We introduce a new class of algebras, the Nakayama oriented pullbacks, obtained from pullbacks of surjective morphisms of algebras A C and B C. We prove that such a pullback is tilted when A and B are hereditary. We also show that stably hereditary algebras respecting the clock condition are Nakayama oriented pullbacks, and we use results about these pullbacks to show when a stably hereditary algebra is tilted or iterated tilted. MSC: 16G20; 16G70; 16S50
IntroductionModule categories and pullbacks of rings and algebras have been studied from many points of view (see, for instance, [1,6,11,12,15,18]), but not from the tilting point of view. In this paper, we introduce a particular class of pullbacks of algebras over an algebraically closed field K , which we call Nakayama oriented pullbacks. These are pullbacks of surjective morphisms of algebras A C and B C with C a hereditary Nakayama algebra. We construct a tilting module T over this kind of pullback, and we compute the endomorphism algebra End T of this module when B is hereditary (see 2.4.6). A first consequence of this result is the principal theorem of this paper:Theorem. Let R be a Nakayama oriented pullback of K -algebra surjective morphisms A C and B C. Suppose that A and B are hereditary. Then R is tilted.We next show that a stably hereditary algebra respecting the clock condition, that is, such that the number of clockwise oriented relations on each cycle of its bound quiver equals the number of counterclockwise oriented relations, can be expressed as a Nakayama oriented pullback. As a second consequence of the main theorem above we give a new proof of Theorem 2.6 of [14] which characterizes iterated tilted and tilted stably hereditary algebras.This paper consists of three sections. The first is devoted to preliminaries, the second to Nakayama oriented pullbacks, and the third to stably hereditary algebras. * Tel.
We prove that a stably hereditary bound quiver algebra A = KQ/I is iterated tilted if and only if (Q, I) satisfies the clock condition, and that in this case it is of type Q. Furthermore, A is tilted if and only if (Q, I) does not contain any doublezero.
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