SolveDiffEq 3.6 for Mac could be downloaded from the developer's website when we last checked. We cannot confirm if there is a free download of this app available.
Please check the Mac app with an antivirus before launch as it is downloaded from the developer's website, and we cannot ensure that it is safe. This free Mac app was originally produced by Bob Delaney. The program lies within Education Tools, more precisely Science.
From the developer:
SolveDiffEq is a standalone application which allows you to numerically solve differential equations and evaluate definite integrals of your choosing.
Using the Runge-Kutta method, SolveDiffEq can solve any number of coupled differential equations of the form
dx(0)/dt = f(0)(t, x(0), x(1), . . . )
dx(1)/dt = f(1)(t, x(0), x(1), . . . )
The Help menu shows how to input a script defining the functions f(0), f(1), . . .
You may want to check out more software for Mac, such as Trapcode Particular, AstroGrav or MoonDock, which might be related to SolveDiffEq.