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Re: Series problem


Tommy Nordgren <f85-tno at telesto.nada.kth.se>
[mg4084] Series problem

Writes
>I want to expand the Cosine only in the expression:
>Cos[b x] Exp[-x^2]/(k^2+x^2) into a taylor series around 0.
>Computing the series in terms of x don't work, because the  
>exponetial and the divisor will be expanded as well, when the  
series >of the Cosine is multiplied by the other factors.

Tommy,
You might like to try

Normal[Series[Cos[b x],{x,0,5}]]  Exp[-x^2]/(k^2+x^2)

	     2  2    4  4
	    b  x    b  x
	1 - ----- + -----
	      2      24
	-----------------
	    2
	   x    2    2
	  E   (k  + x )


The problem arises since

Series[Cos[b x],{x,0,5}]

	     2  2    4  4
	    b  x    b  x        6
	1 - ----- + ----- + O[x]
	      2      24

is a SeriesData object, not a series:

%//InputForm

	SeriesData[x, 0, {1, 0, -b^2/2, 0, b^4/24}, 0, 6, 1]

When this is multiplied by the other terms, Mathematica , as you  
observe, takes the series for the whole expression - the assumption  
being that you wish to work to 0[x] ^5.
Normal changes SeriesData to an ordinary series.

Allan Hayes
haystack.demon.co.uk

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