Re: Series expansion question

• To: mathgroup at smc.vnet.net
• Subject: [mg122707] Re: Series expansion question
• From: "Dr. Wolfgang Hintze" <weh at snafu.de>
• Date: Mon, 7 Nov 2011 05:51:50 -0500 (EST)
• Delivered-to: l-mathgroup@mail-archive0.wolfram.com
• References: <j95pf3\$m7v\$1@smc.vnet.net>

```"Recif" <recif at yahoo.com> schrieb im Newsbeitrag
news:j95pf3\$m7v\$1 at smc.vnet.net...
>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
>

You should use replacements instead of assignments:

gts[a, b] /. {a -> x0 + z, b -> y0 + z}
Out[88]=
g[x0, y0] + z*Derivative[1, 0][g][x0, y0] +
z*(Derivative[0, 1][g][x0, y0] +
z*Derivative[1, 1][g][x0, y0])

Wolfgang

```

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