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MathGroup Archive 2000

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multiple subscripted variables in NDSolve; dsfun

  • To: mathgroup at smc.vnet.net
  • Subject: [mg21979] multiple subscripted variables in NDSolve; dsfun
  • From: "stephen e. schwartz" <ses at bnl.gov>
  • Date: Mon, 7 Feb 2000 13:02:31 -0500 (EST)
  • Sender: owner-wri-mathgroup at wolfram.com

Hello All, this is my first question to this newsgroup (at least in its
present incarnation), so please forgive me if I violate any conventions or
am asking a faq, but I have made what I think is a reasonably diligent
search.

I am attempting to use NDSolve (Mathematica 4.0.1.0, mac) to solve a set of coupled
ODEs in the time variable t.  Initially I had a set of variables a[i][t],
which solve just fine.  The index [i] ranges {1, nc}

Then I wanted to add another set of variables mn[i][t].  Attempts to solve
the set (it is still a set of ODEs) brings the response:

NDSolve::dsfun: "{a[1][t], mn[1][t]} cannot be used as a function"

I had defined the argument of NDSolve as a table,

NDSolve[{eq, ic}, Table[{a[i][t], mn[i][t]}, {i, nc}] where eq, ic are
tables of equations and initial conditions, resp.

If I "fool" Mathematica by renaming the variable mn[i] as a[i + nc] and suitably
modify my equations and initial conditions, and the range if the index, the
set solves just fine, although I am very unhappy about the notation in that
a and mn represent very different variables.

So, am I correct in inferring that the compact notation for a set of
coupled indexed equations (Mathematica book, 4th ed, sec 3.9.7, page 926) works
only for a single variable name?

Is there a more elegant fix?

By the way, I am kind of surprised that a search for dsfun on
http://smc.vnet.net/mathgroupsearch.html
didnt yield any hits.  Was I looking in the wrong place?

My esteem to you all, and thanks in advance; a copy of any response to me,
ses at bnl.gov, would be greatly appreciated.

-steve




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               PLEASE NOTE NEW AREA CODE:  516 --> 631

Stephen E. Schwartz, Senior Scientist                mailto:ses at bnl.gov
Environmental Chemistry Division                  Phone: (631) 344-3100
Brookhaven National Laboratory                    Fax:   (631) 344-2887
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         Home Page: http://www.ecd.bnl.gov/steve/schwartz.html
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