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MathGroup Archive 2007

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Re: piecewise functions from logical relationships (ie. solving with constraints)

  • To: mathgroup at smc.vnet.net
  • Subject: [mg81296] Re: piecewise functions from logical relationships (ie. solving with constraints)
  • From: Benjamin.R.Lewis at gmail.com
  • Date: Tue, 18 Sep 2007 05:57:57 -0400 (EDT)
  • References: <fclb12$fdk$1@smc.vnet.net><fcnk67$rnl$1@smc.vnet.net>

> I dunno how you made Reduce give you that answer (I didn't try very
> long), but, since you have it:
>
> In[1]:= (x==2/5+y&&-2/5<y<1/2)||(x==7/5+y&&-1/2<y<-2/5)/.{Or->List,And[Equal[l_,r_],cond_]:>(l/;cond->r)}
>
> Out[1]= {x/;-(2/5)<y<1/2->2/5+y,x/;-(1/2)<y<-(2/5)->7/5+y}

I'm reluctant to try to "parse" the expression myself because (since
my package won't know the equations and constraints in advance) I
don't know what guarantees exist on the form of the output.

With respect, but as an example of how easily I might overlook some
further possibility: your solution requires a particular ordering of
terms (which appears to be version dependent at best), that the
constraint is non-empty, and that the condition on an individual part
never contains Or.

I would have thought this would be a fairly common problem; can't
Mathematically automatically re-express these somehow?



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