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Question

The 2n vertices of graph G correspond to all subsets a set of size n, for n6. Two vertices of G are adjacent if and only if the corresponding sets intersect in exactly two elements.

The number of vertices of degree zero in G is

A
1
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B
n
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C
n + 1
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D
2n
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Solution

The correct option is C n + 1
Let S contains n elements then S have 2n subsets. Graph G contains 2n vertices.

Let S={v1, v2,...., vn}. Two vertices of G are adjacent if and only if the corresponding sets intersect in exactly two elements.

So |{Vi}{Vj}|=2

For this to happen, the subset must have at least 2 elements.

There are n sets which contains a single elements for V1 to Vn who doesn't intersect another set such that it contains two elements. Therefore the degree of all these n vertices is zero. G also contains a vertex ϕ whose degree is zero. So the number of vertices whose degree is zero is n + 1.

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