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MathGroup Archive 2001

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Re:avoiding neg values in NDSolve

  • To: mathgroup at smc.vnet.net
  • Subject: [mg27371] Re:avoiding neg values in NDSolve
  • From: bghiggins at ucdavis.edu (Brian Higgins)
  • Date: Thu, 22 Feb 2001 02:25:16 -0500 (EST)
  • Organization: The Math Forum
  • References: <96t9gv$prt@smc.vnet.net>
  • Sender: owner-wri-mathgroup at wolfram.com

Michael, The problem lies in the statement

ndsolvevec /. {\[Mu][t] -> m (t + tstart)}

When NDSolve tries to integrate the equations it puts a real number
for t in the LHS of your rule, and thus the rule is not being
implemented the way you want. There are several ways you can solve
this problem

(i) Implement the rule before the NDSolve statement 
ndsolvevec = Join[diffeqvec, ivec]/. {\[Mu][t] -> m (t + tstart)}

(ii) Use  a dummy variable for the argument of \Mu[t] in the
definition of your equations, say \Mu[t1], and then use the following
rule in NDSolve

ndsolvevec /. \[Mu][t1] -> m (t + tstart)

Your equations are then readily integrated without the need to specify
all the NDSolve options.

Cheers

Brian 


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