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squaldig's avatar

How do I show that a simple graph of size n >= 2 always has at least two vertices of the same degree?

Asked by squaldig (5points) February 7th, 2007
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20 Answers

ben's avatar
Here's one idea... just use counting:
ben (9085points)“Great Answer” (0points) Flag as…
ben's avatar
Start by picking the vertex with the highest degree, k
ben (9085points)“Great Answer” (0points) Flag as…
ben's avatar
we can assume k >= 2, (if it's 1, we're done).
ben (9085points)“Great Answer” (0points) Flag as…
ben's avatar
Now we know each connected vertex, k(i), must have a different degree (or we're done)
ben (9085points)“Great Answer” (0points) Flag as…
ben's avatar
so k actually must be > 3
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ben's avatar
and then iterate through k(i) the same way... should be able to outnumber it
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ben's avatar
I'm positive there's a far simpler solution, this was just my first idea... maybe it will help get the wheels turning
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ben's avatar
Yeah, this should work. Just for the second step pick the vertex connected to k with the highest degree
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bhs's avatar
I don't see how that's a proof... It seems like more of a test for verification that this is true?
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ben's avatar
It's an loose idea for a proof
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ben's avatar
you're right, it's not a proof
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bhs's avatar
hmm... I don't think that will work formally... I think you need to start from the N=2 case and use induction to go up
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ben's avatar
Okay, I was close, this proof is indeed much simpler:
ben (9085points)“Great Answer” (0points) Flag as…
ben's avatar
We assume there are no two vertices with the same degree
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ben's avatar
so degrees must go from 0 to n-1 (we have n vertices, and an vertex can't connect to itself)
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ben's avatar
but have a vertex with degree 0 and n-1 is a contradiction... the n-1 must be connected to every other vertex
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ben's avatar
that should do it
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finkelitis's avatar
I think this is a pigeonhole argument: What are the possible degrees? 1 through n-1 (let's assume it's connected, else we just start with the smaller pieces). How many vertices? n. So two of them have the same degree by the pigeonhole principle.
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finkelitis's avatar
what you said, ben
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finkelitis's avatar
essentially
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