Difference between revisions of "Picard groups"

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(Added abelian condition for Picent trivial)
(Removed line saying no known examples of Picent nontrivial when defect group abelian.)
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It was recently shown by Eisele that <math>{\rm Pic}_\mathcal{O}(B)</math> must be finite when <math>B</math> is an <math>\mathcal{O}</math>-block<ref>See [[References#C|[Ei19]]]</ref>. However, in general <math>{\rm Pic}_k(B)</math> for a <math>k</math>-block <math>B</math> may be (and usually is) infinite.   
 
It was recently shown by Eisele that <math>{\rm Pic}_\mathcal{O}(B)</math> must be finite when <math>B</math> is an <math>\mathcal{O}</math>-block<ref>See [[References#C|[Ei19]]]</ref>. However, in general <math>{\rm Pic}_k(B)</math> for a <math>k</math>-block <math>B</math> may be (and usually is) infinite.   
 
At present there are no known examples where <math>{\rm Picent}_\mathcal{O}(B)</math> is nontrivial when <math>B</math> is an <math>\mathcal{O}</math>-block with abelian defect groups.
 
  
  

Revision as of 17:11, 22 November 2019

Definitions

The Picard group of an algebra is related to its automorphism group. Chapter 55 of [CuRe81b] gives an excellent introduction.

Let [math]R[/math] be a commutative ring (with identity) and [math]A[/math] an [math]R[/math]-order. The examples relevant here are finitely generated [math]k[/math] and [math]\mathcal{O}[/math]-algebras, mostly blocks and their basic algebras.

The Picard group [math]{\rm Pic}(A)={\rm Pic}_R(A)[/math] has elements the isomorphism classes of [math]A[/math]-[math]A[/math]-bimodules affording Morita self-eqivalences of [math]A[/math] (such bimodules are called invertible). It forms a group under taking tensor products of bimodules.

The subgroup of [math]{\rm Pic}(A)[/math] consisting of bimodules centralized by the centre [math]Z(A)[/math] is denoted [math]{\rm Picent}(A)[/math] or [math]{\rm Piccent}(A)[/math]. The isomorphism types of both [math]{\rm Pic}(A)[/math] and [math]{\rm Picent}(A)[/math] are Morita invariants.

The group [math]{\rm Aut}(A)={\rm Aut}_R(A)[/math] of algebra automorphisms of [math]A[/math] maps homomorphically to [math]{\rm Pic}(A)[/math], with kernel [math]{\rm Inn}(A)[/math], so [math]{\rm Out}(A)[/math] injects into [math]{\rm Pic}(A)[/math] with finite index. There is equality if [math]A[/math] is a basic algebra.[1]

It was recently shown by Eisele that [math]{\rm Pic}_\mathcal{O}(B)[/math] must be finite when [math]B[/math] is an [math]\mathcal{O}[/math]-block[2]. However, in general [math]{\rm Pic}_k(B)[/math] for a [math]k[/math]-block [math]B[/math] may be (and usually is) infinite.


Notes

  1. For detail see [CuRe81b,Chapter 55]
  2. See [Ei19]