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I have a functional polynomial expression of the form:

expr = f[x] + f[x] g[w] + Conjugate[f[y]] g[z] +
       Conjugate[g[z]] + g[w] + f[x] f[y] g[w] + 1 +
       Conjugate[a] f[t] + Conjugate[b] Conjugate[c]

I would like to write a function that selects terms that are up to linear (i.e. zero and first order) in the functions f and g, eg. in my case the correct output would be:

1 + f[x] + Conjugate[g[z]] + g[w] + Conjugate[a] f[t] +
Conjugate[b] Conjugate[c] 

Note that the functions are complex, and the conjugate terms are also considered. I basically just want to neglect second-order terms and higher.

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You could build rules as in : expr /. {f[x_] f[y_] -> 0, g[x_] g[y_] -> 0}. –  b.gatessucks May 7 '13 at 11:43
    
the expression i put is just a simplification, in the general case it contains a lot more terms with much higher orders, so i would need very many rules –  Andrei May 7 '13 at 12:38
    
Take the Jacobian, zero everything as for the constant term just zero everything in the initial expression. –  Spawn1701D May 7 '13 at 13:29

3 Answers 3

up vote 1 down vote accepted

Maybe this

Replace[Expand@expr, Times[terms__ /; Count[{terms}, _f | _g, Infinity] > 1] -> 0, 1]
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How about this:

(Series[Simplify[expr/.{forg_[x_] -> m forg[x]}, m \[Element] Reals], {m, 0, 1}] // Normal) /. m -> 1

Edit:

If there are other functions within the expression that are not f or g then you have to be a bit more explicit:

(Series[Simplify[expr/.{f[x_] -> m f[x],g[x_]->m g[x]}, m \[Element] Reals], {m, 0, 1}] // Normal) /. m -> 1
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thanks, this works for the above example, but unfortunately my (real) expression also has terms of the form: Conjugate[a]*f[t] and Conjugate[b]*Conjugate[c], for which it fails –  Andrei May 7 '13 at 13:05
    
@user4794, the edit should solve that problem. –  Jonathan Shock May 7 '13 at 22:06

Try this

First harvest the terms:

terms = Cases[expr, (_f | _g) | Conjugate[_g | _f], Infinity]//Union

and then

(D[expr, {terms}] /. {f -> (0 &), g -> (0 &)}).terms + (expr/.Thread[terms -> 0])

Note:

For a faster alternative use CoefficientArrays:

#1 + #2.terms & @@ (Take[CoefficientArrays[expr, terms], 2] // Normal)
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