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