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From Wikipedia, the free encyclopedia
In algebraic geometry, given a Deligne–Mumford stack X, a perfect obstruction theory for X consists of:
The notion was introduced by Kai Behrend and Barbara Fantechi (1997) for an application to the intersection theory on moduli stacks; in particular, to define a virtual fundamental class.
Consider a regular embedding fitting into a cartesian square
where are smooth. Then, the complex
forms a perfect obstruction theory for X.[1] The map comes from the composition
This is a perfect obstruction theory because the complex comes equipped with a map to coming from the maps and . Note that the associated virtual fundamental class is
Consider a smooth projective variety . If we set , then the perfect obstruction theory in is
and the associated virtual fundamental class is
In particular, if is a smooth local complete intersection then the perfect obstruction theory is the cotangent complex (which is the same as the truncated cotangent complex).
The previous construction works too with Deligne–Mumford stacks.
By definition, a symmetric obstruction theory is a perfect obstruction theory together with nondegenerate symmetric bilinear form.
Example: Let f be a regular function on a smooth variety (or stack). Then the set of critical points of f carries a symmetric obstruction theory in a canonical way.
Example: Let M be a complex symplectic manifold. Then the (scheme-theoretic) intersection of Lagrangian submanifolds of M carries a canonical symmetric obstruction theory.
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