Harder–Narasimhan reduction for principal bundles over a compact Kähler manifold |
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Authors: | Boudjemaa Anchouche Hassan Azad Indranil Biswas |
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Institution: | (1) Department of Mathematics, Sultan Qaboos University, Muscat, Oman (e-mail:anchouch@squ.edu.om) , OM;(2) Department of Mathematical Sciences, King Fahd University, Dhahran 31261, Saudi Arabia (e-mail: hassanaz@kfupm.edu.sa) , SA;(3) School of Mathematics, Tata Institute of Fundamental Research, Homi Bhabha Road, Bombay 400005, India (e-mail: indranil@math.tifr.res.in) , IN |
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Abstract: | Generalizing the Harder–Narasimhan filtration of a vector bundle it is shown that a principal G-bundle over a compact K?hler manifold admits a canonical reduction of its structure group to a parabolic subgroup of G. Here G is a complex connected reductive algebraic group; in the special case where , this reduction is the Harder–Narasimhan filtration of the vector bundle associated to by the standard representation of . The reduction of in question is determined by two conditions. If P denotes the parabolic subgroup, L its Levi factor and the canonical reduction, then the first condition says that the principal L-bundle obtained by extending the structure group of the P-bundle using the natural projection of P to L is semistable. Denoting by the Lie algebra of the unipotent radical of P, the second condition says that for any irreducible P-module V occurring in , the associated vector bundle is of positive degree; here is considered as a P-module using the adjoint action. The second condition has an equivalent reformulation which says that for any nontrivial
character of P which can be expressed as a nonnegative integral combination of simple roots (with respect to any Borel subgroup contained
in P), the line bundle associated to for is of positive degree. The equivalence of these two conditions is a consequence of a representation theoretic result proved
here.
Received: 10 November 1999 / Revised version: 31 October 2001 / Published online: 26 April 2002 |
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Keywords: | Mathematics Subject Classification (2000): 32L05 53C05 |
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