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Re: Error in FullSimplify

  • To: mathgroup at smc.vnet.net
  • Subject: [mg122955] Re: Error in FullSimplify
  • From: Bob Hanlon <hanlonr357 at gmail.com>
  • Date: Fri, 18 Nov 2011 06:23:36 -0500 (EST)
  • Delivered-to: l-mathgroup@mail-archive0.wolfram.com
  • References: <201111171104.GAA22782@smc.vnet.net>

This appears to have been fixed in version 8.

$Version

"8.0 for Mac OS X x86 (64-bit) (October 5, 2011)"

expr = (-E^((a + b I) x) + E^((a - b I) x) + E^((-a + b I) x) -
    E^((-a - b I) x));

expr /. {a -> 3, b -> 4} // FullSimplify

-4 I Sin[4 x] Sinh[3 x]

expr /. {a -> 3., b -> 4.} // FullSimplify

2 (Cos[(4. + 3. I) x] - Cosh[(3. + 4. I) x])

% // Rationalize // Simplify

-4 I Sin[4 x] Sinh[3 x]

In both v7 and v8 on a Mac you can simplify this symbolically as follows.

expr // ExpToTrig // Simplify

-4 I Sin[b x] Sinh[a x]


Bob Hanlon


On Thu, Nov 17, 2011 at 6:04 AM,  <"Philipp Wei=DFgraeber"@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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