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Re: Problems with DSolve

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  Re: Problems with DSolve DrMajorBob <btreat1@austin.rr.com> - 2011-06-17 04:07 +0000

#3134 — Re: Problems with DSolve

a) This is kind of like asking  "Who's on first?"  and mathematica  
answers:  "Who's on first."

No, it's kind of like entering invalid syntax and getting it back  
unevaluated.

b) That is NOT the output returned for your input, and

c) Evaluating your input gives an error message (in a different window)  
saying:

DSolve::deqn: Equation or list of equations expected instead of -0.408  
x[t]-(0.355 +(0.645 t)/(223.06 +t)) x[t]+0.311 y[t]+0.046  
z[t]+(x^\[Prime])[t]
  in the first argument {-0.408 x[t]-(0.355 +(0.645 t)/(223.06 +t))  
x[t]+0.311 y[t]+0.046 z[t]+(x^\[Prime])[t],<<4>>,0}
. >>

d) As the error message says, you need equations, not just things you want  
to be zero (if that's what they are). This is closer to the right syntax:

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

but the last three items in your "equation" list were x[0] D, y[0] 0, and  
z[0] 0. I can't guess what you intended them to be, so I haven't changed  
them into equations.

Make all the equations into... umm... equations, and you'll have better  
luck.

e) In your listed output (which Mathematica, I guarantee, did NOT return,  
for ANY input), note that

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]

ALSO is not an equation, since = is Set, and == is Equal.

I suspect this is part of your actual input.

Bobby

On Thu, 16 Jun 2011 03:01:05 -0500, David King <drdavidbking@gmail.com>  
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.
>


-- 
DrMajorBob@yahoo.com

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