Services & ResourcesWolfram ForumsMathGroup Archive

[Date Index] [Thread Index] [Author Index]

DSolve

• To: mathgroup at smc.vnet.net
• Subject: [mg126943] DSolve
• From: Eirik Larsen Følstad <eirik.folstad at q2s.ntnu.no>
• Date: Tue, 19 Jun 2012 03:14:52 -0400 (EDT)
• Delivered-to: l-mathgroup@mail-archive0.wolfram.com

Hi,

I'm trying solve differential equations with DSolve, but get an output that does no fulfill the initial conditions.

Here is the setup running with Mathematica 8.0.1.0 for Students on a Mac OS 10.6.8;


mypara = {lk -> 1/1998, mk -> 1/2, li -> 1/1998, mi -> 1/2};

mymatrix =
 Transpose[{{-li - lk, lk, 0, li}, {mk, -li - mk, li, 0}, {0, 0, 0,
     0}, {mi, 0, lk, -lk - mi}}] //. mypara;


With numeric initial values, I get a result as expected;

res = DSolve[{Inner[Equal,
     N[mymatrix].{u[t], x[t], y[t], z[t]}, {D[u[t], t], D[x[t], t],
      D[y[t], t], D[z[t], t]}, List], u[0] == 1, x[0] == 0, y[0] == 0,
     z[0] == 0 }, {u[t], x[t], y[t], z[t]}, t] // Simplify ;

res //. t -> 0

gives;
{{u[0] -> 1., x[0] -> 0., y[0] -> 2.22045*10^-16, z[0] -> 2.1684*10^-19}}


While with symbolic initial values, I get a result that is not as expected;

res2 = DSolve[{Inner[Equal,
     N[mymatrix].{u[t], x[t], y[t], z[t]}, {D[u[t], t], D[x[t], t],
      D[y[t], t], D[z[t], t]}, List], u[0] == a, x[0] == b, y[0] == c,
     z[0] == d }, {u[t], x[t], y[t], z[t]}, t] // Simplify ;

res2 //. {a -> 1, b -> 0, c -> 0, d -> 0, t -> 0}

gives;
{{u[0] -> 1., x[0] -> -3.37404*10^-16, y[0] -> -3.46945*10^-18, z[0] -> -1.}}

Where the result is not according to the initial result.

What am I doing wrong?


Best regards,
Eirik

• Follow-Ups:
  • Re: DSolve
    • From: Christoph Lhotka <christoph.lhotka@fundp.ac.be>