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Re: Error in FullSimplify

  • To: mathgroup at smc.vnet.net
  • Subject: [mg122943] Re: Error in FullSimplify
  • From: Andrzej Kozlowski <akoz at mimuw.edu.pl>
  • Date: Thu, 17 Nov 2011 07:02:36 -0500 (EST)
  • Delivered-to: l-mathgroup@mail-archive0.wolfram.com
  • References: <201111171104.GAA22782@smc.vnet.net>

1. The error is no longer present in Mathematica 8:

In[17]:= FullSimplify[Rationalize[FullSimplify[-E^(x (a+I b))+E^(x (a-I b))+E^(x (-a+I b))-E^(x (-a-I b))/. a->3./. b->4.]]]

Out[17]= -4 I sin(4 x) sinh(3 x)

2. FullSimplify depends on such things as exact cancellations, so using it with approximate numbers is not a good idea. However, it has to be admitted that Mathematica is getting much better at dealing with such situations, and it is getting hard to find examples to illustrate the pitfalls.

Andrzej Kozlowski


On 17 Nov 2011, at 12:04, Philipp Wei=DFgraeber at smc.vnet.net wrote:

> Hello
>
> I have experienced some strange error in the FullSimplify-function in
> Mathematica 7.
>
> Input:
> (-E^((a+b I)x)+E^((a-b I)x)+E^((-a+b I)x)-E^((-a-b I)x))/.a->3 /.b->4 //
> FullSimplify
> (-E^((a+b I)x)+E^((a-b I)x)+E^((-a+b I)x)-E^((-a-b I)x))/.a->3. /.b->4.
> // FullSimplify
>
> Output:
> -4 I Sin[4 x] Sinh[3 x]
> -2 I Sin[4. x] Sinh[3. x]
>
> And the later result is wrong.
>
> What happens here, how can this be explained?
>
> CU
>
> Philipp
>




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