NDSolve and Systems of DEs

*To*: mathgroup at smc.vnet.net*Subject*: [mg74177] NDSolve and Systems of DEs*From*: "e-touch\ at libero\.it" <e-touch at libero.it>*Date*: Tue, 13 Mar 2007 04:03:54 -0500 (EST)

Hello everybody! In order to better understand the behaviour of NDSolve, I decided to try to= solve a more simple system of equations...take a look here eq1--d/dt p[x,t]==== k dd/dx2 p[x,t] eq2--d/dt n[x,t]==== k dd/dx2 n[x,t] eq3--d/dx El[x,t]====p[x,t]-n[x,t] with: p[x,0]====10*UnitStep[-x]; p[100,t]====0; p[-100,t]====10; n[x,0]====10*UnitStep[x]; n[100,t]====10; n[-100,t]====0; El[x,0]====0; Eq1 and Eq2 are simple diffusion equations and El[x,t] it's just a function= that I tried to couple with the other two... All I get are error messages... "NDSolve::pdord: Some of the functions have zero differential order so the = \ equations will be solved as a system of differential-algebraic equations." "NDSolve::bcart: Warning: An insufficient number of boundary conditions hav= e \ been specified for the direction of independent variable x. Artificial \ boundary effects may be present in the solution." "NDSolve::icfail: Unable to find initial conditions which satisfy the resid= ual \ function within specified tolerances. Try giving initial conditions for bo= th \ values and derivatives of the functions." The strange thing is that if I derive the funcion El[x,t] respect to time i= nstead of x...so with eq3--d/dt El[x,t]====p[x,t]-n[x,t]; El[x,0]====0; ...it works!!! Could anyone explain this to me??? Thanks everybody Matteo ------------------------------------------------------ Leggi GRATIS le tue mail con il telefonino i-mode=99 di Wind http://i-mode.wind.it13mar07