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Re: Why Indeterminate?

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On Apr 30, 5:54 am, Themis Matsoukas <tmatsou...@me.com> wrote:
> Consider this expression:
>
> A[a_List, x_] := x (1 - x)  \!\(
> \*UnderoverscriptBox[\(\[Sum]\), \(j = 1\), \(2\)]
> \*FractionBox[\(a[[j]]
> \*SuperscriptBox[\((1 - 2\ x)\), \(j - 1\)]\), \(1 -
>      a[[3]] \((1 - 2  x)\)\)]\)
> a = Range[3];
>
> Evaluation at x=0.5 gives
>
> A[a, 0.5]
>
> Indeterminate
>
> ..but I can get the right answer if I use
>
> A[a, x] /. x -> 0.5
>
> 0.25
>
> What puzzles me is that there is no obvious indeterminacy in the original expression at x=0.5.
>
> Thanks
>
> Themis

Themis,

   The message generated is Power::indet: Indeterminate expression
0.^0 encountered. >>
   A closer look shows that in your sum, when the index j is = 1, you
have the expression (1-2*0.5)^(1-1) = 0^0
this is indeterminate, since we dont know anything about how x and j
are related to eachother to do any sort of L'Hopital calculation to
resolve the problem.
When you run it for arbitrary x, it evaluates (1-2x)^(1-1) as
(1-2x)^0, and evaluates this to 1, a perfectly good assumption for all
(1-2x) =/= 0. This is the source of the error, but its hard to say if
its actually a problem or not without context.
-Stefan Salanski

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Thread

Why Indeterminate? Themis Matsoukas <tmatsoukas@me.com> - 2011-04-30 09:54 +0000
  Re: Why Indeterminate? David Bailey <dave@removedbailey.co.uk> - 2011-05-01 10:20 +0000
  Re: Why Indeterminate? Stefan <wutchamacallit27@gmail.com> - 2011-05-01 10:21 +0000

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