Series expansion question

*To*: mathgroup at smc.vnet.net*Subject*: [mg122696] Series expansion question*From*: Recif <recif at yahoo.com>*Date*: Sun, 6 Nov 2011 05:56:39 -0500 (EST)*Delivered-to*: l-mathgroup@mail-archive0.wolfram.com

I defined a function: gts[x_, y_] := Normal[Series[g[x, y], {x, x0, 1}, {y, y0, 1}]] and when I use it with 'a' and 'b' undefined, I get the value I expect: gts[a, b] g[x0,y0] + (a-x0) g^(1,0)[x0,y0] + (b-y0) g^(0,1)[x0,y0] ... However when I assign values to a and b: a = x0 + z b = y0 + z gts[a,b] gives me an error: General::ivar: "x0+z is not a valid variable" etc. I would appreciate any help to fix this. Thanks. Recif

**Follow-Ups**:**Re: Series expansion question***From:*Bob Hanlon <hanlonr357@gmail.com>