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Re: Error in FullSimplify

  • To: mathgroup at smc.vnet.net
  • Subject: [mg122942] Re: Error in FullSimplify
  • From: "Dr. Wolfgang Hintze" <weh at snafu.de>
  • Date: Thu, 17 Nov 2011 07:02:25 -0500 (EST)
  • Delivered-to: l-mathgroup@mail-archive0.wolfram.com
  • References: <ja2psp$ma5$1@smc.vnet.net>

This can indeed be easily explained. You didn't take apropriate care of 
operator priorities.
In your case the last FullSimplify acts on 4. which is most probably 
not what you had intended.
Hence either put brackets around the whole expression or use the 
function form FullSimplify[]

In[136]:=
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]
Out[136]=
-4*I*Sin[4*x]*Sinh[3*x]


In[139]:=
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.]
Out[139]=
-4*I*Sin[4.*x]*Sinh[3.*x]

Hence it was not an error in FullSimplify but an error in the usage of 
it.

Best regards,
Wolfgang



"Philipp Weißgraeber" <=?ISO-8859-15?Q?Philipp_Wei=DFgraeber?=> schrieb 
im Newsbeitrag news:ja2psp$ma5$1 at smc.vnet.net...
> 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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