It helps for visualization. Yes, I made first mistakes with matrix multiplication and i corrected since.

My wish is to have a function like f0 rolling around f0 (C2 rolling around C1 in the graphic), thus the frenet system. My second curvature, to calculate, is like f0 but in frenet system of the initial f0, name it C2 in the graphic, same equation than C1, but in C2 system (frenet system).

Then I want to do the same a third time, having C3 (same curvature as always but in C2 frenet system now). And I want to have C3 formal equation.

I tried the code you propose and get a point going along C1, which means the Frenet system is ok. But I don't get visualization of C2, and the system complains about some values.

So much thanks for helping the poor lad I am,