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Problems with DSolve

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  Problems with DSolve David King <drdavidbking@gmail.com> - 2011-06-16 08:01 +0000
    Re: Problems with DSolve "Kevin J. McCann" <Kevin.McCann@umbc.edu> - 2011-06-17 04:10 +0000

#3120 — Problems with DSolve

Hello Mathematica Forum,

I am a new forum contributer, so it is nice to meet you.  I was hoping
at least one of you could help me out.  I am attempting to find an
exact solution to a system of differential equations

I  type in the following Dsolve  function:

 sol=DSolve[{x'[t] -(0.355+(0.645t/(t+223.06)))x[t]+0.311y[t]
+0.046z[t] - 0.149x[t] -0.259x[t],y'[t] 0.149x[t] -
0.311y[t],z'[t] 0.259x[t] -0.046z[t],x[0] D,y[0] 0,z[0] 0},
{x[t],y[t],z[t]},t]

So far, Mathematica  returns the following:

Out[7]:=  DSolve[{x=A2[t] -0.408 x[t]+(-0.355-(0.645 t)/(223.06 +t)) x[t]
+0.311 y[t]+0.046 z[t],y=A2[t] 0.149 x[t]-0.311 y[t],z=A2[t] 0.259
x[t]-0.046 z[t],x[0] D,y[0] 0,z[0] 0},{x[t],y[t],z[t]},t]

Which is what my question is.... This is kind of like asking  "Who's on
first?"  and mathematica answers:  "Who's on first."  Which is what my
question is....  I am pretty sure that my syntax is correct.  Anyway,
I was hoping that you might be able to knock this one out of the
park.

I have a numerical solution, just not an exact solution.

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#3146

Your syntax for the differential equations is incorrect. Note that you 
need to have stuff == stuff. Often the second "stuff" is a zero. Also, 
unless it is some copy issue, you initial conditions don't read 
correctly, e.g. x[0] D. I think you mean x[0]==D. However, note that "D" 
is a the name of the derivative function; so, you shouldn't use it. Use 
"d" or some other. Check the Help on DSolve.

Kevin

On 6/16/2011 4:01 AM, David King wrote:
> Hello Mathematica Forum,
>
> I am a new forum contributer, so it is nice to meet you.  I was hoping
> at least one of you could help me out.  I am attempting to find an
> exact solution to a system of differential equations
>
> I  type in the following Dsolve  function:
>
>   sol=DSolve[{x'[t] -(0.355+(0.645t/(t+223.06)))x[t]+0.311y[t]
> +0.046z[t] - 0.149x[t] -0.259x[t],y'[t] 0.149x[t] -
> 0.311y[t],z'[t] 0.259x[t] -0.046z[t],x[0] D,y[0] 0,z[0] 0},
> {x[t],y[t],z[t]},t]
>
> So far, Mathematica  returns the following:
>
> Out[7]:=  DSolve[{x=A2[t] -0.408 x[t]+(-0.355-(0.645 t)/(223.06 +t)) x[t]
> +0.311 y[t]+0.046 z[t],y=A2[t] 0.149 x[t]-0.311 y[t],z=A2[t] 0.259
> x[t]-0.046 z[t],x[0] D,y[0] 0,z[0] 0},{x[t],y[t],z[t]},t]
>
> Which is what my question is.... This is kind of like asking  "Who's on
> first?"  and mathematica answers:  "Who's on first."  Which is what my
> question is....  I am pretty sure that my syntax is correct.  Anyway,
> I was hoping that you might be able to knock this one out of the
> park.
>
> I have a numerical solution, just not an exact solution.
>

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