most recent 30 from http://mathoverflow.net 2013-05-19T10:36:56Z http://mathoverflow.net/feeds/question/93862 http://www.creativecommons.org/licenses/by-nc/2.5/rdf http://mathoverflow.net/questions/93862/simplicial-complex-made-of-central-idempotents-of-an-algebra Simon Lentner 2012-04-12T12:35:23Z 2012-04-12T13:17:31Z

<p>Let $A$ be an algebra, say over $\mathbb{C}$ and finite-dimensional, but not necessary semisimple. I have the strong feeling, which I would like to prove and use, about the following rather natural approach.</p> <p>Consider the set $S$ of central idempotents in $A$, then I want to turn the set of central idempotents $\Delta$ into an <strong>abstract simplicial complex</strong>. Two idempotents $e_1,e_2$ are connected iff $e_1\in e_2A$, simplices consist of completely connected subset and and the highest-dimensional simplices hence are flags associated to a decomposition of $A$ into central <strong>primitive</strong> idempotents. This is much how one defines buildings as simplicial complexes e.g. of subspaces of a finite vector space....</p> <p>Is this OK? I'm not familiar with calculating with central prinimtive idempotents - especially I'm concerned, <strong>if any decomposistion into central primitive idempotents has the same cardinality??</strong></p> <p>..<strong>if yes</strong>,it seems so natural, something similar ought to exist (surely more advanced ;-) - <strong>any reference?</strong> I would probably also suffice with a related construction and/or I want to find out more established facts about such an "algebraic simplicial complex / building"...Thanks for your help in advance.</p>

http://mathoverflow.net/questions/93862/simplicial-complex-made-of-central-idempotents-of-an-algebra/93863#93863 Benjamin Steinberg 2012-04-12T12:47:28Z 2012-04-12T13:17:31Z

<p>The central idempototents of a finite dimensional algebra form a finite Boolean algebra with 1 as the max and the central primitive idempotents as the atoms. The decomposition of 1 into central idempotents is thus unique. So the central idempotents are the face lattice of a simplex. The order is $e\leq f$ if $e\in fA$. Your simplicial complex would be the order complex of this Boolean algebra and so would be the barycentric subdivision of a simplex. </p> <p><strong>Added details.</strong> The boolean algebra operations are given by $e\wedge f=ef$, $e\vee f=e+f-ef$ and $\neg e=1-e$. The finiteness follows, for example, because one can look at the regular representation of the algebra $A$ by matrices and observe that the central idempotents form a commutative semigroup of idempotent matrices. Such a semigroup is simultaneously diagonalizable and there are only $2^n$ diagonal idempotent $n\times n$ matrices.</p>