Blocks of small defect

On the source algebra equivalence class of blocks with cyclic defect groups, I

We investigate the source algebra class of a p-block with cyclic defect groups of the group algebra of a finite group. By the work of Linckelmann this class is parametrized by the Brauer tree of the block together with a sign function on its vertices and an endo-permutation module of a defect group. We prove that this endo-permutation module can be read off from the character table of the group. We also prove that this module is trivial for all cyclic p-blocks of quasisimple groups with a simple quotient which is a sporadic group, an alternating group, a group of Lie type in defining character…

Counting characters in blocks of solvable groups with abelian defect group

If $G$ is a solvable group and $p$ is a prime, then the Fong-Swan theorem shows that given any irreducible Brauer character $ϕ$ of $G$, there exists a character $χ\in \irrg$ such that $χ^o = ϕ$, where $^o$ denotes the restriction of $χ$ to the $p$-regular elements of $G$. We say that $χ$ is a {\it{lift}} of $ϕ$ in this case. It is known that if $ϕ$ is in a block with abelian defect group $D$, then the number of lifts of $ϕ$ is bounded above by $|D|$. In this paper we give a necessary and sufficient condition for this bound to be achieved, in terms of local information in a subgroup $V$ determi…

The inductive Alperin-McKay and blockwise Alperin weight conditions for blocks with cyclic defect groups

We verify the inductive blockwise Alperin weight (BAW) and the inductive Alperin-McKay (AM) conditions introduced by the second author for blocks of finite quasisimple groups with cyclic defect groups. Furthermore we establish a criterion that describes conditions under which the inductive AM condition for blocks with abelian defect groups implies the inductive BAW condition for those blocks.

Orders of units in integral group rings and blocks of defect $1$

We show that if the Sylow $p$-subgroup of a finite group $G$ is of order $p$, then the normalized unit group of the integral group ring of $G$ contains a normalized unit of order $pq$ if and only if $G$ contains an element of order $pq$, where $q$ is any prime. We use this result to answer the Prime Graph Question for most sporadic simple groups and some simple groups of Lie type, including a new infinite series of such groups. Our methods are based on the understanding of blocks of cyclic defect and Young tableaux combinatorics.

Splendid Morita equivalences for principal blocks with semidihedral defect groups

We classify principal blocks of finite groups with semidihedral defect groups up to splendid Morita equivalence. This completes the classification of all principal $2$-blocks of tame representation type up to splendid Morita equivalence and shows that Puig's Finiteness Conjecture holds for such blocks.

Profinite群块的块Pro-fusion系统与具有无限二面体亏群的块

Block pro-fusion systems for profinite groups and blocks with infinite dihedral defect groups

本文引入profinite群块的块pro-fusion系统，证明Puig幂零块结构定理的profinite版本，进而得出以无限二面体pro-2群为亏群的块仅一个Morita等价类的结论。

Descent of equivalences for blocks with Klein four defect groups

We show that the splendid Rickard complexes for blocks with Klein four defect groups constructed by Rickard and Linckelmann descend to non-split fields. As a corollary, Navarro's refinement of the Alperin-McKay conjecture holds for blocks with a Klein four defect group. We also prove that splendid Morita equivalences between blocks and their Brauer correspondents (if exist) descend to non-split situations.

On the Alperin-McKay conjecture for 2-blocks of maximal defect

In this paper, we show that the Alperin-McKay conjecture holds for 2-blocks of maximal defect. A major step in the proof is the verification of the inductive Alperin-McKay condition for the principal 2-block of groups of Lie type in odd characteristic.

Blocks of profinite groups with cyclic defect group

We demonstrate that the blocks of a profinite group whose defect groups are cyclic have a Brauer tree algebra structure analogous to the case of finite groups. We show further that the Brauer tree of a block with defect group $\mathbb{Z}_{p}$ is of star type.

Conjectures of Alperin and Broue for 2-blocks with elementary abelian defect groups of order 8

Using the classification of finite simple groups we prove Alperin's weight conjecture and the character theoretic version of Broue's abelian defect group conjecture for 2-blocks of finite groups with an elementary abelian defect group of order 8.

