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Student Support Forum: 'Bug in Series/SeriesCoefficient' topicStudent Support Forum > General > Archives > "Bug in Series/SeriesCoefficient"

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Tom
07/21/08 10:50am

I am relatively new to Mathematica and relatively bad at mathematics, so this may be all my fault. Still, I've uncovered some mysterious behavior in Series/SeriesCoefficient that seems like a bug to me. Can anyone comment on the problem? I am using Mathematica for Students 6.0.1.0, MacOS X/Intel.

Here is the suspicious behavior. I am trying to find the coefficient for the second-order term in a Taylor expansion of a simple function around 0.5:

In[1]:= SeriesCoefficient[q Log[1 + xi/(x + c)], {x, 0.5, 2}]

Out[1]= 0

In fact all terms of order higher than 2 have a zero coefficient according to Mathematica. This would be great for my problem, but I don't think it's true. Here I try to find the second order term's coefficient by hand:

In[2]:= FullSimplify[1/(2!) D[q Log[1 + xi/(x + c)], {x, 2}] /. x -> 0.5]

Out[2]= (0.5 q xi (1. + 2. c + xi)) / ((0.5 + c)^2 (0.5 + c + xi)^2)

Note that I've modified the output display here: text mode Mathematica won't display properly with the variable-width font that this forum uses.

Anyway, if you evaluate this result for, say, c->1, xi->1, and q->1, you get a nonzero value, which seems to contradict the results from SeriesCoefficient. What's going on?

I've attached a notebook that presents the same example shown above.

Thanks,
--Tom

Attachment: series.nb, URL: ,

Subject (listing for 'Bug in Series/SeriesCoefficient')
Author Date Posted
Bug in Series/SeriesCoefficient Tom 07/21/08 10:50am
Re: Bug in Series/SeriesCoefficient Forum Modera... 07/21/08 12:54pm
Re: Re: Bug in Series/SeriesCoefficient Tom 07/21/08 1:31pm
Re: Re: Bug in Series/SeriesCoefficient Tom 07/21/08 1:33pm
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