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Re: Problems with DSolve

  • To: mathgroup at
  • Subject: [mg119672] Re: Problems with DSolve
  • From: DrMajorBob <btreat1 at>
  • Date: Fri, 17 Jun 2011 00:07:47 -0400 (EDT)
  • References: <>
  • Reply-to: drmajorbob at

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  

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

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

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  
  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  

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.


On Thu, 16 Jun 2011 03:01:05 -0500, David King <drdavidbking at>  

> 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 at

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