A question on interval arithmetic

• To: mathgroup at smc.vnet.net
• Subject: [mg43691] A question on interval arithmetic
• From: Oliver Friedrich <oliver.friedrich at tzm.de>
• Date: Tue, 30 Sep 2003 16:42:40 -0400 (EDT)
• Organization: TZ Mikroelektronik
• Sender: owner-wri-mathgroup at wolfram.com

```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.

How can I avoid this problem?

Oliver Friedrich

```

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