NDSolve in matrix form

• To: mathgroup at smc.vnet.net
• Subject: [mg120460] NDSolve in matrix form
• From: gcarlson <gcarlson at xannah.org>
• Date: Sun, 24 Jul 2011 03:19:03 -0400 (EDT)
• Delivered-to: l-mathgroup@mail-archive0.wolfram.com

```I am trying to numerically solve systems of ODEs using Mathematica 7.

The desired accuracy that I am striving for depends on the number of
solution functions so I want to structure my solution algorithm to
allow different numbers of solution functions. Solving the problem as
a system of ODEs in matrix form seems the logical choice:

dX/dt = f[X[t]],

where f[X[t]] is a specified function, and X[t] is a vector of
solution functions {x1[t],x2[t],...}. The initial conditions are
specified by a vector X[0] = {x1[0],x2[0],...}.

In my problem, the number of solution functions will usually vary from
3 to 7, but could be any number.

One approach that doesn't work is:

f := {f[[1]][t], f[[2]][t], f[[3]][t]}
fInit := {0, 0, 2}
mat := {(-3*f[[1]][t] - f[[2]][t]), 26.5*f[[1]][t] - f[[2]][t] -
f[[1]][t]*f[[3]][t], f[[1]][t]*f[[2]][t] - f[[3]][t]}
solution =  NDSolve[{Derivative[1][f][t] == mat, f[0] == fInit},
f, {t, 0, 17}]

I don't know if the problem is in the way f is specified or in my
expression for NDSolve.

I'd appreciate any assistance.

Thanks and regards,

Glenn

```

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