Iskanje po repozitoriju

Opis: A k-regular graph of girth g is called an edge-girth-regular graph, or an egr-graph for short, if each of its edges is contained in exactly λ distinct g-cycles. An egr-graph is called extremal for the triple (k, g, λ) if has the smallest possible order. We prove that some graphs arising from incidence graphs of finite planes are extremal egr-graphs. We also prove new lower bounds on the order of egr-graphs.
Ključne besede: edge-girth-regular graph, cage problem, finite biaffine planes
Objavljeno v RUP: 03.11.2025; Ogledov: 314; Prenosov: 2
Celotno besedilo (547,76 KB)

Opis: A cycle in a graph G is said to be primitive from its vertex x if at least one of its edges does not belong to any shorter cycle that passes through x. This type of cycle and an associated notion of extended neighborhoods play a key role in message-passing algorithms that compute spectral properties of graphs with short loops. Here, we investigate such primitive cycles and graphs without long primitive cycles in a more traditional graph-theoretic framework. We show that a cycle is primitive from all its vertices if and only if it is isometric. We call a cycle fully redundant cycles if it is not primitive from any of its vertices and show that fully redundant cycles, in particular, are not edge short, i.e., they cannot be represented as the edge-disjoint union of a single edge and two shortest paths in G. The families Rk and Lk of graphs with all cycles of length at least k + 1 being fully redundant and not edge-short, respectively, coincide for k = 3 and k = 4. In these graphs, all cycles of length at least k + 1 are pantochordal, i.e., each of their vertices is incident with a chord. None of these results generalizes to k ≥ 5. Moreover, R₃ = L₃ turn out to be the block graphs, and R₄ = L₄ are the graphs with complete multi-partite blocks. The cographs, finally, are shown to form a proper subset of R₅.
Ključne besede: edge-short cycle, chord, block-graph, complete multipartite graph, wheel graphs, cographs, geodesic cycles, Hamiltonian cycles
Objavljeno v RUP: 03.11.2025; Ogledov: 238; Prenosov: 0
Celotno besedilo (478,50 KB)

Opis: The family OG(4) consisting of graph-group pairs (Γ, G), where Γ is a finite, connected, 4-valent graph admitting a G-vertex-, and G-edge-transitive, but not G-arc-transitive action, has recently been examined using a normal quotient methodology. A subfamily of OG(4) has been identified as ‘basic’, due to the fact that all members of OG(4) are normal covers of at least one basic pair. We provide an explicit classification of those basic pairs (Γ, G) which have at least two independent cyclic G-normal quotients (these are G-normal quotients which are not extendable to a common cyclic normal quotient).
Ključne besede: half-arc-transitive, vertex-transitive graph, edge-transitive graph, normal cover, cycle graph
Objavljeno v RUP: 21.10.2025; Ogledov: 327; Prenosov: 1
Celotno besedilo (398,19 KB)

Opis: A finite simple graph Γ is called a Nest graph if it is regular of valency 6 and admits an automorphism ρ with two orbits of the same length such that at least one of the subgraphs induced by these orbits is a cycle. We say that Γ is core-free if no non-trivial subgroup of the group generated by ρ is normal in Aut(Γ). In this paper, we show that, if Γ is edge-transitive and core-free, then it is isomorphic to one of the following graphs: the complement of the Petersen graph, the Hamming graph H(2,4), the Shrikhande graph and a certain normal 2-cover of K_{3,3} by ℤ_2^4.
Ključne besede: bicirculant, edge-transitive, primitive permutation group
Objavljeno v RUP: 10.09.2025; Ogledov: 374; Prenosov: 3
Celotno besedilo (466,20 KB)

Opis: We present some new discoveries about graphs that are half-arc-transitive (that is, vertex- and edge-transitive but not arc-transitive). These include the recent discovery of the smallest half-arc-transitive 4-valent graph with vertex-stabiliser of order 4, and the smallest with vertex-stabiliser of order 8, two new half-arc-transitive 4-valent graphs with dihedral vertex-stabiliser ▫$D_4$▫ (of order 8), and the first known half-arc-transitive 4-valent graph with vertex-stabiliser of order 16 that is neither abelian nor dihedral. We also use half-arc-transitive group actions to provide an answer to a recent question of Delorme about 2-arc-transitive digraphs that are not isomorphic to their reverse.
Ključne besede: graph, edge-transitive, vertex-transitive, arc-transitive, half arc-transitive
Objavljeno v RUP: 31.12.2021; Ogledov: 2548; Prenosov: 23
Celotno besedilo (333,06 KB)