The 2n vertices of graph G correspond to all subsets a set of size n, for n≥6. 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 V1toVn 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.