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arXiv math/0602626math.AGmath.AC
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分支簇的完备模空间

Complete moduli spaces of branchvarieties

Valery Alexeev, Allen Knutson

摘要:

具有固定希尔伯特多项式的P^n的子簇空间是不完备的。Grothendieck将“簇”放松为“概型”,定义了完备化,给出了P^n的子概型的完备Hilbert概型.相反,我们将“sub”放宽为“branch”,其中_ P^n的_branchvariety被定义为具有到_ P^n的有限态射的_reduced_(尽管可能可约)方案。我们的主要定理是:具有固定Hilbert多项式和i维分支的全次数的P^n的分支簇的模栈是一个具有有限稳定子的真(完全的和分离的)Artin栈,并且有一个粗糙的模空间,它是一个真代数空间.分支簇的族比子概型的族有更多的局部常数不变量;例如,连通分支的个数是一个新的不变量。在特征为0时,可以将这个计数扩展到将Z标记的有根森林与任何branchvariety相关联。

英文摘要:

The space of subvarieties of P^n with a fixed Hilbert polynomial is not complete. Grothendieck defined a completion by relaxing "variety" to "scheme", giving the complete_Hilbert scheme_ of subschemes of P^n with fixed Hilbert polynomial. We instead relax "sub" to "branch", where a_branchvariety of_ P^n is defined to be a_reduced_ (though possibly reducible) scheme_with a finite morphism to_ P^n. Our main theorems are that the moduli stack of branchvarieties of P^n with fixed Hilbert polynomial and total degrees of i-dimensional components is a proper (complete and separated) Artin stack with finite stabilizer, and has a coarse moduli space which is a proper algebraic space. Families of branchvarieties have many more locally constant invariants than families of subschemes; for example, the number of connected components is a new invariant. In characteristic 0, one can extend this count to associate a Z-labeled rooted forest to any branchvariety.

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