%swe.jura.ch/lcp/cours/sp/mathematica.pdf 

input mp-solid

%16''
figureespace(-10u,-20u,10u,10u);
Initialisation(1500,60,30,5);
incolor:=gris;
arcenciel:=true;
vardef f(expr x)=(-x*cos(x),x*sin(x),-3x)
enddef;

vardef g(expr x)=(6/7)*x
enddef;

vardef fp(expr x)=(-cos(x)+x*sin(x),sin(x)+x*cos(x),-3)/Norm((-cos(x)+x*sin(x),sin(x)+x*cos(x),-3))
enddef;

vardef fd(expr x)=(2*sin(x)+x*cos(x),2*cos(x)-x*sin(x),0)/Norm((2*sin(x)+x*cos(x),2*cos(x)-x*sin(x),0))
enddef;

vardef b(expr x)=fp(x) Vectprod fd(x)
enddef;


draw Sparam("f(u)+g(u)*(cos(v)*fd(u)+sin(v)*b(u))",0,9*pi/2,pi/12,0,2*pi,pi/18);
finespace;
end