Newbie Question About An Error Message

*To*: mathgroup at smc.vnet.net*Subject*: [mg55443] Newbie Question About An Error Message*From*: Adam Hartshorne <oracle3001 at yahoo.com>*Date*: Wed, 23 Mar 2005 05:34:54 -0500 (EST)*Organization*: University of Warwick, UK*Sender*: owner-wri-mathgroup at wolfram.com

Hi All, I am a real newbie to mathematica and have just encountered the following error message and I have no idea why and what it means. I have written a function to calculate the abs of a vector as follows abs[x_] := Sqrt[x[[1]]^2+x[[2]]^2+x[[3]]^2] then i was doing the following fine until a few moments ago and mathematica starting complaining. I am to my knowledge not asking anything different to what I asked it before, but I don't know enough about mathematica to know much more. Ok then I try D[(abs[{x-xi}])^2,x] Which is know the answer to is 2(x-xi), and mathematica has given me the answer. However now it is saying, maybe I have changed my input and no am typing something wrong. Part::partw: Part 2 of {x - xi} does not exist. Part::partw: Part 3 of {x - xi} does not exist. Any help will be much appreciated, Adam

**Follow-Ups**:**Re: Newbie Question About An Error Message***From:*"Matthias Bode" <lvsaba@hotmail.com>

**Re: Newbie Question About An Error Message***From:*DrBob <drbob@bigfoot.com>

**Re: Newbie Question About An Error Message***From:*Sseziwa Mukasa <mukasa@jeol.com>