Blocks with abelian defect groups of rank 2 and one simple module

In this paper, we investigate the block that has an abelian defect group of rank $2$ and its Brauer correspondent has only one simple module. We will get an isotypy between the block and its Brauer correspondent. It will generalize the result of Kessar and Linckelmann (\cite{KL}).

Blocks with a generalized quaternion defect group and three simple modules over a 2-adic ring

We show that two blocks of generalized quaternion defect with three simple modules over a sufficiently large $2$-adic ring $\mathcal O$ are Morita-equivalent if and only if the corresponding blocks over the residue field of $\mathcal O$ are Morita-equivalent. As a corollary we show that any two blocks defined over $\mathcal O$ with three simple modules and the same generalized quaternion defect group are derived equivalent.

2-blocks with an abelian defect group and a freely acting cyclic inertial quotient

We study blocks with an abelian defect group and a cyclic inertial quotient acting freely but not transitively. We prove that when p=2, such blocks are inertial, i.e. basic Morita equivalent to their Brauer correspondent. Together with a result of the second author on Singer cycle actions on homocyclic defect groups, this completes the classification of 2-blocks with a cyclic inertial quotient acting freely on an abelian defect group.

Blocks with quaternion defect group over a 2-adic ring: the case \tilde{A}_4

Except for blocks with a cyclic or Klein four defect group, it is not known in general whether the Morita equivalence class of a block algebra over a field of prime characteristic determines that of the corresponding block algebra over a p-adic ring. We prove this to be the case when the defect group is quaternion of order 8 and the block algebra over an algebraically closed field k of characteristic 2 is Morita equivalent to $k\tilde A_4$. The main ingredients are Erdmann's classification of tame blocks and work of Cabanes and Picaronny on perfect isometries between tame blocks.

关于具有四元数亏格群的2-块

On $2$-blocks with quaternion defect groups

该研究确定了任意2幂阶且具有四元数亏格群的2-块的Morita等价类，完成了驯服表示型块的Donovan猜想的证明。

2-blocks with abelian defect groups

We give a classification, up to Morita equivalence, of 2-blocks of quasi-simple groups with abelian defect groups. As a consequence, we show that Donovan's conjecture holds for elementary abelian 2-groups, and that the entries of the Cartan matrices are bounded in terms of the defect for arbitrary abelian 2-groups. We also show that a block with defect groups of the form $C_{2^m} \times C_{2^m}$ for $m \geq 2$ has one of two Morita equivalence types and hence is Morita equivalent to the Brauer correspondent block of the normaliser of a defect group. This completes the analysis of the Morita eq…

Broué's Conjecture for 2-blocks with elementary abelian defect groups of order 32

The first author has recently classified the Morita equivalence classes of 2-blocks B of finite groups with elementary abelian defect group of order 32. In all but three cases he proved that the Morita equivalence class determines the inertial quotient of B. We finish the remaining cases by utilizing the theory of lower defect groups. As a corollary, we verify Broué's Abelian Defect Group Conjecture in this situation.

The classification of the trivial source modules in blocks with cyclic defect groups

Relying on the classification of the indecomposable liftable modules in arbitrary blocks with non-trivial cyclic defect groups we give a complete classification of the trivial source modules lying in such blocks, describing in particular their associated path on the Brauer tree of the block in the sense of Janusz (1969). The appendix contains a description of the minimal distance from an arbitrary non-projective indecomposable liftable module to the boundary of the stable Auslander-Reiten quiver of the block.

惯性2块与阿贝尔缺陷群

Inertial 2-blocks with abelian defect groups

本文证明了具有阿贝尔缺陷群的2块为惯性块，方法基于缺陷群的结构分析。

Morita equivalence classes of blocks with elementary abelian defect groups of order 32

We describe a general technique to classify blocks of finite groups, and we apply it to determine Morita equivalence classes of blocks with elementary abelian defect groups of order 32 with respect to a complete discrete valuation ring with an algebraically closed residue field of characteristic two. As a consequence we verify that a conjecture of Harada holds on these blocks.

