Re: solving equations
- To: mathgroup at smc.vnet.net
- Subject: [mg108555] Re: solving equations
- From: Bill Rowe <readnews at sbcglobal.net>
- Date: Tue, 23 Mar 2010 04:23:04 -0500 (EST)
On 3/22/10 at 2:41 AM, arbiadr at gmail.com (Maria Davis) wrote: >The equations presented in the file are the result of another software, >I know that they contain much redunduncy, but I must resolve them. For >example, the given equations are: >a=Min(c, d) >c=infinity It is hard for me to determine whether you realize the above are not equations in Mathematica or not. As equations, the above would be: a == Min[c,d] c == Infinity >I want mathematica to solve the system above and returns a=d I don't >understand why "Min" can not be reduced, so, please, is there any >solution? Not without more information. The problem is d can be anything including Infinity. Until d has a value that can be compared in a useful manner with Infinity, Mathematica will correctly return Min[d,Infinity] unevaluated as Min[d, Infinity] which is obviously not what you want. I can get Mathematica to do as you want doing the following: In[1]:= eq1 = a == Min[c, d]; c = Infinity; In[3]:= eq1 /. Min[a_, Infinity] :> a Out[3]= a == d Here, I've used Set (=) to set the value of c to Infinity. That way, Mathematica's evaluator will replace all occurrences of c with Infinity. Then I've used a replacement rule to transform Min[d,Infinity] to d. Note, when doing this last, I am no longer necessarily doing valid mathematics. For example, In[4]:= a + 1 /. b_ + _ :> b + 2 Out[4]= a+2 Demonstrating Mathematica will happily replace 1 with 2 using replacement rules even though In[5]:= 1 == 2 Out[5]= False