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Re: problem with reduce

  • To: mathgroup at smc.vnet.net
  • Subject: [mg100431] Re: problem with reduce
  • From: dh <dh at metrohm.com>
  • Date: Wed, 3 Jun 2009 05:28:15 -0400 (EDT)
  • References: <h00d04$phb$1@smc.vnet.net>


Hi,

  consider the equation: jP == j - C[2]]. As j does not appear anywhere 

else j and jp are completely undetermined.

Now consider: i == 1 + C[1] && N == 1 + C[1] + C[2]. As i and N do not 

appear anywhere else, these equation do not add any info.

Finally: iP == 1 + C[1] + C[2] simply says that ip is an integer >=1.

Therefore, the solution is: ip is an integer >=1 and jP an arbitrary number.

Daniel





olfa wrote:

> Hi mathematica community,

> I have to solve this example

> In[88]:= Reduce[Exists[{C[1], C[2]},

>   Element[{C[1], C[2]}, Integers] && C[1] >= 0 && C[2] >= 0 &&

>    i == 1 + C[1] && N == 1 + C[1] + C[2] && iP == 1 + C[1] + C[2] &&

>    jP == j - C[2]], {iP, jP}, Backsubstitution -> True]

> 

> During evaluation of In[88]:= Reduce::nsmet: This system cannot be

> solved with the methods available to \

> Reduce. >>

> 

> I have observed that when I remove Element[{C[1], C[2]}, Integers] && C

> [1] >= 0 && C[2] >= 0 like that:

> In[89]:= Reduce[

>  Exists[{C[1], C[2]},

>   i == 1 + C[1] && N == 1 + C[1] + C[2] && iP == 1 + C[1] + C[2] &&

>    jP == j - C[2]], {iP, jP}, Backsubstitution -> True]

> 

> it gives me an output which is:

> 

> Out[89]= iP == N && jP == i + j - N

> 

> 

> I need to keep the information that Element[{C[1], C[2]}, Integers] &&

> C[1] >= 0 && C[2] >= 0  but reduce tells me that it cannot solve this

> system. why and how to deal with this problem?

> thank you.

> 




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