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Avoid this error "NDSolve::deqn: Equation or list of equations expected "?

Ser Man
Posted 8 years ago
POSTED BY: Ser Man
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Ser Man
Ser Man
Posted 8 years ago

Thanks for the update. I take Mathematica can't solve this PDE then.

Thanks!
POSTED BY: Ser Man

Hello,

op = Function[r*D[#, r] + Tan[Phi] D[#, Phi]];
eq = I*Nest[op, CapitalPsi[r, Phi], 3] == 2*CapitalPsi[r, Phi]*r^2/Cos[Phi]^5;
sol = NDSolve[{eq, CapitalPsi[0, Phi] == 1, 
Derivative[1, 0][CapitalPsi][0, Phi] == 0, 
Derivative[2, 0][CapitalPsi][0, Phi] == 10, 
Derivative[3, 0][CapitalPsi][0, Phi] == 0, CapitalPsi[r, 0] == 1, 
Derivative[0, 1][CapitalPsi][r, 0] == 0}, 
CapitalPsi, {r, 0, 10}, {Phi, 0, 10}]

If I Execute this above code MMA give me:

NDSolve::femcmsd: The spatial derivative order of the PDE may not exceed two.

See information about this error Here!

op = Function[r*D[#, r] + Tan[Phi] D[#, Phi]];
eq = I*Nest[op, CapitalPsi[r, Phi], 3] == 2*CapitalPsi[r, Phi]*r^2/Cos[Phi]^5;
sol = NDSolve[{eq, CapitalPsi[0, Phi] == 1, 
Derivative[1, 0][CapitalPsi][0, Phi] == 0, 
Derivative[2, 0][CapitalPsi][0, Phi] == 10, 
Derivative[3, 0][CapitalPsi][0, Phi] == 0, CapitalPsi[r, 0] == 1, 
Derivative[0, 1][CapitalPsi][r, 0] == 0}, 
CapitalPsi, {r, 0, 10}, {Phi, 0, 10},Method -> {"PDEDiscretization" -> {"MethodOfLines", 
 "SpatialDiscretization" -> "FiniteElement"}}]

If I Execute this above code MMA give me:

NDSolve::ivone: Boundary values may only be specified for one independent variable. Initial values may only be specified at one value of the other independent variable.

See information about this error Here!

In summary, you must implement your own code to solve the PDE equation.
POSTED BY: Mariusz Iwaniuk
Anonymous User
Anonymous User
Posted 8 years ago

i'm guessing you are cut and pasting this diff'e'q from somewhere and do not yet have the basic mathematica skills to "run it".

i'm unsure what the Nest does not knowing the variables. i'm not solving the diff'e'q by hand "for fun".

But i do see (variables) op and sol are assigned by =, and once that occurs (we assume a value is obtained) we know values cannot be assigned values. Use ? or ?? to show the current value of a variable.
POSTED BY: Anonymous User
Ser Man
Ser Man
Posted 8 years ago
POSTED BY: Ser Man

The symbol op is not an equation. The equation is the next line:

op = Function[r*D[#, r] + Tan[\[Phi]] D[#, \[Phi]]];
equation = 
  I*Nest[op, \[CapitalPsi][r, \[Phi]], 3] == 
   2 \[CapitalPsi][r, \[Phi]]*r^2/Cos[\[Phi]]^5;
NDSolve[{equation,...
POSTED BY: Gianluca Gorni
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