Simplifying a second order eq. system tests=PRIORITY_NO_NAME version=2.55

*To*: mathgroup at smc.vnet.net*Subject*: [mg45928] Simplifying a second order eq. system tests=PRIORITY_NO_NAME version=2.55*From*: "Gunnar Lindenblatt" <Gunnar.Lindenblatt at pobox.com>*Date*: Fri, 30 Jan 2004 04:15:54 -0500 (EST)*Sender*: owner-wri-mathgroup at wolfram.com

Hi, I just switched from another system to Mathematica 5. While all "complicated" operations (like importing Wave-Files, filtering, parameter fitting, plotting) just work fine, I still have problems with the "basics" like solving or even simplifying equations: For example, to get the telegraph equation by self-induction and capacitive coupling: (One can solve this problem on the space of a postage stamp...) In[1] := Remove["Global`*"] In[2] := myEqn1 = -Dt[u,x] == r i + l Dt[i,t] In[3] := myEqn2 = -Dt[i,x] == s u + c Dt[u,t] Direct approach: Using "Solve" In[4] := Solve[{myEqn1,myEqn2}, {Dt[u,{x,2}]}] results an empty set of solutions: {{}} Second try: Using "Reduce" In[5] := Reduce[{myEqn1,myEqn2}, {Dt[u,{x,2}]}] results: Reduce::nsmet: This system cannot be solved with the methods available to Reduce. Third try: Using "Eliminate" In[6] := Eliminate[{myEqn1,myEqn2}, {Dt[i,x],Dt[i,t]}] results: True That's fine! However, it does not really help me... (By the way, the result should be: Dt[u,{x,2}]== r s u + (r c + l s) Dt[u,t] + l c Dt[u,{t,2}]) Any ideas? -- Perhaps this problem is too simple for Mathematica, so it rejects any help ;-) - Gunnar -- Gunnar Lindenblatt e-mail: Gunnar.Lindenblatt at pobox.com

**Follow-Ups**:**Re: Simplifying a second order eq. system***From:*Daniel Lichtblau <danl@wolfram.com>

**Re: Simplifying a second order eq. system tests=PRIORITY_NO_NAME version=2.55***From:*Andrzej Kozlowski <akoz@mimuw.edu.pl>