Why is this the obvious? (graph Theory)

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SUMMARY

The discussion centers on the application of the Perron-Frobenius theorem in the context of connected graphs. Participants express confusion regarding the clarity of the theorem's implications for connected graphs, specifically in relation to eigenvalues and eigenvectors. The theorem states that for a connected graph G, the first eigenvalue is simple and its eigenspace is generated by a strictly positive eigenvector. Additionally, the discussion highlights the property of bipartite graphs, where altering the sign of every other entry in an eigenvector yields an eigenvector associated with the negative of the first eigenvalue.

PREREQUISITES
  • Understanding of graph theory concepts, particularly connected graphs and bipartite graphs.
  • Familiarity with the Perron-Frobenius theorem and its implications for eigenvalues and eigenvectors.
  • Knowledge of Schrödinger operators and their relationship to graph theory.
  • Basic linear algebra, specifically eigenvalues and eigenvectors.
NEXT STEPS
  • Study the Perron-Frobenius theorem in detail, focusing on its applications to connected graphs.
  • Research the properties of bipartite graphs and their eigenvalues.
  • Explore the relationship between Schrödinger operators and graph theory.
  • Learn about eigenvector transformations and their implications in graph theory.
USEFUL FOR

Mathematicians, theoretical computer scientists, and students studying graph theory or linear algebra, particularly those interested in eigenvalue problems and their applications in connected and bipartite graphs.

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http://www.win.tue.nl/~aeb/srgbk/node10.html

Under the 'For connected graphs all is clear from above'

I've been asked to prove this for connected graphs, but I don't see how this is so clear?

Can anyone help me? Thanks
 
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It says "it is clear from the above" but you have cut off the "above"! What did it say before that theorem?
 
HallsofIvy said:
It says "it is clear from the above" but you have cut off the "above"! What did it say before that theorem?

Well I googled my problem and found that link after some pages. I'm not sure what it said.

But it probably was something like the Perron Frobenius theorem.

From my notes I have (Perron Frobenius Thereom)

'Let A be in the set of all Schrödinger operators, and suppose a graph G is connected.

Then eigenvalue(1) is simple (i.e. has multiplicity 1) and its eigenspace is generated by an eigenvector that is strictly positive everywhere.'My general question that I was asked to do came from this email he sent me:

'These graphs are bipartite, meaning that if you change the sign of every other entry in an eigenvector then you get an eigenvector associated with negative the first eigenvalue (prove this!)'
 

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