Re: Apparently easy ODE

*To*: mathgroup at smc.vnet.net*Subject*: [mg22940] Re: Apparently easy ODE*From*: Jens-Peer Kuska <kuska at informatik.uni-leipzig.de>*Date*: Fri, 7 Apr 2000 02:54:26 -0400 (EDT)*Organization*: Universitaet Leipzig*References*: <8chb0j$9af@smc.vnet.net>*Sender*: owner-wri-mathgroup at wolfram.com

Hi, your problem is not correct a) for e[x] you have a *first* order equation -- you can't give a inital value for e'[0] ! you must supply a inital value for e[0] b) for p[x] you try top solve a boundary and for e[x] an initial value problem. NDSolve[] can't do this c) you can try to implement a shooting or multiple shooting method but you system is incredible stiff Regards Jens Kurt Taretto wrote: > > Hi, I'm having some problems solving PDE's, for example the folowing > notebook > > Clear["Global`*"]; > > (* constants definitions *) > > Es =11.8*8.85418`50*10^-14; > q = 1.60218`50*10^-19; > > Nd = 1.0`50*^16; g = 1.0`50*^-4; G = 1.0`50*^20; u= 500.0`50; DD = > 25.0`50; > k1 = q/Es; > > solution = NDSolve[{e'[x] == k1 p[x], > > G - u e'[x] p[x] - u e[x] p'[x] + DD p''[x] == 0, > > e'[0] == 0, p[g] == 1.0`50*^10, p'[0] == 0}, > > {e, p}, {x, 0, g}, WorkingPrecision -> 20]; > > Plot[p[x] /. solution, {x, g/100, g}, PlotRange -> All]; > > causes an error message, "Cannot find starting value for the variable > x.", and obviously no solution is given. Apparently this error message > is about the internals of the algorithm, but I can't figure out what I'm > doing wrong. Any help on this would be appreciated. > > Thanks! > > Kurt Taretto