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I want to prove whether it is possible for a graph to have different degrees for each vertex. I think that it can be possible with an example, but I can't prove it with mathematics.

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Yes it is possible. Consider a graph with zero or one vertex. –  LostInMath Oct 11 '11 at 14:49
    
Please notice that the singular is "vertex" and the plural is "vertices". I changed "vertice" to "vertex" in the question. –  Michael Hardy Oct 11 '11 at 15:55
    
i was confused when i wrote it and i am not a native english speaker –  Parhs Oct 11 '11 at 19:23
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2 Answers 2

up vote 2 down vote accepted

Every simple graph with at least two vertices has at least two vertices of the same degree.

http://www.student.math.uwaterloo.ca/~math239/winter2008/t7sol.pdf

http://www.mymathforum.com/viewtopic.php?f=27&t=20418

This was among the first result when searching for "two vertices" "same degree". You can also find many books containing this claim.

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i googled it but didint find anything..however this seems true for vertices that do not have loop. –  Parhs Oct 11 '11 at 15:01
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If you are talking of simple graphs then clearly in any connected component containing n(>1) vertices the n vertex degrees will have degrees among the numbers $\{1,2,3\cdots n-1\}$ and so by the pigeonhole principle at least 2 vertices will have the same degree. The conclusion is false if we consider graphs with loops or with multiple edges. enter image description here

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thank you ,this was the example i was thinking of! –  Parhs Oct 11 '11 at 15:53
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