Re: NDSolve with functions of vectors

*To*: mathgroup at smc.vnet.net*Subject*: [mg82873] Re: NDSolve with functions of vectors*From*: Jens-Peer Kuska <kuska at informatik.uni-leipzig.de>*Date*: Fri, 2 Nov 2007 03:24:59 -0500 (EST)*Organization*: Uni Leipzig*References*: <fgca49$9ho$1@smc.vnet.net>*Reply-to*: kuska at informatik.uni-leipzig.de

Hi, U[x_List] := {2*x[[2]], x[[1]]/3, x[[3]]} X0 = {1, 1, 1}; and sol = NDSolve[{a'[s] == U[a[s]], a[0] == X0}, a[s], {s, 0, 1}] will do it. If yoy wish to use the path length you have to transform a[s]== a[Sqrt[x[s]^2+y[s]^2+z[s]^2]]. Regards Jens ohn Lee wrote: > Hi, > I'm trying to integrate a function with NDSolve (ExplicitRungeKutta method), and it doesn't seem to recognize vector variables (I keep getting an incorrect dimension error). In addition, I would like to integrate over an absolute distance and I don't know how to do that in Mathematica. For simplicity's sake, I will post a random example of what I'm talking about: > U[x] = {2*x[[2]],x[[1]]/3,x[[3]]} is the derivative > X0 = {1,1,1} is the initial condition > so if a'[s] = U[x[s]], I'd like to solve for a[s] where s is the total distance. And s is to be integrated from 0 to 6 with a[0]=X0. > Ultimately, I have a derivative that depends on a 3-component position vector. I want to integrate that derivative over a total distance traveled from a starting point. > > Any advice would be helpful. Thanks, > John >