, 2014 at 14:03
  • $\begingroup$ @Pickett If I'm honest, I wasn't (and still not am) really sure if this question would survive, as this is a pretty trivial use of Series $\endgroup$
    – sebhofer
    Commented Aug 12, 2014 at 15:45

1 Answer 1

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Depending on your specific needs the straightforward way to do this is

Series[p[x+e,y-e,t],{e,0,1}]

or

Series[p[x+ex,y-ey,t],{ex,0,1},{ey,0,1}]

If you want to extract the terms proportional to $\epsilon$ you can get the correct coefficients with

SeriesCoefficient[p[x + e, y - e, t], {e, 0, 1}]

or if that suits you better with

CoefficientList[Series[p[x + e, y - e, t], {e, 0, 1}], e][[2]]
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