DSolve

• To: mathgroup at smc.vnet.net
• Subject: [mg126943] DSolve
• 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

```

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