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+4 votes

Let $S$ be a set of $n$ elements $\left\{1, 2,....., n\right\}$ and $G$ a graph with 2$^{n}$ vertices, each vertex corresponding to a distinct subset of $S$. Two vertices are adjacent iff the symmetric difference of the corresponding sets has exactly $2$ elements. Note: The symmetric difference of two sets $R_{1}$ and  $R_{2}$ is defined as $\left ({R_{1}}\setminus{R_{2}} \right)$ $\cup $ $\left ({R_{2}}\setminus{R_{1}} \right)$

  1. Every vertex in $G$ has the same degree. What is the degree of a vertex in $G$?
  2. How many connected components does $G$ have?
asked in Set Theory & Algebra by Veteran (56k points)  
edited by | 295 views

2 Answers

+3 votes
Best way to solve this for GATE is to take $n=2$ and $n=3$ and we get degree of each vertex = ${}^nC_2$ and no. of connected components = $2$.

Lets do it more formally.

It is clear {} should get connected to all $2$ element subsets (and not to any other) of $S$. So, degree of the corresponding vertex is ${}^nC_2$ as we have ${}^nC_2$ ways of choosing 2 elements from $n$. So, answer to first part must be this as it is given degree of all vertices are same.

Now, for the second part, from the definition of $G$ all the vertices of cardinality $k$ will be disconnected from all the vertices of cardinality $k-1$. This is because either all the $k-1$ elements must be same in both or $k-2$ elements must be same in both or else the symmetric difference will be more than $2$. Now if $k-1$ elements are same, symmetric difference will just be $1$. If $k-2$ elements are same, we have one element in one set not in other and $2$ elements in other set not in this, making symmetric difference $3$. Thus symmetric difference won't be $2$ for any vertices of adjacent cardinality  making them disconnected.

All the vertices of same cardinality will be connected - when just one element differs. Also, vertices with cardinality difference 2 will be connected- when 2 new elements are in one vertex. Thus we will be getting $2$ connected components in total.

answered by Veteran (272k points)  
@arjun sir.. i am not being able to understand this. what is the better way to understand this? or which resourse i should refer to learn this question topic?
+2 votes

set {1,2,3} 

power set { {}, 1 ,2,3, {1,2}, {2,3} ,{1,3}, {1,2,3,} }

according to property above diagram is gnerated .

Now You can ans both the questions.

answered by Veteran (21.2k points)  
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