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MathGroup Archive 2000

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Re: multiple subscripted variables in NDSolve; dsfun

  • To: mathgroup at smc.vnet.net
  • Subject: [mg22009] Re: [mg21979] multiple subscripted variables in NDSolve; dsfun
  • From: "stephen e. schwartz" <ses at bnl.gov>
  • Date: Thu, 10 Feb 2000 02:25:52 -0500 (EST)
  • References: <200002071802.NAA08387@smc.vnet.net>
  • Sender: owner-wri-mathgroup at wolfram.com

Hello Hartmut, you correctly identified the problem.  Thank you!  I
flattened the tables in my lists of initial conditiions, equations, and
arguments of NDSolve and voila.

My faith is restored.

-steve


At 10:00 AM +0100 2/8/00, Hartmut Wolf wrote:
>stephen e. schwartz schrieb:
>>
>
>> 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?
>>
>
>Dear Stephen,
>
>if you rework the example in §3.5.10 of The Mathematica Book
>
>In[1]:= DSolve[{y[x] == -z'[x], z[x] == -y'[x]}, {y, z}, x]
>
>to your manner of denoting the variables
>
>In[2]:=
>DSolve[{a[1][t] == -mn[1]'[t], mn[1][t] == -a[1]'[t]}, {a[1], mn[1]}, t]
>
>you'll see it isn't that. Instead you introduced Table
>
>In[3]:=
>DSolve[{a[1][t] == -mn[1]'[t], mn[1][t] == -a[1]'[t]},
>  Table[{a[i], mn[i]}, {i, 1}], t]
>
>and it stops working  -- because now you no longer have a flat list of
>variables.
>So
>
>In[4]:=
>DSolve[{a[1][t] == -mn[1]'[t], mn[1][t] == -a[1]'[t]},
>  Flatten[Table[{a[i], mn[i]}, {i, 1}]], t]
>
>
>Kind regards, Hartmut



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Stephen E. Schwartz, Senior Scientist                mailto:ses at bnl.gov
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