# Tagged Questions

Questions on the symbolic (DSolve, DifferentialRoot) and numerical (NDSolve) solutions of differential equations in Mathematica.

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### failure of code with Helmholtz equation with point source

I am new to the finite elements package in Mathematica. I have a system of equations, one of which is a Helmholtz equation with a point source in the interior of a bounded domain. The following is the ...
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### Changing variable in numerical solution

I solved numerically the differential equation: ...
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### Can I use operators of the form $\frac{d^2}{dx^2}+i\frac{d}{dx}$ inside NDEigensystem?

I am currently trying to find the eigenfunctions of Hamiltonians of the form $\hat H=\frac12 \hat p^2 +a\hat p +f(\hat x)$, and I thought I would give NDEigensystem ...
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### Different results from ode45 and NDSolve?

I am solving nonlinear ODE + integration groups using NDSolve of mma and ode45 of matlab. At last, I found that they give different results, even after I added RungeKutta method, interpolationorder, ...
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### Naive question on ODE numerical solving and periodic initial conditions

I have a second-order nonlinear ODE for which I would like to find a numerical solution, with periodic initial (boundary) conditions. Can this be done directly with ...
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### NDSolve for complex variables

I am trying to simulate a multiple pendulum system with damping as follows: ...
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### Error message when running NDSolveValue

I am trying to solve a set of differential equations but keep getting an error. See message below "NDSolveValue::ndnum: Encountered non-numerical value for a derivative at t=0" The system that I ...
127 views

### How to plot an Eddington-Finkelstein diagram

I need to plot the same thing as what is shown on the third picture of the site below (sorry, I'm unable to poste that picture here) : http://ion.uwinnipeg.ca/~vincent/4500.6-001/Cosmology/...
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### NDSolve and differentiation of Abs

I have found several questions about the derivative of Abs and how it is not defined in the complex plane. What I have not found yet is a precise and simple ...
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### Neumann and Dirichlet eigenvalue problem - Eigenfunctions on the square [closed]

I don't know if I am in a good place to ask my question, but I'd like someone to create a visual support for a problem of mathematics. I was wondering what is the fundamental different between a ...
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### Stop Integration in NDSolve

The following code executes properly for some choices of initial points and parameter c, but fails for other choices. Could the failure be due to computation outside of the defined t, x, and y ranges? ...
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### Ion motion in uniform axial magnetic field and radial electric field [closed]

I'm trying to model an ion's 3D motion in a constant/uniform axial magnetic field and a radial electric field (similar to a cylindrical magnetron, but two positive parallel center electrodes offset by ...
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### Evaluating solution from NDSolve at a specific point

sol = NDSolve[{eq, iv}, {f[x, t]}, {x, 0, 1}, {t, 0, 10}]; sol[0,1] I want to evaluate the solution from NDSolve at any given point, but when I do the above, ...
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### Speeding up WhenEvent + NDSolve (Performance tuning and handling instabilities)

I am solving a non-linear partial differential equation using NDSolve. The equation models a liquid film on an isothermal solid substrate. As a result of various fluid physics mechanisms such as ...
131 views

### Bug in NDSolve/WhenEvent?

Bug introduced in v10. I'm fairly sure the following is a bug, and I would normally just report it to WRI. However, this is related to my answer to When using NDsolve, how to determine the ...
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### DSolve non linear DE

probably my problem i´m gonna ask seems ridiculous and quite basic. I want to solve an equation with the following code, where a,b,c are parameters. But surprisingly they don´t appear in the solution. ...
Consider the two below ODEs with following dependent B.C. In the boundary conditions $k$ is an unknown. $$y_1''(x)=x$$ $$y_2''(y)=0$$ $$y_1(1)=y_2(1)$$ $$y_1(-k)=y_2(-k)$$ \frac{\text{dy}_1(1)}{\...