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objects induce autoequivalences of triangulated categories by taking cones, called spherical
twist (see [2–4]).However, in the literature little can be found on classifying spherical objects
in some triangulated category. The aim of this paper is to determine all spherical objects, as
well as tilting objects, of the multiplicity free Brauer tree algebra with two edges.
To each finite connected tree one can associate a Brauer tree algebra combinatorially. In
the graph one exceptional vertex with positive multiplicity is allowed. When the multiplicity is
one, we say the Brauer tree algebra is multiplicity free. Brauer tree algebras are an important
class of finite dimensional, representation finite and symmetric algebras. For example, they
describe the blocks of the group algebra of finite groups with cyclic defect groups (see [5,
Theorem 6.5.5]).
Note that a connected tree with two edges and without exceptional vertex is unique. Hence
the multilplicity free Brauer tree algebra with two edges is unique, up to Morita equivalence.
It is the path algebra of a 2-cycle modulo all paths of length ≥ 3. Rouquier and Zimmermann
[6] studied the derived Picard group of this algebra and found generators which satisfy braid
group relations. This enables us to classify tilting objects and spherical objects in the derived
category, see Theorem 2.1. Indeed, the spherical objects are precisely the indecomposable direct
summands of tilting objects. For this we use Aihara’s result [7] that for representation finite
and symmetric algebras, pretilting objects are also partial tilting. Moreover, we introduce the
socalled n-complexes to describe the spherical objects, as well as the tilting objects, up to shift
and symmetry, see Theorem 3.8.
The paper is organized as follows. In Section 2 we recall the definition of spherical objects,
tilting objects and derived Picard groups, and and collect some known results about Brauer
tree algebras. In section 3 we concentrate on the multiplicity free Brauer tree algebra with two
edges and give a classification of spherical objects and tilting objects. In section 4 we define
n-complex and describe the spherical objects, as well as the tilting objects explicitly.
Throughout the paper k is an algebraically closed field, D = Hom
k
(−, k) the usual k-dual
and ⊗ = ⊗
k
when the subscript is dropped. For a finite dimensional k-algebra A, write modA
for the category of finite dimensional left A-modules, D
b
(A) the bounded derived category
of modA, and K
b
(projA) the bounded homotopy category of finite dimensional projective A-
modules. It is natural to view K
b
(projA) as a full subcategory of D
b
(A). The Hom-space
and endomorphism space in D
b
(A) or K
b
(projA) will be simply denoted by Hom
A
(−, −) and
End
A
(−) respectively.
1 Preliminaries
In this section we recall the definition of spherical objects, tilting objects and derived Picard
groups, and collect some known results about Brauer tree algebras. As a corollary we prove
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