Concave polygon a convex polygon. As with any simple polygon, the sum of the internal angles of a concave polygon is ("n" − 2) radians, equivalently 180°×("n" − 2), where "n" is the number of sides. It is always possible to partition a concave polygon into a set of convex polygons. A polynomial-time algorithm for finding a decomposition into as few convex polygons as possible is described by . A triangle can never be concave, but there exist concave polygons with "n" sides for any "n" > 3. An example of a concave quadrilateral is the dart. At least one interior angle
Luneburg lens one side, and formula_14 only on the other: The parameter formula_19 is a constant for any given ray, but differs between rays passing at different distances from the centre of the lens. For rays passing through the centre, it is zero. In some special cases, such as for Maxwell's fish-eye, this first order equation can be further integrated to give a formula for formula_14 as a function or formula_13. In general it provides the relative rates of change of formula_14 and formula_13, which may be integrated numerically to follow the path of the ray through the lens. Luneburg lens A