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Hall-Littlewood polynomials and vector bundles on the Hilbert scheme

Let $E$ be the bundle defined by applying a polynomial representation of $GL_n$ to the tautological bundle on the Hilbert scheme of $n$ points in the complex plane. By a result of Haiman, the Cech cohomology groups $H^i(E)$ vanish for all $i>0$. It follows that the equivariant Euler characteristic with respect to the standard two-dimensional torus action has nonnegative coefficients in the torus variables $z_1,z_2$, because they count the dimensions of the weight spaces of $H^0(E)$. We derive a very explicit asymmetric formula for this Euler characteristic which has this property, by expanding known contour integral formulas for the Euler characteristic stemming from the quiver description in $z_2$, and calculating the coefficients using Jing's Hall-Littlewood vertex operator with parameter $z_1$.