Strongly Closed Subgraphs in a Regular Thick Near Polygon

AbstractIn this paper we show that a regular thick near polygon has a tower of regular thick near sub-polygons as strongly closed subgraphs if the diameter d is greater than the numerical girth g

Distance-regular graphs

This is a survey of distance-regular graphs. We present an introduction to distance-regular graphs for the reader who is unfamiliar with the subject, and then give an overview of some developments in the area of distance-regular graphs since the monograph 'BCN' [Brouwer, A.E., Cohen, A.M., Neumaier, A., Distance-Regular Graphs, Springer-Verlag, Berlin, 1989] was written.Comment: 156 page

3-bounded property in a triangle-free distance-regular graph

Let $\Gamma$ denote a distance-regular graph with classical parameters $(D, b, \alpha, \beta)$ and $D\geq 3$. Assume the intersection numbers $a_1=0$ and $a_2\not=0$. We show $\Gamma$ is 3-bounded in the sense of the article [D-bounded distance-regular graphs, European Journal of Combinatorics(1997)18, 211-229].Comment: 13 page

The nonexistence of regular near octagons with parameters (s, t, t(2), t(3)) = (2,24,0,8)

Let S be a regular near octagon with s + 1 = 3 points per line, let t + 1 denote the constant number of lines through a given point of S and for every two points x and y at distance i is an element of {2, 3} from each other, let t(i) + 1 denote the constant number of lines through y containing a (necessarily unique) point at distance i - 1 from x. It is known, using algebraic combinatorial techniques, that (t(2), t(3), t) must be equal to either (0, 0, 1), (0, 0, 4), (0, 3, 4), (0, 8, 24), (1, 2, 3), (2, 6, 14) or (4, 20, 84). For all but one of these cases, there is a unique example of a regular near octagon known. In this paper, we deal with the existence question for the remaining case. We prove that no regular near octagons with parameters (s, t, t(2), t(3)) = (2, 24, 0, 8) can exist

Terwilliger algebraの表現とその応用

金沢大学理学部本研究の最大の研究成果はTerwilliger algebraのnonthin表現の研究においてbreakthroughがあったことである。このbreakthoughに的をしぼり、いかにしてnonthin表現の表現論を完成しようとしているかを説明する。その他の研究成果については、代表的なものとして、cyclotomic schemeのTerwilliger algebraの研究が整数論との関連のもとに深まったこと、spin modelとquantum groupとTerwilliger algebraとの係わりが見出されたこと、type II matrixの構造に新知見が得られたことなどの先駆的仕事をあげるにとどめる。本研究ではnonthincaseにおいて、classical parameterを持つP-and Q-polynomial schemaのT-algerbaの表現について、以下のような成果が得られた(ただし、ここではclassical parameterを通常より変数が1個少ない意味に解釈している)。endpoint 1のirreducible T-moduleはladder basisという非常に良い性質をもつ(Hobart-伊藤)。最も簡単なparameterの場合、irreducible T-moduleは、Onsager algebraの有限次元既約表現から求まる(伊藤-田辺-Terwilliger)。以上の結果を一般の場合に拡張するための基本となる構造定理がT-moduleに対して得られ、Onsager algebraのq-analogue(q-Onsager algebra)が定義された(伊藤-田辺-Terwilliger)。以上により、classical parameterの場合、問題はq-Onsager algebraの有限次元既約表現に帰着する。diameterが3のとき、q-Onsager algebraの有限次元既約表現は、affine quantom algebra U_g(sl_2)のtype(1,1)表現から求まる(伊藤-田辺-Benkart-Terwilliger、論文準備中)。これが初めに述べたbreakthroughであり、この結果を一般のdiameterに拡張するのが、今後の研究の目標となる。The major outcome of this research project, which will be discussed in detail later, is that there was a breakthrough in the area of the non thin representations of Terwilliger algebras of P- and Q- polynomial type. Among others are some pioneering works on the Terwilliger algebras of cyclotomic schemes and the Jacobi sums, on relations between spin models, quantum groups and Terwilliger algebras, on the structure of type II matrices. Let T be a Terwilliger algebra of P- and Q-polynomial type with classical parameters, where the classical parameters mean the ones with one less variables than usual. We obtained the following results.The irreducible T-modules of endpoint 1 have a ladder basis (Hobart-Ito). In the simplest case of parameters, irreducible T-modules are determined via finite dimensional irreducible representations of On sager algebras (Ito-Tanabe-Terwilliger). A basic theorem is obtained for the structure of T-modules, enabling us to deal with the general case by defining the q-analogue of an On sager algebra (q-On sager algebra) (Ito-Tanabe-Terwilliger).Thus in the case of classical parameters, the problem of irreducible T-modules is reduced to the determination of finite dimensional irreducible representations of q-Onsager algebras. If the diameter is 3, finite dimensional irreducible representations of q-0n sager algebras are determined via the type (1,1) representations of the affine quantum algebra U_q (sl_2). This is the breakthrough mentioned at the beginning and we are aiming at generalizing it to arbitrary diameters.研究課題/領域番号:10440003, 研究期間(年度):1998 – 2001出典：「Terwilliger algebraの表現とその応用」研究成果報告書　課題番号10440003（KAKEN：科学研究費助成事業データベース（国立情報学研究所））（https://kaken.nii.ac.jp/ja/grant/KAKENHI-PROJECT-10440003/）を加工して作

On Q-polynomial regular near 2d-gons

We discuss thick regular near 2d-gons with a Q-polynomial collinearity graph. For da parts per thousand yen4, we show that apart from Hamming near polygons and dual polar spaces there are no thick Q-polynomial regular near polygons. We also show that no regular near hexagons exist with parameters (s, t (2), t) equal to (3, 1, 34), (8, 4, 740), (92, 64, 1314560), (95, 19, 1027064) or (105, 147, 2763012). Such regular near hexagons are necessarily Q-polynomial. All these nonexistence results imply the nonexistence of distance-regular graphs with certain classical parameters. We also discuss some implications for the classification of dense near polygons with four points per line

3-manifolds efficiently bound 4-manifolds

It is known since 1954 that every 3-manifold bounds a 4-manifold. Thus, for instance, every 3-manifold has a surgery diagram. There are several proofs of this fact, including constructive proofs, but there has been little attention to the complexity of the 4-manifold produced. Given a 3-manifold M of complexity n, we show how to construct a 4-manifold bounded by M of complexity O(n^2). Here we measure ``complexity'' of a piecewise-linear manifold by the minimum number of n-simplices in a triangulation. It is an open question whether this quadratic bound can be replaced by a linear bound. The proof goes through the notion of "shadow complexity" of a 3-manifold M. A shadow of M is a well-behaved 2-dimensional spine of a 4-manifold bounded by M. We prove that, for a manifold M satisfying the Geometrization Conjecture with Gromov norm G and shadow complexity S, c_1 G <= S <= c_2 G^2 for suitable constants c_1, c_2. In particular, the manifolds with shadow complexity 0 are the graph manifolds.Comment: 39 pages, 21 figures; added proof for spin case as wel