The 'Harder-Narasimhan trace' and unitarity of the KZ/Hitchin connection: genus 0

• Autores: Trivandrum Ramakrishnan Ramadas
• Localización: Annals of mathematics, ISSN 0003-486X, Vol. 169, Nº 1, 2009, págs. 1-39
• Idioma: inglés
• DOI: 10.4007/annals.2009.169.1
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• Resumen

  • Let a reductive group G act on a projective variety X+, and suppose given a lift of the action to an ample line bundle ?. By definition, all G-invariant sections of ? vanish on the nonsemistable locus X+nss. Taking an appropriate normal derivative defines a map H0(X+,?)G ? H0(Sµ,Vµ)G, where Vµ is a G-vector bundle on a G-variety Sµ. We call this the Harder-Narasimhan trace. Applying this to the Geometric Invariant Theory construction of the moduli space of parabolic bundles on a curve, we discover generalisations of �Coulomb-gas representations�, which map conformal blocks to hypergeometric local systems. In this paper we prove the unitarity of the KZ/Hitchin connection (in the genus zero, rank two, case) by proving that the above map lands in a unitary factor of the hypergeometric system. (An ingredient in the proof is a lower bound on the degree of polynomials vanishing on partial diagonals.) This elucidates the work of K. Gawedzki.