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 A.Domarkas

 We draw  "Yin&yang".  See
http://en.wikipedia.org/wiki/Yin_and_yang

(%i1) load(draw)$
(%i2) wxdraw2d( proportional_axes=xy,
          x_voxel = 30,
          y_voxel = 30,
          fill_color = grey,
        region(y<=sqrt(-2*x-x^2) and x^2+y^2<=4, x,-2,2, y,-2,2),
        region(y<=-sqrt(2*x-x^2) and x^2+y^2<=4, x,-2,2, y,-2,2),
        color=black,
        implicit(x^2+y^2=4,x,-2,2, y,-2,2));
(%t2)  << Graphics >>
(%o2)

 "Yin&yang" :
(%i3) wxdraw2d(
proportional_axes=xy,
        x_voxel = 30,
        y_voxel = 30,
        fill_color = black,
        region(x>=-sqrt(-2*y-y^2) and x^2+y^2<=4, x,-2,2, y,-2,2),
        region(x>=sqrt(2*y-y^2) and x^2+y^2<=4, x,-2,2, y,-2,2),
        color=black,
        implicit(x^2+y^2=4,x,-2,2, y,-2,2),
        point_type =filled_circle,
        point_size=5,
        points([[0,1]]),
        color=white,
        points([[0,-1]])
);
(%t3)  << Graphics >>
(%o3)

Aleksas D.

 Aleksas D.