Iskanje po repozitoriju

Opis: A vertex colouring of a graph is called asymmetric if the only automorphism which preserves it is the identity. Tucker conjectured that if every automorphism of a connected, locally finite graph moves infinitely many vertices, then there is an asymmetric colouring with 2 colours. This conjecture was recently confirmed by Babai, using the heavy machinery of the classification of finite simple groups. We make progress towards a purely combinatorial proof of this conjecture in the special case of graphs with bounded maximal degree. More precisely, using only elementary combinatorial methods, we prove that if every automorphism of a connected graph with maximal degree Δ moves infinitely many vertices, then there is an asymmetric colouring using O(\sqrt(Δ) log(Δ)) colours. This is the first improvement which does not depend on the classification of finite simple groups over the trivial bound of O(Δ).
Ključne besede: infinite graph, asymmetric colouring, infinite motion
Objavljeno v RUP: 22.10.2025; Ogledov: 82; Prenosov: 2
Celotno besedilo (299,41 KB)

Opis: We introduce a notion of wreath product of two gain graphs (Γ_1, ψ_1, G_1) and (Γ_2, ψ_2, G_2), producing a gain graph over the direct product group G_2|V_Γ1| × G_1, whose underlying graph is the classical wreath product of graphs Γ_1≀Γ_2. By composition with a suitable group homomorphism, our construction produces a signed graph when the two factors are signed graphs. We prove that the wreath product is stable under switching isomorphism. By using group representations, we are able to perform spectral computations on the wreath product: in particular, we determine its largest and its smallest eigenvalue, and we give a description of the spectrum when the first factor is a complex unit complete balanced or antibalanced gain graph, and the second factor is circulant. Finally, when G_1 is a group of permutations of the vertex set of the first factor, and the group G_2 is abelian, we give an alternative definition producing a gain graph over the group wreath product G_1≀G_2, which turns out to be stable under switching equivalence of the second factor, when the first factor is balanced.
Ključne besede: gain graph, signed graph, wreath product of graphs, wreath product of groups, circulant gain graph, mixed Kronecker product, π-spectrum
Objavljeno v RUP: 22.10.2025; Ogledov: 64; Prenosov: 0
Celotno besedilo (492,42 KB)

Opis: Median graphs are connected graphs in which for all three vertices there is a unique vertex that belongs to shortest paths between each pair of these three vertices. To be more formal, a graph G is a median graph if, for all μ, u, v ∈ V(G), it holds that |I(μ, u) ∩ I(μ, v) ∩ I(u, v)| = 1 where I(x, y) denotes the set of all vertices that lie on shortest paths connecting x and y. In this paper we are interested in a natural generalization of median graphs, called k-median graphs. A graph G is a k-median graph, if there are k vertices μ1, …, μk ∈ V(G) such that, for all u, v ∈ V(G), it holds that |I(μ_i, u) ∩ I(μ_i, v) ∩ I(u, v)| = 1, 1 ≤ i ≤ k. By definition, every median graph with n vertices is an n-median graph. We provide several characterizations of k-median graphs that, in turn, are used to provide many novel characterizations of median graphs.
Ključne besede: median graph, convexity, meshed and quadrangle property, modular, interval
Objavljeno v RUP: 22.10.2025; Ogledov: 48; Prenosov: 0
Celotno besedilo (520,78 KB)

Opis: Given two graphs F_1 and F_2, their size Ramsey number, denoted by r̂(F_1, F_2), is the minimum number of edges of a graph G such that for any edge coloring of G by colors red and blue, G contains either a red copy of F1 or a blue copy of F2. In this paper, we deal with the size Ramsey number of star forests (disjoint union of stars) and following a conjecture by Burr, Erdős, Faudree, Rousseau, and Schelp in 1978, we determine the exact value of r̂(⊔_{i = 1}^s K_{1, ni}, ⊔_{i = 1}^t K_{1, mi}) in several cases including when either m_i’s and n_i’s are odd, or s = 1 or s = 2 and n_1 = n_2.
Ključne besede: size Ramsey number, star forest, Ramsey minimal graph
Objavljeno v RUP: 21.10.2025; Ogledov: 74; Prenosov: 0
Celotno besedilo (282,24 KB)

Opis: In PARTITION INTO COMPLEMENTARY SUBGRAPHS (COMP-SUB) we are given a graph G = (V, E), and an edge set property Π, and asked whether G can be decomposed into two graphs, H and its complement H̄, for some graph H, in such a way that the edge cut [V(H), V(H̄)] satisfies the property Π. Motivated by previous work, we consider COMP-SUB(Π) when the property Π=PM specifies that the edge cut of the decomposition is a perfect matching. We prove that COMP-SUB(PM) is GI-hard when the graph G is C_5-free or G is {C_k ≥ 7, C̄_k ≥ 7}-free. On the other hand, we show that COMP-SUB(PM) is polynomial-time solvable on hole-free graphs and on P5-free graphs. Furthermore, we present characterizations of COMP-SUB(PM) on chordal, distance-hereditary, and extended P_4-laden graphs.
Ključne besede: graph partitioning, complementary subgraphs, perfect matching, matching cut, graph isomorphism
Objavljeno v RUP: 21.10.2025; Ogledov: 66; Prenosov: 1
Celotno besedilo (420,94 KB)