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arXiv 2003.14051math.OAmath.DS
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可分解部分作用

Decomposable partial actions

Fernando Abadie, Eusebio Gardella, Shirly Geffen

摘要:

定义了离散群在$C^*$ -代数上的部分作用的分解性质。可分解的部分系统在实践中自然出现,许多常见的部分动作可以分解成具有分解性质的部分动作。例如,有限群的任何部分作用都是可分解系统的迭代扩张。具有分解性质的部分行动总是可全球化的和可接受的,不管行动组是什么,它们的全球化可以用某些全球子系统来明确描述。还进行了它们的交叉积的直接计算。我们发现,部分行动的分解性质的行为在许多方面像全球行动的有限群(即使当代理组是无限的),这使得他们的研究特别容易。例如,在不动点代数上存在一个典型的忠实条件期望,而且它是交叉积中的一个角。(Both对于有限群的部分作用,这些事实中的一个一般为假。)作为应用,我们证明了具有分解性质的拓扑部分作用的自由性等价于它的不动点代数是Morita等价于它的交叉积。我们还通过例子表明,这失败的一般部分行动的有限群。

英文摘要:

We define the decomposition property for partial actions of discrete groups on $C^*$-algebras. Decomposable partial systems appear naturally in practice, and many commonly occurring partial actions can be decomposed into partial actions with the decomposition property. For instance, any partial action of a finite group is an iterated extension of decomposable systems. Partial actions with the decomposition property are always globalizable and amenable, regardless of the acting group, and their globalization can be explicitly described in terms of certain global sub-systems. A direct computation of their crossed products is also carried out. We show that partial actions with the decomposition property behave in many ways like global actions of finite groups (even when the acting group is infinite), which makes their study particularly accessible. For example, there exists a canonical faithful conditional expectation onto the fixed point algebra, which is moreover a corner in the crossed product in a natural way. (Both of these facts are in general false for partial actions of finite groups.) As an application, we show that freeness of a topological partial action with the decomposition property is equivalent to its fixed point algebra being Morita equivalent to its crossed product. We also show by example that this fails for general partial actions of finite groups.

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