مجتمع هاي ساده گون تجزيه پذير رأسي نظير به گراف هاي مسير

Vertex Decomposable Simplicial Complexes Associated to Path Graphs

شناخت مجتمع ­هاي ساده ­گون تجزيه ­پذير رأسي به واسطۀ خواص جبري و توپولوژيكي ­اي كه دارند از جمله مسائل مهم در جبر جابه‌جايي تركيبياتي به‌ شمار مي ­رود. در اين راستا معرفي خانواده­ هايي از مجتمع­ هاي ساده­ گون با اين خاصيت بسيار مورد توجه است. در اين مقاله مجتمع ساده­ گون استنلي-ريزنر نظير به ايده ­آل t-خوشه­ اي گراف­ هاي مكمل مسير بررسي شده است. براي اين خانواده از مجتمع­ هاي ساده ­گون، مجموعۀ رويه ­هاي آن‌ها را به‌طور دقيق مشخص كرده و با استفاده از اين موضوع نشان مي­دهيم اين دسته از مجتمع­ هاي ساده ­گون داراي خاصيت تجزيه ­پذيري رأسي هستند. در واقع با توجه به محض بودن آن ها ثابت مي­ شود كه حلقۀ استنلي-ريزنر آن‌ها داراي خاصيت كوهن-مكالي است. از آن‌ جا كه 2-خوشه ايده­آل‌ها همان ايده ­آل­هاي يالي گراف ­ها هستند، اين دسته از مجتمع­ هاي ساده ­گون شامل خانوادۀ مجتمع ­هاي ساده­ گون مستقل ­هاي گراف مكمل مسير هستند. در پايان به‌عنوان نتيجه نشان مي­ دهيم­ كه ايده­آل -مستقل‌هاي گراف مكمل مسير يك ايده ­آل جدا شوندۀ رأسي است و جداساز بتي آن را ارائه مي ­دهيم.

Vertex decomposability of a simplicial complex is a combinatorial topological concept which is related to the algebraic properties of the Stanley-Reisner ring of the simplicial complex. This notion was first defined by Provan and Billera in 1980 for k-decomposable pure complexes which is known as vertex decomposable when . Later Bjorner and Wachs extended this concept to non-pure complexes. Being defined in an inductive way, vertex decomposable simplicial complexes are considered as a well behaved class of complexes and has been studied in many research papers. Because of their interesting algebraic and topological properties, giving a characterization for this class of complexes is of great importance and is one of the main problems in combinatorial commutative algebra. In this regard obtaining families of simplicial complexes with this property is of great interest. In this paper we present a new family of vertex decomposable simplicial complexes, which is associated to the t-clique ideal of the complement of path graphs. The t-clique ideal is a natural generalization of the concept of the edge ideal of a graph. For a graph G, a complete subgraph of G with t vertices is called a t-clique of G. The ideal generated by the monomials of degree t such that the induced subgraph of G on the set is a complete graph, is called the t-clique ideal of G. We consider the Stanley- Reisner simplicial complex of the ideal , where is a path graph of order n. For such a simplicial complex , we obtain the set of facets of and using this characterization we show that every such simplicial complex is vertex decomposable, whose shedding vertex is an endpoint of the path graph. Indeed, any simplicial complex in this family is Cohen-Macaulay, since it is pure. Since edge ideals of graphs are in fact 2-clique ideals, this family of simplicial complexes contains the independence complexes of complement of path graphs. Finally, as a consequence it is shown that the t-independence ideal of the complement of a path graph is vertex splittable and its Betti splitting is presented Material and methods To prove the vertex decomposability of , first we characterize the set of facets of . This helps us to find a shedding vertex for this simplicial complex and then by an inductive approach the vertex decomposability has been proved. Results and discussion For positive integers and , we show that a subset F of the vertex set of is a facet of if and only if and every component of the induced subgraph is a path graph of even order. Using this characterization, it is shown that any endpoint of the path graph is a shedding vertex of and is vertex decomposable. Moreover, it is proved that the ideal has a Betti splitting. Conclusion The following conclusions were drawn from this research.