
doi: 10.1007/bf02921824
Let \(T\) be the \(n\)-dimensional real torus \(\mathbb R^{n}/\mathbb Z^{n}\) and \(\beta T_{\mathbb C}=\mathbb R^{n}\times\mathbb R^{n}/\mathbb Z^{n}=(\mathbb C^{*})^{n}\) its complexification. Take a Kähler form \(\omega\) on \(T_{\mathbb C}\) which is invariant under the natural \(T\)-action on \(T_{\mathbb C}\). A straightforward calculation shows that \(\omega=\sqrt{-1}\partial\overline{\partial}F\) with a \(T\)-invariant function \(F\in C^{\infty}(\mathbb R^{n})\). It is shown that the \(T\)-action on \(T_{\mathbb C}\) preserving \(\omega\) is Hamiltonian and that its \(T\)-invariant moment map, considered as a map \(\Phi:\mathbb R^{n}\to \mathbb R^{n}\), satisfies the equation \(\varphi=\frac{1}{2}\operatorname {grad}F\). The Kähler form \(\omega\) represents the Chern class of the trivial holomorphic line bundle \(L\) on \(T_{\mathbb C}\). Let \(\nabla\) be a connection on \(L\) with curvature form \(\omega\). The \(T\)-action on \(T_{\mathbb C}\) can be lifted to a linear representation of \(T\) on \(H(L)\), the vector space of all holomorphic sections \(s\) in(\(L,\nabla\)), which is in a natural way isomorphic to the vector space \(\mathcal O(T_{\mathbb C})\) of holomorphic functions on \(T_{\mathbb C}\) (\(s\in H(L)\Leftrightarrow s\in C^{\infty}(T_{\mathbb C})\) and \(\nabla_{v}s=0\) for every antiholomorphic vector field \(v\) on \(T_{\mathbb C}\)). Therefore each irreducible representation of \(T\) occurs with multiplicity one in \(H(L)\). Given a \(T\)-invariant Hermitian metric on \(L\), the subspace \(H_{\omega}\) of \(L^{2}\)-sections \(s\in H(L)\) is \(T\)-invariant. The weights of \(T\) can be identified with the elements of the lattice \(\mathbb Z^{n}\subset \mathbb R^{n}\backsimeq {\mathfrak t}^{*}\). It is shown that the eigenspace \(H_{\lambda}(L),\lambda\in\mathbb Z^{n}\), is contained in \(H_{\omega}\) if and only if \(\lambda\in\Phi(\mathbb R^{n})\).
Representation theory for linear algebraic groups, Kähler manifolds, Complex Lie groups, group actions on complex spaces, Kähler form, moment map
Representation theory for linear algebraic groups, Kähler manifolds, Complex Lie groups, group actions on complex spaces, Kähler form, moment map
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