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MathGroup Archive 2004

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Re: What does +- mean?

  • To: mathgroup at smc.vnet.net
  • Subject: [mg47581] Re: What does +- mean?
  • From: drbob at bigfoot.com (Bobby R. Treat)
  • Date: Fri, 16 Apr 2004 05:21:33 -0400 (EDT)
  • References: <c5lfbm$bq2$1@smc.vnet.net>
  • Sender: owner-wri-mathgroup at wolfram.com

Look at the output of

Series[Sqrt[-1/2 + x^2], {x, 0, 4}];
% // FullForm

FullForm[SeriesData[x, 0, 
   {Complex[0, 1]*1/Sqrt[2], 0, 
    (Complex[0, -1])*1/Sqrt[2], 0, 
    (Complex[0, Rational[-1, 2]])*
     1/Sqrt[2]}, 0, 5, 1]]

and pick out the second term:

Times[Complex[0, -1], Power[2, Rational[-1, 2]]]
-(I/Sqrt[2])

Notice the Complex number is negative and the Real part is positive,
but "-+" has been resolved to "-" rather than "+-" -- as it should be.

I'd say this is a bug, but it only affects DISPLAY of the Series.

Bobby

Lautaro Vergara <lvergara at lauca.usach.cl> wrote in message news:<c5lfbm$bq2$1 at smc.vnet.net>...
> Hi,
> 
> I have found (v. 5.0) an extrange thing: when one expands, e.g.,
> 
> Series[Sqrt[-1/2+x^2],{x,0,4}]
> 
> there appear a +- sign (which is correctly not present when one changes
> -1/2 by -1)
> 
> is this a bug or a feature?
> 
> Thanks
> 
> L. Vergara


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