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G is a complete graph with V-vertices and E-edges. Then which of the following is false ?

  • A.G is V-colourable
  • B.\(E = \frac{V(V - 1)}{2}\)
  • C.G cannot be planar for any value of V and E
  • D.G is always connected

Answer: C

The false statement is G cannot be planar for any value of V and E.

The complete graphs \(K_{1}\) to \(K_{4}\) are all planar: \(K_{4}\) can be drawn as a triangle with a vertex in the middle joined to the three corners, with no edges crossing. Only from \(K_{5}\) onwards is a complete graph non-planar, by Kuratowski's theorem, so the statement fails for small V.

The other statements are true. A complete graph on V vertices needs exactly V colours, since every vertex is adjacent to every other, so it is V-colourable. Each pair of vertices is joined by one edge, giving \(E = V(V-1)/2\). And every vertex is adjacent to every other, so the graph is always connected.

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