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size(200); import graph3; currentprojection=perspective(5,4,4); real R=3; real a=1; /* import solids; revolution torus=revolution(reverse(Circle(R*X,a,Y,32)),Z,90,345); surface s=surface(torus); */ triple f(pair t) { return ((R+a*cos(t.y))*cos(t.x),(R+a*cos(t.y))*sin(t.x),a*sin(t.y)); } surface s=surface(f,(radians(90),0),(radians(345),2pi),8,8,Spline); draw(s,green);