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- W4300457488 abstract "The Moore bound constitutes both an upper bound on the order of a graph of maximum degree $d$ and diameter $D=k$ and a lower bound on the order of a graph of minimum degree $d$ and odd girth $g=2k+1$. Graphs missing or exceeding the Moore bound by $epsilon$ are called {it graphs with defect or excess $epsilon$}, respectively. While {it Moore graphs} (graphs with $epsilon=0$) and graphs with defect or excess 1 have been characterized almost completely, graphs with defect or excess 2 represent a wide unexplored area. Graphs with defect (excess) 2 satisfy the equation $G_{d,k}(A) = J_n + B$ ($G_{d,k}(A) = J_n-B$), where $A$ denotes the adjacency matrix of the graph in question, $n$ its order, $J_n$ the $ntimes n$ matrix whose entries are all 1's, $B$ the adjacency matrix of a union of vertex-disjoint cycles, and $G_{d,k}(x)$ a polynomial with integer coefficients such that the matrix $G_{d,k}(A)$ gives the number of paths of length at most $k$ joining each pair of vertices in the graph. In particular, if $B$ is the adjacency matrix of a cycle of order $n$ we call the corresponding graphs emph{graphs with cyclic defect or excess}; these graphs are the subject of our attention in this paper. We prove the non-existence of infinitely many such graphs. As the highlight of the paper we provide the asymptotic upper bound of $O(frac{64}3d^{3/2})$ for the number of graphs of odd degree $dge3$ and cyclic defect or excess. This bound is in fact quite generous, and as a way of illustration, we show the non-existence of some families of graphs of odd degree $dge3$ and cyclic defect or excess. Actually, we conjecture that, apart from the Mobius ladder on 8 vertices, no non-trivial graph of any degree $ge 3$ and cyclic defect or excess exists." @default.
- W4300457488 created "2022-10-03" @default.
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- W4300457488 date "2010-10-27" @default.
- W4300457488 modified "2023-09-28" @default.
- W4300457488 title "On graphs with cyclic defect or excess" @default.
- W4300457488 doi "https://doi.org/10.48550/arxiv.1010.5841" @default.
- W4300457488 hasPublicationYear "2010" @default.
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