PN triangle division

Hey guys
I am a beginner to OpenGL. now I am involved in a project. which require us to use a PN triangle to divide a cube
to be more precise. given a cube(obj file). we need to divide each triangle(two triangles per face of the cube) into more smaller triangles according to different number indicated by user
here is a pic of the division [ATTACH=CONFIG]759[/ATTACH]
but when when I generate the result according to the steps of PN triangle(it has 6 control points.) the result division of a triangle is not the one I want. who can give me a hint . how to do the PN triangle so the division is the same as the pic showed here
thank you very much