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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