
Journal of Lie Theory 19 (2009), No. 3, 463481 Copyright Heldermann Verlag 2009 LUDecomposition of a Noncommutative Linear System and Jacobi Polynomials Alfredo O. Brega CIEMFaMAF, Universidad Nacional de Córdoba, Ciudad Universitaria, 5000 Córdoba, Argentina brega@famaf.unc.edu.ar Leandro R. Cagliero CIEMFaMAF, Universidad Nacional de Córdoba, Ciudad Universitaria, 5000 Córdoba, Argentina cagliero@famaf.unc.edu.ar [Abstractpdf] \def\a{{\frak a}} \def\g{{\frak g}} \def\k{{\frak k}} We obtain the LUdecomposition of a non commutative linear system of equations that, in the rank one case, characterizes the image of the Lepowsky homomorphism $U(\g)^{K}\to U(\k)^{M} \otimes U(\a)$. Although this system can not be expressed as a single matrix equation with coefficients in $U(\k)$, it turns out that obtaining a triangular system equivalent to it, can be reduced to obtaining the LUdecomposition of a matrix $\widetilde M_0$ with entries in a polynomial algebra. We prove that both the Lpart and Upart of $\widetilde M_0$ are expressed in terms of Jacobi polynomials. Moreover, each entry of the Lpart of $\widetilde M_0$ and of its inverse is given by a single ultraspherical Jacobi polynomial. This fact yields a biorthogonality relation between the ultraspherical Jacobi polynomials. Keywords: Noncommutative LUfactorization, Jacobi polynomials, Kinvariants in the enveloping algebra of g, Lepowsky homomorphism. MSC: 33C45, 22E46; 33C05, 16S30 [ Fulltextpdf (229 KB)] for subscribers only. 