doughnut_exact

doughnut_exact, a Fortran90 code which evaluates an exact solution of a system of ordinary differential equations (ODE) whose trajectories lie on a torus or doughnut, and for which an exact solution is known.

Licensing:

The information on this web page is distributed under the MIT license.

Languages:

doughnut_exact is available in a C version and a C++ version and a Fortran90 version and a MATLAB version and an Octave version and a Python version.

Related Data and codes:

doughnut_exact_test

doughnut_ode, a Fortran90 code which sets up and solves a system of ordinary differential equations (ODE) whose trajectories lie on a torus or doughnut, and for which an exact solution is known.

f90_exact, a Fortran90 code which evaluates exact solutions to a few selected examples of ordinary differential equations (ODE) and partial differential equations (PDE).

Reference:

1. John D Cook,
   Differential equation on a doughnut,
   https://www.johndcook.com/blog/2025/10/08/diffeq-donut/
   Posted 08 October 2025.
2. Richard Parris,
   A Three-Dimensional System with Knotted Trajectories.
   The American Mathematical Monthly,
   Volume 84, Number 6 , June/July 1977, pages 468-469.

Source Code:

• doughnut_exact.f90, the source code.
• doughnut_exact.sh, compiles the source code.