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XPP (XPPAUT is another name; I (Bard Ermentrout) will use the two interchangeably) is a tool for solving 
differential equations, 
difference equations, 
delay equations, 
functional equations, 
boundary value problems, and 
stochastic equations. 
It evolved from a chapter written by John Rinzel and me on the qualitative theory of nerve membranes and eventually became a commercial product for MSDOS computers called PHASEPLANE. It is now available as a program running under X11 and Windows. 
The code brings together a number of useful algorithms and is extremely portable.
                
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