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LOCAL LIMITATIONS OF THE Ext FUNCTOR DO NOT EXIST

Published online by Cambridge University Press:  30 January 2006

S. O. SMALØ
Affiliation:
Norwegian University of Science and Technology, Department of Mathematical Sciences, N-7491 Trondheim, [email protected]
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Abstract

In this note it is shown that for $k$ a field, and for the four-dimensional algebra $\Lambda=k\langle x,y\rangle /\langle x^2,y^2,xy+qyx\rangle$ when $q^n\neq 1,0$ for all $n$, there exist a two-dimensional module $M$ and a family of two-dimensional modules $M_i$, $i=1,2,\ldots$, such that $\dim_k\Ext^i_\Lambda(M,M_j)=1$ for $i$ equal to 0, $j$ and $j+1$, and $\dim_k\Ext^i_\Lambda(M,M_j)=0$ otherwise. This is probably the most straightforward example giving a negative answer to a question raised by Maurice Auslander.

Type
Papers
Copyright
The London Mathematical Society 2006

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Footnotes

The paper was written when the author was visiting the Mittag-Leffler institute in Stockholm.