Problem with NDSOLVE in Mathematica

[nikolafmf]

Mathematica says:

"NDSolve::alliv: "The function x[t] was specified without dependence on all the independent variables. Each function must depend on all the independent variables.""

But function x depends only on t, there is no other variable specified in the code.

What could be the problem? I attach .nb file below.

 

[Niles]

Hi

Try this:

NDSolve[{
  y'[t] == -y[t], 
  x'[t] == x[t] + y[t],
  x[0] == 0, y[0] == 1},
 {x, y}, {t, 0, 1},
 Method -> "ExplicitMidpoint", "StartingStepSize" -> 1/10]

 

[nikolafmf]

It works. Thank you very much :)

 

[nikolafmf]

But for similar system with four variables Mathematica says this:

"NDSolve has computed initial values that give a zero residual for the \
differential-algebraic system, but some components are different from \
those specified. If you need them to be satisfied, it is recommended \
that you give initial conditions for all dependent variables and \
their derivatives."

What could be the cause of this and how to avoid it?

 

[alphy]

It appears that the problem is related to the way the function x is defined in the code. In order for NDSolve to work properly, all functions must be explicitly dependent on all independent variables. This means that the function x must also have a dependence on another variable, even if it is just a constant.

One solution could be to redefine the function x to include a dependence on a constant, such as x[t_, k_] := t + k. This way, NDSolve will have all the necessary information to solve the equation.

Another potential solution could be to check for any typos or missing information in the code. It is possible that there is another variable that was meant to be included in the function definition, but was accidentally left out.

Overall, it is important to carefully check the code and make sure that all functions are properly defined with dependencies on all independent variables in order for NDSolve to work correctly.