# Erdős–Gallai theorem

Erdős–Gallai theorem The Erdős–Gallai theorem is a result in graph theory, a branch of combinatorial mathematics. It provides one of two known approaches to solving the graph realization problem, ou seja. it gives a necessary and sufficient condition for a finite sequence of natural numbers to be the degree sequence of a simple graph. A sequence obeying these conditions is called "graphic". The theorem was published in 1960 by Paul Erdős and Tibor Gallai, depois de quem é nomeado.

Conteúdo 1 Declaração 2 Provas 3 Relation to integer partitions 4 Graphic sequences for other types of graphs 5 Versão mais forte 6 Generalização 7 Veja também 8 References Statement A sequence of non-negative integers {estilo de exibição d_{1}geq cdots geq d_{n}} can be represented as the degree sequence of a finite simple graph on n vertices if and only if {estilo de exibição d_{1}+cdots +d_{n}} is even and {soma de estilo de exibição _{i=1}^{k}d_{eu}leq k(k-1)+soma _{i=k+1}^{n}min(d_{eu},k)} holds for every k in {displaystyle 1leg kleg n} .

Proofs It is not difficult to show that the conditions of the Erdős–Gallai theorem are necessary for a sequence of numbers to be graphic. The requirement that the sum of the degrees be even is the handshaking lemma, already used by Euler in his 1736 paper on the bridges of Königsberg. The inequality between the sum of the {estilo de exibição k} largest degrees and the sum of the remaining degrees can be established by double counting: the left side gives the numbers of edge-vertex adjacencies among the {estilo de exibição k} highest-degree vertices, each such adjacency must either be on an edge with one or two high-degree endpoints, a {estilo de exibição k(k-1)} term on the right gives the maximum possible number of edge-vertex adjacencies in which both endpoints have high degree, and the remaining term on the right upper bounds the number of edges that have exactly one high degree endpoint. Desta forma, the more difficult part of the proof is to show that, for any sequence of numbers obeying these conditions, there exists a graph for which it is the degree sequence.

The original proof of Erdős & Gallai (1960) was long and involved. Choudum (1986) cites a shorter proof by Claude Berge, based on ideas of network flow. Choudum instead provides a proof by mathematical induction on the sum of the degrees: he lets {estilo de exibição t} be the first index of a number in the sequence for which {estilo de exibição d_{t}>d_{t+1}} (or the penultimate number if all are equal), uses a case analysis to show that the sequence formed by subtracting one from {estilo de exibição d_{t}} and from the last number in the sequence (and removing the last number if this subtraction causes it to become zero) is again graphic, and forms a graph representing the original sequence by adding an edge between the two positions from which one was subtracted.

Tripathi, Venugopalan & West (2010) consider a sequence of "subrealizations", graphs whose degrees are upper bounded by the given degree sequence. They show that, if G is a subrealization, and i is the smallest index of a vertex in G whose degree is not equal to di, then G may be modified in a way that produces another subrealization, increasing the degree of vertex i without changing the degrees of the earlier vertices in the sequence. Repeated steps of this kind must eventually reach a realization of the given sequence, provando o teorema.

Relation to integer partitions Aigner & Triesch (1994) describe close connections between the Erdős–Gallai theorem and the theory of integer partitions. Deixar {displaystyle m=sum d_{eu}} ; then the sorted integer sequences summing to {estilo de exibição m} may be interpreted as the partitions of {estilo de exibição m} . Under majorization of their prefix sums, the partitions form a lattice, in which the minimal change between an individual partition and another partition lower in the partition order is to subtract one from one of the numbers {estilo de exibição d_{eu}} and add it to a number {estilo de exibição d_{j}} that is smaller by at least two ( {estilo de exibição d_{j}} could be zero). As Aigner and Triesch show, this operation preserves the property of being graphic, so to prove the Erdős–Gallai theorem it suffices to characterize the graphic sequences that are maximal in this majorization order. They provide such a characterization, in terms of the Ferrers diagrams of the corresponding partitions, and show that it is equivalent to the Erdős–Gallai theorem.

Graphic sequences for other types of graphs Similar theorems describe the degree sequences of simple directed graphs, simple directed graphs with loops, and simple bipartite graphs (Berger 2012). The first problem is characterized by the Fulkerson–Chen–Anstee theorem. The latter two cases, which are equivalent, are characterized by the Gale–Ryser theorem.

Stronger version Tripathi & Vijay (2003) proved that it suffices to consider the {estilo de exibição k} th inequality such that {displaystyle 1leq k

Generalization A finite sequences of nonnegative integers {estilo de exibição (d_{1},cdots ,d_{n})} com {estilo de exibição d_{1}geq cdots geq d_{n}} is graphic if {soma de estilo de exibição _{i=1}^{n}d_{eu}} is even and there exists a sequence {estilo de exibição (c_{1},cdots ,c_{n})} that is graphic and majorizes {estilo de exibição (d_{1},cdots ,d_{n})} . This result was given by Aigner & Triesch (1994). Mahadev & Peled (1995) reinvented it and gave a more direct proof.

See also Havel–Hakimi algorithm References Aigner, Martinho; Triesch, Eberhard (1994), "Realizability and uniqueness in graphs", Matemática Discreta, 136 (1-3): 3-20, doi:10.1016/0012-365X(94)00104-Q, MR 1313278. Barrus, M. D.; Hartke, S. G.; Jao, Kyle F.; Oeste, D. B. (2012), "Length thresholds for graphic lists given fixed largest and smallest entries and bounded gaps", Matemática Discreta, 312 (9): 1494–1501, doi:10.1016/j.disc.2011.05.001 Berger, Annabell (2012), The connection between the number of realizations for degree sequences and majorization, arXiv:1212.5443, Bibcode:2012arXiv1212.5443B Choudum, S. UMA. (1986), "A simple proof of the Erdős–Gallai theorem on graph sequences", Bulletin of the Australian Mathematical Society, 33 (1): 67-70, doi:10.1017/S0004972700002872, MR 0823853. Floresta, P.; Gallai, T. (1960), "Gráfok előírt fokszámú pontokkal" (PDF), Matematikai Lapok, 11: 264–274 Mahadev, N. V. R.; Peled, você. N. (1995), Threshold graphs and related topics, Elsevier Tripathi, Amitabha; Vijay, Sujith (2003), "A note on a theorem of Erdős & Gallai", Matemática Discreta, 265 (1-3): 417–420, doi:10.1016/s0012-365x(02)00886-5, MR 1969393 Tripathi, Amitabha; Venugopalan, Sushmita; Oeste, Douglas B. (2010), "A short constructive proof of the Erdős–Gallai characterization of graphic lists", Matemática Discreta, 310 (4): 843–844, doi:10.1016/j.disc.2009.09.023, MR 2574834 Categorias: Paul ErdősTheorems in graph theory

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