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Register a new userCalorons, Nahms equations on S^1 and bundles over P^1xP^1
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The moduli space of solutions to Nahms equations of rank (k,k+j) on the circle, and hence, of SU(2) calorons of charge (k,j), is shown to be equivalent to the moduli of holomorphic rank 2 bundles on P^1xP^1 trivialized at infinity with c_2=k and equipped with a flag of degree j along P^1x{0}. An explicit matrix description of these spaces is given by a monad construction
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Nahms equations are viewed in a more general context where they appear as a vector field on a moduli space of co-Higgs bundles on the projective line. Zeros of this vector field correspond to torsion-free sheaves on a singular spectral curve which we translate in terms of a smooth curve in three-dimensional projective space. We also show how generalizations of Nahms equations are required when the spectral curve is non-reduced and deduce the existence of non-classical conserved quantities in this situation.
This paper is a continuation of our article (European J. Math., https://doi.org/10.1007/s40879-020-00419-8). The notion of a poor complex compact manifold was introduced there and the group $Aut(X)$ for a $P^1$-bundle over such a manifold was proven to be very Jordan. We call a group $G$ very Jordan if it contains a normal abelian subgroup $G_0$ such that the orders of finite subgroups of the quotient $G/G_0$ are bounded by a constant depending on $G$ only. In this paper we provide explicit examples of infinite families of poor manifolds of any complex dimension, namely simple tori of algebraic dimension zero. Then we consider a non-trivial holomorphic $P^1$-bundle $(X,p,Y)$ over a non-uniruled complex compact Kaehler manifold $Y$. We prove that $Aut(X)$ is very Jordan provided some additional conditions on the set of sections of $p$ are met. Applications to $P^1$-bundles over non-algebraic complex tori are given.
We introduce and study (strict) Schottky G-bundles over a compact Riemann surface X, where G is a connected reductive algebraic group. Strict Schottky representations are shown to be related to branes in the moduli space of G-Higgs bundles over X, and we prove that all Schottky $G$-bundles have trivial topological type. Generalizing the Schottky moduli map introduced in Florentino to the setting of principal bundles, we prove its local surjectivity at the good and unitary locus. Finally, we prove that the Schottky map is surjective onto the space of flat bundles for two special classes: when G is an abelian group over an arbitrary X, and the case of a general G-bundle over an elliptic curve.
Let $X$ be a set of $K$-rational points in $P^1 times P^1$ over a field $K$ of characteristic zero, let $Y$ be a fat point scheme supported at $ X$, and let $R_Y$ be the bihomogeneus coordinate ring of $Y$. In this paper we investigate the module of Kaehler differentials $Omega^1_{R_Y/K}$. We describe this bigraded $R_Y$-module explicitly via a homogeneous short exact sequence and compute its Hilbert function in a number of special cases, in particular when the support $X$ is a complete intersection or an almost complete intersection in $P^1 times P^1$. Moreover, we introduce a Kaehler different for $Y$ and use it to characterize reduced fat point schemes in $P^1 times P^1$ having the Cayley-Bacharach property.
We study the asymptotics as $puparrow 2$ of stationary $p$-harmonic maps $u_pin W^{1,p}(M,S^1)$ from a compact manifold $M^n$ to $S^1$, satisfying the natural energy growth condition $$int_M|du_p|^p=O(frac{1}{2-p}).$$ Along a subsequence $p_jto 2$, we show that the singular sets $Sing(u_{p_j})$ converge to the support of a stationary, rectifiable $(n-2)$-varifold $V$ of density $Theta_{n-2}(|V|,cdot)geq 2pi$, given by the concentrated part of the measure $$mu=lim_{jtoinfty}(2-p_j)|du_{p_j}|^{p_j}dv_g.$$ When $n=2$, we show moreover that the density of $|V|$ takes values in $2pimathbb{N}$. Finally, on every compact manifold of dimension $ngeq 2$ we produce examples of nontrivial families $(1,2) i pmapsto u_pin W^{1,p}(M,S^1)$ of such maps via natural min-max constructions.
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