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Register a new userThe Terwilliger algebra of the halved cube
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Let $Dgeq 3$ denote an integer. For any $xin mathbb F_2^D$ let $w(x)$ denote the Hamming weight of $x$. Let $X$ denote the subspace of $mathbb F_2^D$ consisting of all $xin mathbb F_2^D$ with even $w(x)$. The $D$-dimensional halved cube $frac{1}{2}H(D,2)$ is a finite simple connected graph with vertex set $X$ and $x,yin X$ are adjacent if and only if $w(x-y)=2$. Fix a vertex $xin X$. The Terwilliger algebra $mathcal T=mathcal T(x)$ of $frac{1}{2}H(D,2)$ with respect to $x$ is the subalgebra of ${rm Mat}_X(mathbb C)$ generated by the adjacency matrix $A$ and the dual adjacency matrix $A^*=A^*(x)$ where $A^*$ is a diagonal matrix with $$ A^*_{yy}=D-2w(x-y) qquad hbox{for all $yin X$}. $$ In this paper we decompose the standard $mathcal T$-module into a direct sum of irreducible $mathcal T$-modules.
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The adjacency matrix of a symplectic dual polar graph restricted to the eigenspaces of an abelian automorphism subgroup is shown to act as the adjacency matrix of a weighted subspace lattice. The connection between the latter and $U_q(sl_2)$ is used to find the irreducible components of the standard module of the Terwilliger algebra of symplectic dual polar graphs. The multiplicities of the isomorphic submodules are given.
We provide a Hopf algebra structure on the space of superclass functions on the unipotent upper triangular group of type D over a finite field based on a supercharacter theory constructed by Andre and Neto. Also, we make further comments with respect to types B and C. Type A was explores by M. Aguiar et. al (2010), thus this paper is a contribution to understand combinatorially the supercharacter theory of the other classical Lie types.
We find by applying MacMahons partition analysis that all magic labellings of the cube are of eight types, each generated by six basis elements. A combinatorial proof of this fact is given. The number of magic labellings of the cube is thus reobtained as a polynomial in the magic sum of degree $5$. Then we enumerate magic distinct labellings, the number of which turns out to be a quasi-polynomial of period 720720. We also find the group of symmetry can be used to significantly simplify the computation.
The Ramsey number r(K_3,Q_n) is the smallest integer N such that every red-blue colouring of the edges of the complete graph K_N contains either a red n-dimensional hypercube, or a blue triangle. Almost thirty years ago, Burr and ErdH{o}s conjectured that r(K_3,Q_n) = 2^{n+1} - 1 for every n in N, but the first non-trivial upper bound was obtained only recently, by Conlon, Fox, Lee and Sudakov, who proved that r(K_3,Q_n) le 7000 cdot 2^n. Here we show that r(K_3,Q_n) = (1 + o(1)) 2^{n+1} as n to infty.
For the Drinfeld double $D_n$ of the Taft algebra $A_n$ defined over an algebraically closed field $mathbb k$ of characteristic zero using a primitive $n$th root of unity $q in mathbb k$ for $n$ odd, $nge3$, we determine the ribbon element of $D_n$ explicitly. We use the R-matrix and ribbon element of $D_n$ to construct an action of the Temperley-Lieb algebra $mathsf{TL}_k(xi)$ with $xi = -(q^{frac{1}{2}}+q^{-frac{1}{2}})$ on the $k$-fold tensor product $V^{otimes k}$ of any two-dimensional simple $D_n$-module $V$. When $V$ is the unique self-dual two-dimensional simple module, we develop a diagrammatic algorithm for computing the $mathsf{TL}_k(xi)$-action. We show that this action on $V^{otimes k}$ is faithful for arbitrary $k ge 1$ and that $mathsf{TL}_k(xi)$ is isomorphic to the centralizer algebra $text{End}_{D_n}(V^{otimes k})$ for $1 le kle 2n-2$.
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