Let A be your favorite finite dimensional algebra, and Pi be a sets of representatives for the indecomposible projectives (or PIMs, if you like). Then we have the Cartan matrix C of the algebra, whose entries are dimHom(Pi,Pj). You can think of this as the matrix of the Euler form on the Grothendieck group K0(A−pmod) of projective A-modules.
Now, if the algebra A has finite global dimension, then we can define the classes of the simple heads of these Li in K0(A−pmod) as integer linear combinations of the Pi's, and [Pi] and [Li] are dual bases in the Euler form. That is, the matrix C is integer valued and has integer-valued inverse, i.e. it has determinant 1.
To what degree is the converse of this true? Is there a weaker hypothesis than finite global dimension itself such that det(C)=1 and that hypothesis will imply finite global dimension?
The application I have in mind for this is a little more complex. I'd like to consider a graded version of this question. So, let A be a graded algebra such that each degree is finite dimensional (and let say the appearing gradings are bounded below). The the graded version of C is well-defined in mathbbZ((q)), and similarly, if every simple has a resolution by projectives where only finitely many projectives generated in a given degree appear, this implies that this matrix has an inverse in mathbbZ((q)), that is determinant with leading coefficient 1.
The same question as above: can I use a hypothesis like the graded Cartan matrix having determinant with integral leading coefficient to conclude the existence of such a projective resolution?
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