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MathGroup Archive 2006

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Re: Beginner--[Please Help]How to get coefficient list issued from a Series Command

  • To: mathgroup at smc.vnet.net
  • Subject: [mg70173] Re: Beginner--[Please Help]How to get coefficient list issued from a Series Command
  • From: dimmechan at yahoo.com
  • Date: Fri, 6 Oct 2006 01:58:53 -0400 (EDT)
  • References: <eg2e8k$73m$1@smc.vnet.net>

Hello.

What you need is very easy. But in order to do programming I need to
modify a little your K list.
For example the first element of the list, a1+b1 x +c1 x^2  was
modified to a[1]+b[1]x+c[1]x^2 which
apart for issues of programming is the proper one since as said you
deal with power series.

First define the list

lst = (a[#1] + b[#1]*x + c[#1]*x^2 & ) /@ Range[3]
{a[1] + x*b[1] + x^2*c[1], a[2] + x*b[2] + x^2*c[2], a[3] + x*b[3] +
x^2*c[3]}

Then use Cases

Cases[lst, a[_], Infinity]
{a[1], a[2], a[3]}
Cases[lst, b[_], Infinity]
{b[1], b[2], b[3]}
Cases[lst, c[_], Infinity]
{c[1], c[2], c[3]}

and here is the desired user-defined function

selectCoeff[lis_List, x_] := Cases[lis, x[_], Infinity]

(*check*)

selectCoeff[lst, a]
{a[1], a[2], a[3]}

If you insist on taking the result as {a1,a2,a3} (bad for you...) type

% /. {a[1] -> a1, a[2] -> a2, a[3] -> a3}
{a1, a2, a3}

Regards
Dimitris


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