Lemma 55.5.12. Nonexistence of proper subgraphs of the form
\xymatrix{ \bullet \ar@{-}[r] & \bullet \ar@{-}[r] & \bullet \ar@{-}[r] \ar@{-}[d] & \bullet \ar@{-}[r] & \bullet \\ & & \bullet \ar@{-}[d] \\ & & \bullet }
Assume n > 7. There do not exist 7 distinct (-2)-indices f, g, h, i, j, k, l such that a_{fg}, a_{gh}, a_{ij}, a_{jh}, a_{kl}, a_{lh} are nonzero.
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