The minimum number of colours that is sufficient to vertex-colour any planar graph is_______. 

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Concept:

A planar graph is a graph in which no two edges cross each other. A vertex coloring of a graph is an assignment of colors to the vertices of graph such that adjacent vertices have different colors.

Explanation:

According to the property of planar graph and four colour theorems. Maximum number of colours that are needed to vertex-colour any planar graph is 4.

Example:

In this graph, only 3 colours are enough to colour this planar graph.

F1 R.S Pallavi 23.12.19 D1

But if in this graph, there is an edge from a to d also, then three colours are not enough to vertex-colour this graph.

F1 R.S Pallavi 23.12.19 D2

In this graph, 4 colours are needed to vertex- colour this graph.

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