Re: Series problem

*To*: mathgroup at smc.vnet.net*Subject*: [mg4133] Re: [mg4084] Series problem*From*: Allan Hayes <hay at haystack.demon.co.uk>*Date*: Fri, 7 Jun 1996 02:06:13 -0400*Sender*: owner-wri-mathgroup at wolfram.com

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 ==== [MESSAGE SEPARATOR] ====