xppaut

Phase Plane Plus Auto: Solves many kinds of equations
  http://www.math.pitt.edu/~bard/xpp/xpp.html
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XPPAUT is a tool for solving * differential equations, * difference equations, * delay equations, * functional equations, * boundary value problems, and * stochastic equations.

The code brings together a number of useful algorithms and is extremely portable. All the graphics and interface are written completely in Xlib which explains the somewhat idiosyncratic and primitive widgets interface.