Abstract

Locally projective graphs can be viewed as an axiomatised version of point-line graphs of the symplectic (thick) and orthogonal (thin) dual polar spaces over $\mathrm{GF}<!--FUNCTION\; APPLICATION-->(2)$. Further examples come from Petersen and tilde geometries of the sporadic simple groups $M_{22}$, $M_{23}$, $M_{24}$, $He$, ${Co}_2$, ${Co}_1$, $J_4$, $BM$, $M$. The paper contributes to the classification of simply connected locally projective graphs. Such classification was accomplished for the thin case by S. V. Shpectorov and the author in 2002 with the essential use of results by V. I. Trofimov proved in a series of papers published in the 1990s. The graphs of the orthogonal dual polar spaces form the only nontrivial infinite series in the thin case. The orthogonal graphs are densely embedded in the symplectic graphs. The natural generalisation of this notion places in a general setting the famous embeddings

among Petersen and tilde geometries. It was recently proved by the author that graphs from a large class contain thin densely embedded subgraphs. In the present paper we characterise the symplectic dual polar graphs in the class of locally projective graphs with densely embedded subgraphs. The well-known special behaviour of the first cohomology group of the linear groups $L_n(2)$ for $n=3$ and $4$ makes the proof a bit nontrivial but, unlike in the thin case, does not bring any exceptional examples.

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Mathematical Subject Classification

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