The Tangent Space at a Special Symplectic Instanton Bundle on $P_{2n+1}$

Abstract

Mathematical instanton bundles on $P_3$ have their analogues in rank--$2n$ instanton bundles on odd dimensional projective spaces $P_{2n+1}$. The families of special instanton bundles on these spaces generalize the special 'tHooft bundles on $P_3$. We prove that for a special symplectic instanton bundle $E$ on $P_{2n+1}$ with $c_2=k$ $h^{1}End(E)=4(3n-1)k+(2n-5)(2n-1)$. Therefore the dimension of the moduli space of instanton bundles grows linearly in $k$. The main difference with the well known case of $P_3$ is that $h^{2}End(E)$ is nonzero, in fact we prove that it grows quadratically in $k$. Special symplectic instanton bundles turn out to be singular points of the moduli space. Such bundles $E$ are $SL(2)$--invariant and the result is obtained regarding the cohomology groups of $E$ as $SL(2)$--representations.

13 pages, LaTex (Reason for resubmission: correction of a Tex misprint in the title page)