       Re: Problems with DSolve

• To: mathgroup at smc.vnet.net
• Subject: [mg119684] Re: Problems with DSolve
• From: "Kevin J. McCann" <Kevin.McCann at umbc.edu>
• Date: Fri, 17 Jun 2011 00:09:56 -0400 (EDT)
• References: <itcd8j\$cuk\$1@smc.vnet.net>

```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 D. I think you mean x==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 D,y 0,z 0},
> {x[t],y[t],z[t]},t]
>
> So far, Mathematica  returns the following:
>
> Out:=  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 D,y 0,z 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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