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let $\mathbf{A}$ be adjacency matrix of line graph $T$ and $T$ is a tree. I want to show that $\operatorname{null}(\mathbf{A})\ge 1$.

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I have to use (D^t)D=A+2I.But I don't know to how – Ravid Nov 11 2011 at 7:39
What is D ? – Greg Martin Nov 11 2011 at 7:51
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 When you posted this question to m.se (math.stackexchange.com/questions/79214/…), you were asked what you had tried. You never replied. Better to go back there and engage with the people who would help you if you'd meet them halfway. – Gerry Myerson Nov 11 2011 at 11:17
D is incidence matrix – Ravid Nov 11 2011 at 14:28

closed as too localized by Dan Petersen, Gjergji Zaimi, Alain Valette, Igor Rivin, Andreas Blass Nov 11 2011 at 13:05

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