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R. M. Green

Publications and source records attributed to R. M. Green.

At least 37 records · Page 2Linked to original sources
arXiv2008

On the non-generic representation theory of the symplectic blob algebra

This paper reports some advances in the study of the symplectic blob algebra. We find a presentation for this algebra. We find a minimal poset for this as a quasi-hereditary algebra. We discuss how to reduce the number of parameters defining the algebra from 6 to 4 (or even 3) without loss of representation theoretic generality. We then find some non-semisimple specialisations by calculating Gram determinants for certain cell modules (or standard modules) using the good parametrisation defined. We finish by considering some quotients of specialisations of the symplectic blob algebra which are isomorphic to Temperley--Lieb algebras of type $A$.

math.RT↗
arXiv2007

Representations of Lie algebras arising from polytopes

We present an extremely elementary construction of the simple Lie algebras over the complex numbers in all of their minuscule representations, using the vertices of various polytopes. The construction itself requires no complicated combinatorics and essentially no Lie theory other than the definition of a Lie algebra; in fact, the Lie algebras themselves appear as by-products of the construction.

math.RT↗
arXiv2007

Full heaps and representations of affine Kac--Moody algebras

We give a combinatorial construction, not involving a presentation, of almost all untwisted affine Kac--Moody algebras modulo their one-dimensional centres in terms of signed raising and lowering operators on a certain distributive lattice ${\Cal B}$. The lattice ${\Cal B}$ is constructed combinatorially as a set of ideals of a ``full heap'' over the Dynkin diagram, which leads to a kind of categorification of the positive roots for the Kac--Moody algebra. The lattice ${\Cal B}$ is also a crystal in the sense of Kashiwara, and its span affords representations of the associated quantum affine algebra and affine Weyl group. There are analogues of these results for two infinite families of twisted affine Kac--Moody algebras, which we hope to treat more fully elsewhere. By restriction, we obtain combinatorial constructions of the finite dimensional simple Lie algebras over $\complex$, except those of types $E_8$, $F_4$ and $G_2$. The Chevalley basis corresponding to an arbitrary orientation of the Dynkin diagram is then represented explicitly by raising and lowering operators. We also obtain combinatorial constructions of the spin modules for Lie algebras of types $B$ and $D$, which avoid Clifford algebras, and in which the action of Chevalley bases on the canonical bases of the modules may be explicitly calculated.

math.CO↗
arXiv2007

Generalized Jones traces and Kazhdan--Lusztig bases

We develop some applications of certain algebraic and combinatorial conditions on the elements of Coxeter groups, such as elementary proofs of the positivity of certain structure constants for the associated Kazhdan--Lusztig basis. We also explore some consequences of the existence of a Jones-type trace on the Hecke algebra of a Coxeter group, such as simple procedures for computing leading terms of certain Kazhdan--Lusztig polynomials. The sequel to this paper is math.QA/0509363.

math.QA↗
arXiv2007

On the Markov trace for Temperley--Lieb algebras of type $E_n$

We show that there is a unique Markov trace on the tower of Temperley--Lieb type quotients of Hecke algebras of Coxeter type $E_n$ (for all $n \geq 6$). We explain in detail how this trace may be computed easily using tom Dieck's calculus of diagrams. As applications, we show how to use the trace to show that the diagram representation is faithful, and to compute leading coefficients of certain Kazhdan--Lusztig polynomials.

math.QA↗
arXiv2007

On the maximally clustered elements of Coxeter groups

We continue the study of the maximally clustered elements for simply laced Coxeter groups which were recently introduced by Losonczy. Such elements include as a special case the freely braided elements of Losonczy and the author, which in turn constitute a superset of the $iji$-avoiding elements of Fan. Our main result is to classify the MC-finite Coxeter groups, namely those Coxeter groups having finitely many maximally clustered elements. Remarkably, any simply laced Coxeter group having finitely many $iji$-avoiding elements also turns out to be MC-finite.

math.CO↗
arXiv2006

Constructing cell data for diagram algebras

We show how the treatment of cellularity in families of algebras arising from diagram calculi, such as Jones' Temperley--Lieb wreaths, variants on Brauer's centralizer algebras, and the contour algebras of Cox et al (of which many algebras are special cases), may be unified using the theory of tabular algebras. This improves an earlier result of the first author (whose hypotheses covered only the Brauer algebra from among these families).

math.RA↗
arXiv2006

Star reducible Coxeter groups

We define ``star reducible'' Coxeter groups to be those Coxeter groups for which every fully commutative element (in the sense of Stembridge) is equivalent to a product of commuting generators by a sequence of length-decreasing star operations (in the sense of Lusztig). We show that the Kazhdan--Lusztig bases of these groups have a nice projection property to the Temperley--Lieb type quotient, and furthermore that the images of the basis elements $C'_w$ (for fully commutative $w$) in the quotient have structure constants in ${\Bbb Z}^{\geq 0}[v, v^{-1}]$. We also classify the star reducible Coxeter groups and show that they form nine infinite families (types $A_n$, $B_n$, $D_n$, $E_n$, $F_n$, $H_n$, affine $A_{n-1}$ for $n$ odd, affine $C_{n-1}$ for $n$ even, and the case where the Coxeter graph is complete), with two exceptional cases (of ranks 6 and 7). This paper is the sequel to math.QA/0509362.

