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MathGroup Archive 2000

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Re: Q: NDSolve

  • To: mathgroup at smc.vnet.net
  • Subject: [mg21785] Re: Q: NDSolve
  • From: Paul Abbott <paul at physics.uwa.edu.au>
  • Date: Thu, 27 Jan 2000 22:57:08 -0500 (EST)
  • Organization: University of Western Australia
  • References: <85mmgb$268@smc.vnet.net>
  • Sender: owner-wri-mathgroup at wolfram.com

Christoph Handel wrote:

>  is there a nice way to solve an equation like this:
>
> x'[t] == aMatrix[t] . x[t]
>
> where x ist a Vector.
>
> I tried something like this:
>
> NDSolve[{x'[t]== aMatrix[t] . x[t], x[0]=={1,2,3...}},x,{t,0,42}]

Here is a "reasonably" nice way which makes use of Through and Thread:

For the vector

  x = {a, b, c};

and the matrix

  A[t_] = {{1, 2 t, 7}, {4, 3, 9 - t^2}, {0, 0, 1}};

the set of differential equations can be written as

  eqn = Thread[D[Through[x[t]], t] == A[t] . Through[x[t]]]

Specifying the initial conditions,

  ics = Thread[Through[x[0]] == {1, 0, 2}]

we can now use NDSolve directly:

  NDSolve[Join[eqn, ics], x, {t, 0, 1}]

Cheers,
    Paul



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