# Outer Planer Graph

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**OUTER PLANAR GRAPHS**: A planar graph is said to be outer planar if i(G) = 0. For example, cycles, trees, K4 – x.

**(a) Maximal outer planar graph**: An outer planar graph G is maximal outer planar if no edge can be added without losing outer planarity.

**For example,**

**(b) Minimally non-outer planar graphs**: A planar graph G is said to be minimally non outer planar if i(G) = 1

**CROSSING NUMBER**: The crossing number C(G) of a graph G is the minimum number of crossing of its edges among all drawings of G in the plane. A graph is planar if and only if C(G) = 0. Since K_{4} is planar C(K_{4}) = 0 for p ≤ 4. On the other hand C(K_{5}) = 1. Also K_{3}, 3 is non planar and can be drawn with one crossing.