Re: Re: Simplifying constants...bug?
- To: mathgroup at smc.vnet.net
- Subject: [mg18619] Re: [mg18559] Re: [mg18489] Simplifying constants...bug?
- From: "Wolf, Hartmut" <hwolf at debis.com>
- Date: Tue, 13 Jul 1999 01:01:37 -0400
- Organization: debis Systemhaus
- References: <7m3l22$shp@smc.vnet.net> <199907100618.CAA03008@smc.vnet.net.>
- Sender: owner-wri-mathgroup at wolfram.com
Hello Alan
calvitti at boes.ces.cwru.edu schrieb:
>
> here's some intersting behavior in 3.0:
>
> (a + c*d)/(b + c*d) //. c*d -> -z
>
> gives (as expected):
>
> (a - z)/(b - z)
>
> yet
>
> (a + c*d)/(c*d) //. c*d -> -z
>
> no longer simplifes the denominator:
>
> (a+b-z)/(c*d)
>
> anyone know why?
>
Well Alan, this is a very common problem, when using Mathematica.
What you input (and think) is not always what you get. To see that (and
to get used thinking in Mathematica structures, I recommend to often) look at
the expression with FullForm:
In[3]:= (a + c*d)/(c*d) //FullForm
Out[3]//FullForm=
Times[Power[c,-1],Power[d,-1],Plus[a,Times[c,d]]]
So Times[c,d] can't match the 'denominator' (there is no division in
Mathematica!). Knowing that you can try:
In[4]:= (a + c*d)/(c*d) //. {c*d -> -z, 1/(c*d)->-1/z}
admitted ...that's ugly. See, the following also won't work :
In[10]:= a+(c*d)^5 /. c*d-> -z
Out[10]= a + c^5*d^5
But that will help you a little bit further:
In[18]:= (a + c*d)/(c*d) //. c^n_. * d^n_. :> (-z)^n
Out[18]= -((a - z)/z)
In[19]:= a + (c*d)^5 /. c^n_. * d^n_. :> (-z)^n
Out[19]= a - z^5
---regards, hw
- References:
- Re: Simplifying constants...bug?
- From: calvitti@boes.ces.cwru.edu
- Re: Simplifying constants...bug?