math.QA↗
arXiv2006

Full heaps and representations of affine Weyl groups

We use the author's combinatorial theory of full heaps (defined in math.QA/0605768) to categorify the action of a large class of Weyl groups on their root systems, and thus to give an elementary and uniform construction of a family of faithful permutation representations of Weyl groups. Examples include the standard representations of affine Weyl groups as permutations of ${\Bbb Z}$ and geometrical examples such as the realization of the Weyl group of type $E_6$ as permutations of 27 lines on a cubic surface; in the latter case, we also show how to recover the incidence relations between the lines from the structure of the heap. Another class of examples involves the action of certain Weyl groups on sets of pairs $(t, f)$, where $t \in {\Bbb Z}$ and $f$ is a function from a suitably chosen set to the two-element set $\{+, -\}$. Each of the permutation representations corresponds to a module for a Kac--Moody algebra, and gives an explicit basis for it.

math.CO↗
arXiv2006

Presenting affine q-Schur algebras

We obtain a presentation of certain affine q-Schur algebras in terms of generators and relations. The presentation is obtained by adding more relations to the usual presentation of the quantized enveloping algebra of type affine gl_n. Our results extend and rely on the corresponding result for the q-Schur algebra of the symmetric group, which were proved by the first author and Giaquinto.

math.QA↗
arXiv2004

Acyclic heaps of pieces, II

We characterize and classify the "regular classes of heaps" introduced by the author using ideas of Fan and of Stembridge. The irreducible objects fall into five infinite families with one exceptional case.

math.CO↗
arXiv2004

Schubert varieties and free braidedness

We give a simple necessary and sufficient condition for a Schubert variety $X_w$ to be smooth when $w$ is a freely braided element of a simply laced Weyl group; such elements were introduced by the authors in a previous work (math.CO/0301104). This generalizes in one direction a result of Fan concerning varieties indexed by short-braid avoiding elements. We also derive generating functions for the freely braided elements that index smooth Schubert varieties. All results are stated and proved only for the simply laced case.

math.CO↗
arXiv2003

Freely braided elements in Coxeter groups, II

We continue the study of freely braided elements of simply laced Coxeter groups, which we introduced in a previous work (math.CO/0301104). A known upper bound for the number of commutation classes of reduced expressions for an element of a simply laced Coxeter group is shown to be achieved only when the element is freely braided; this establishes the converse direction of a previous result. It is also shown that a simply laced Coxeter group has finitely many freely braided elements if and only if it has finitely many fully commutative elements.

math.CO↗
arXiv2003

Freely braided elements in Coxeter groups

We introduce a notion of "freely braided element" for simply laced Coxeter groups. We show that an arbitrary group element $w$ has at most $2^{N(w)}$ commutation classes of reduced expressions, where $N(w)$ is a certain statistic defined in terms of the positive roots made negative by $w$. This bound is achieved if $w$ is freely braided. In the type $A$ setting, we show that the bound is achieved only for freely braided $w$.

math.CO↗
arXiv2003

Acyclic heaps of pieces, I

Heaps of pieces were introduced by Viennot and have applications to algebraic combinatorics, theoretical computer science and statistical physics. In this paper, we how certain combinatorial properties of heaps studied by Fan and by Stembridge are closely related to the properties of a certain linear map $\partial_E$ associated to a heap $E$. We examine the relationship between $\partial_E$ and $\partial_F$ when $F$ is a subheap of $E$. This approach allows neat statements and proofs of results on certain associative algebras (generalized Temperley--Lieb algebras) that are otherwise tricky to prove. The key to the proof is to interpret the structure constants of the aforementioned algebras in terms of the maps $\partial$.

math.CO↗
arXiv2003

On rank functions for heaps

Motivated by work of Stembridge, we study rank functions for Viennot's heaps of pieces. We produce a simple and sufficient criterion for a heap to be a ranked poset and apply the results to the heaps arising from fully commutative words in Coxeter groups.

math.CO↗
arXiv2002

Standard modules for tabular algebras

We introduce cell modules for the tabular algebras defined in a previous work (math.QA/0107230); these modules are analogous to the representations arising from left Kazhdan--Lusztig cells. The standard modules of the title are constructed in an elementary way by suitable tensoring of the cell modules. We show how a certain extended affine Hecke algebra of type $A$ equipped with its Kazhdan--Lusztig basis is an example of a tabular algebra, and verify that in this case our standard modules coincide with other standard modules defined in the literature.

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