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MathGroup Archive 2003

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Re: A question on interval arithmetic

  • To: mathgroup at
  • Subject: [mg43734] Re: A question on interval arithmetic
  • From: Olaf Rogalsky <olaf.rogalsky at>
  • Date: Thu, 2 Oct 2003 02:51:45 -0400 (EDT)
  • References: <blcq5l$p5f$>
  • Sender: owner-wri-mathgroup at

Oliver Friedrich wrote:
> Hallo,
> the resistance of 2 resistors in parallel is r1*r2/(r1+r2). Now I want to
> introduce tolerances in the resistors and ask for the range of resistance
> of the combination. One may think that e.g
> (r1*r2)/(r1+r2)/.{r1->Interval[{10,20}],r2->Interval[{20,40}]}
> would lead to the correct result, but there's a trap. If I replace the
> expressions by the intervals, Mathematica evaluates the new expression assuming
> that all four intervals are independant from each other. And that's not
> correct. Taken either the minimum or the maximum from a certain interval ,
> Mathematica should stick to that, because it is nonsense to take Min[r1] and Max
> [r1] within the same expression, r1 can have only one value at a time.

In[1]:= r1*r2/(r1+r2) /. {r1 -> Interval[{min1,max1}], r2 -> Interval[{min2,max2}]}
In[2]:= FullSimplify[%]
In[3]:= % /. {min1 -> 10, max1 -> 20, min2 -> 20, max2 -> 40}
Out[4]:= Interval[{20/3, 40/3}]
I Dr. Olaf Rogalsky                         Institut f. Theo. Physik I I
I Tel.: 09131 8528440                       Univ. Erlangen-Nuernberg   I
I Fax.: 09131 8528444                       Staudtstrasse 7 B3         I
I rogalsky at  D-91058 Erlangen           I

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