Imagine three identical infinite cylinders, which center lines are the coordinate axes in 3D. How many corners does their finite intersection region have?
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The coordinates of the corners satisfy the system of equations ⎩ ⎪ ⎨ ⎪ ⎧ x 2 + y 2 = R 2 y 2 + z 2 = R 2 z 2 + x 2 = R 2 , where R is the radius of the cylinders. With simple manipulation the solutions of the system are ⎩ ⎪ ⎨ ⎪ ⎧ x = ± R / 2 y = ± R / 2 z = ± R / 2 . Also, there're 6 corners which are intersections of two elipses. So, there're 6 + 8 = 1 4 corners.