arXivDaily arXiv每日学术速递 周一至周五更新
arXiv 2012.07900math.RAmath.AGmath.ATmath.GR
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关于交换环上代数的生成元个数

On the number of generators of an algebra over a commutative ring

Uriya A. First, Zinovy Reichstein, Ben Willams

摘要:

O.福斯特说,如果$R$是一个诺特环的Krull维$d$,那么任何投射$R$ -模的秩$n$可以产生的$d+n$元素。S. Chase和R.斯旺随后证明了这个界限是尖锐的:存在不能由少于$d+n$元素生成的例子。我们把投射$R$ -模看作是无单位的$R$ -代数的$R$ -形式,其中任意两个元素的乘积是$0$ .前两个作者将福斯特定理推广到其他代数的形式(不一定是交换的,结合的或酉的); Shukla和第三作者然后表明,这种广义福斯特界是最佳的étale代数。本文证明了$k$ -代数的$R$ -形式的生成元个数的新的上、下界,其中$k$是一个无限域,$R$在$k$上有有限超越度$d$.特别是,我们表明,与预期相反,对于大多数类型的代数,广义福斯特界是远离最优。我们的结果是特别详细的Azumaya代数的情况下。我们的证明是基于重新解释的问题,作为一个问题近似的分类堆栈$BG$,其中$G$是自同构群的代数问题,由一定形式的代数空间。

英文摘要:

A theorem of O. Forster says that if $R$ is a noetherian ring of Krull dimension $d$, then any projective $R$-module of rank $n$ can be generated by $d+n$ elements. S. Chase and R. Swan subsequently showed that this bound is sharp: there exist examples that cannot be generated by fewer than $d+n$ elements. We view projective $R$-modules as $R$-forms of the non-unital $R$-algebra where the product of any two elements is $0$. The first two authors generalized Forster's theorem to forms of other algebras (not necessarily commutative, associative or unital); A. Shukla and the third author then showed that this generalized Forster bound is optimal for étale algebras. In this paper, we prove new upper and lower bound on the number of generators of an $R$-form of a $k$-algebra, where $k$ is an infinite field and $R$ has finite transcendence degree $d$ over $k$. In particular, we show that, contrary to expectations, for most types of algebras, the generalized Forster bound is far from optimal. Our results are particularly detailed in the case of Azumaya algebras. Our proofs are based on reinterpreting the problem as a question about approximating the classifying stack $BG$, where $G$ is the automorphism group of the algebra in question, by algebraic spaces of a certain form.

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