2.4. The model torus fibrations as Kähler manifolds [03EY]
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2.4. The model torus fibrations as Kähler manifolds
The -torus fibration (more naturally, a -torsor) will depend on additional phase multi-parameter , where all have values in . First, we form (trivial) affine torus bundles over the affine open sets by identifying the fibers with the affine tori
The gluing maps are independent of a base point in an overlap and given there by the natural projection . This defines the torsor .
The topology of the total space of the torsor is determined by the combinatorics of , i.e., independent of and as long as is in the right secondary cone . In particular, all are diffeomorphic to each other, though not canonically.
Since the linear parts of the transition maps are the same for the base and for the fibers, the tangent space at any point in splits canonically as
This allows to define a canonical (integrable) almost complex structure on as
Given a Riemannian metric on , one can define the pullback metric on which is, in fact, Kähler. If, in addition, satisfy the real Monge-Ampère equation, the induced metric on is Ricci-flat.
There is another slightly different description of the torus bundle over a Kähler affine manifold (cf. [KS01]), which is useful when considering limiting behavior of Calabi-Yau degenerations. We define a torus fibration as the quotient of the total space of the tangent bundle by the integral lattice spanned by , where are the affine coordinates. This torus bundle carries canonical complex structure and the pullback metric which comes from the splitting as before.
But in order to make a connection with the previous picture and with the geometry of toric hypersurfaces we need to twist the complex structure on by the element of associated with the phase parameters . The resulting Kähler manifold can be canonically identified with constructed by the first method starting with the -rescaled Kähler affine structure.