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Abstract: An (r, z; g)-mixed graph is a graph containing both edges and darts satisfying the regularity property that each vertex of the graph is incident to r edges, z ingoing and z outgoing darts (called total regularity), and being of oriented girth g, i.e., containing an oriented cycle of length g, and no shorter oriented cycles. The problem addressed in this paper is analogous to the Cage Problem and calls for determining the orders of the smallest totally regular (r, z; g)-mixed graphs. We derive several upper and lower bounds on the orders of such minimal graphs, study the relations between these extremal graphs and their non-oriented or digraphical counterparts, and focus on properties of totally regular mixed graphs obtained by replacing some of the edges of the incidence graphs of projective and biaffine planes by darts. We also introduce two constructions based on introducing additional edges or darts into induced subgraphs of these incidence graphs.
Keywords: totally regular mixed graph, girth, projective plane, biaffine plane
Published in RUP: 04.06.2026; Views: 437; Downloads: 24
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Abstract: An edge-girth-regular graph egr(n, k, g, λ) is a k-regular graph of order n, girth g and with the property that each of its edges is contained in exactly λ distinct g-cycles. We present new families of edge-girth regular graphs arising from generalized quadrangles and pencils of elliptic quadrics. An egr(n, k, g, λ) is called extremal for the triple (k, g, λ) if n is the smallest order of any egr(n, k, g, λ). We give new lower bounds for the order of extremal edge-girth-regular graphs using properties of the eigenvalues of the adjacency matrix of a graph.
Keywords: cage problem, extremal graph theory, generalized polygons, ovoids
Published in RUP: 22.10.2025; Views: 1090; Downloads: 7
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