Re: Problems integrating InterpolatingFunction

*To*: mathgroup at smc.vnet.net*Subject*: [mg116074] Re: Problems integrating InterpolatingFunction*From*: DrMajorBob <btreat1 at austin.rr.com>*Date*: Tue, 1 Feb 2011 06:52:19 -0500 (EST)

Oops, I had fixed it, but sent a bad version anyway! This works: Clear[x, y, t] x[t_] = x[t] /. First@NDSolve[{x''[t] == -x[t]^3 + x[t], x'[0] == 0, x[0] == 1.42}, x[t], {t, 0, 100}]; y[t_] = Derivative[-1][x][t]; Plot[{x@t, y@t}, {t, 0, 100}] Bobby On Mon, 31 Jan 2011 13:08:24 -0600, Roland Franzius <roland.franzius at uos.de> wrote: > Am 31.01.2011 19:42, schrieb DrMajorBob: >> You wrote Y as its own integral, which causes $RecursionLimit errors. >> >> You probably meant something like >> >> Clear[x, y] >> x[t_] = x[t] /. >> NDSolve[{x''[t] == -x[t]^3 + x[t], x'[0] == 0, x[0] == 1.42}, >> x[t], {t, 0, 100}] // First >> y[t_] = Derivative[-1][y][t] >> >> Bobby > > which is not really a better approach :-( > > > Roland > >> On Mon, 31 Jan 2011 02:23:31 -0600, Roland Franzius >> <roland.franzius at uos.de> wrote: >> >>> Am 30.01.2011 01:43, schrieb Sergio Miguel Terrazas Porras: >>>> Hello group, >>>> >>>> I use NDSolve for a nonlinear differential equation, and I get an >>>> InterpolatingFunction, as expected. >>>> >>>> I can plot it, evaluate it, etc. >>>> >>>> The problem I have is that now I need to integrate 1/(the square of >>>> the InterpolatingFunction), and I get nothing but the input back. >>>> >>>> Any ideas? >>>> >>>> Thanks in advance. >>>> >>>> Sergio Terrazas >>>> >>> >>> Yoy can integrate InterpolatingFunction explitely using Derivative of >>> negative order >>> >>> eg the critical soltion to the equations of motion of an anharmonic >>> pendulum >>> >>> X[t_] = x[t] /. >>> NDSolve[{x''[t] == -x[t]^3 + x[t], x'[0] == 0, x[0] == 1.42}, >>> x[t], {t, 0, 100}] // First >>> >>> Y[t_]=Derivative[-1][Y][t] >>> >> >> > -- DrMajorBob at yahoo.com