Splendid Morita equivalences for principal blocks with generalised quaternion defect groups

We prove that splendid Morita equivalences between principal blocks of finite groups with dihedral Sylow $2$-subgroups realised by Scott modules can be lifted to splendid Morita equivalences between principal blocks of finite groups with generalised quaternion Sylow $2$-subgroups realised by Scott modules.

Picard groups for some blocks with TI defect groups

We calculate the Picard groups for principal blocks $B$ with TI defect groups and cyclic inertial quotient. The methods used generalize results on self stable equivalences and take advantage of the existence of equivalences given by Green correspondence in this setting. In particular, we show that $\text{Pic}(B)=\mathcal{E}(B)$, giving more evidence for a conjecture on basic auto-Morita equivalences.

Morita equivalence classes of principal blocks with elementary abelian defect groups of order 64

We classify the Morita equivalence classes of principal blocks with elementary abelian defect groups of order 64 with respect to a complete discrete valuation ring with an algebraically closed residue field of characteristic two.

On defect groups for generalized blocks of the symmetric group

In a paper of 2003, B. Külshammer, J. B. Olsson and G. R. Robinson defined $\ell$-blocks for the symmetric groups, where $\ell >1$ is an arbitrary integer. In this paper, we give a definition for the defect group of the principal $\ell$-block. We then check that, in the Abelian case, we have an analogue of one of M. Broué's conjectures.

Donovan's conjecture and blocks with abelian defect groups

We give a reduction of Donovan's conjecture for abelian groups to a similar statement for quasisimple groups. Consequently we show that Donovan's conjecture holds for abelian $2$-groups.

On blocks stably equivalent to a quantum complete intersection of dimension 9 in characteristic 3 and a case of the abelian defect group conjecture

Using a stable equivalence due to Rouquier, we prove that Broue's abelian defect group conjecture holds for 3-blocks of defect 2 whose Brauer correspondent has a unique isomorphism class of simple modules. The proof makes use of the fact, also due to Rouquier, that a stable equivalence of Morita type between self-injective algebras induces an isomorphism between the connected components of the outer automorphism groups of the algebras.

$2$-Blocks whose defect group is homocyclic and whose inertial quotient contains a Singer cycle II

We consider $2$-blocks of finite groups with defect group $D=Q \times R$ and inertial quotient $\mathbb{E}$ where $Q \cong (C_{2^m})^n$, $R \cong C_{2^r}$, and $\mathbb{E}$ contains a Singer cycle of $\operatorname{Aut}(Q)$ (an element of order $2^n-1$). We classify such blocks up to Morita equivalence when either $\mathbb{E}$ is cyclic or $r=1$. We achieve a partial classification when $r>1$ and $E$ is non-cyclic.

具有循环亏群的块中的自对偶不可分解模

The self-dual indecomposable modules in blocks with cyclic defect groups

本文研究具有循环亏群的有限群 $p$-块中的自对偶不可分解模，并确定每个模是辛型还是正交型。

Lower defect groups and vertices of simple modules

We compare lower defect groups associated with $p$-regular classes and vertices of simple modules for a block of a finite group algebra. We show that lower defect groups are contained in vertices of simple modules after suitable reordering. Moreover, for a block of principal type, we show that a $p$-regular lower defect group which contains a vertex of simple module is a defect group of the block.

具有超特殊亏格群 $p_+^{1+2}$ 的块的Morita等价类

Morita equivalence classes of blocks with extraspecial defect groups $p_+^{1+2}$

本研究刻画了 $p\geq5$ 时具有超特殊亏格群 $p_+^{1+2}$ 的块的Morita等价类，验证了相关猜想对这类 $p$-群成立，还将 $p=3$ 时的Donovan猜想约化，并以 $p=5$ 为例列出对应等价类。

Splendid Morita equivalences for principal 2-blocks with dihedral defect groups

Given a dihedral $2$-group $P$ of order at least~8, we classify the splendid Morita equivalence classes of principal $2$-blocks with defect groups isomorphic to $P$. To this end we construct explicit stable equivalences of Morita type induced by specific Scott modules using Brauer indecomposability and gluing methods; we then determine when these stable equivalences are actually Morita equivalences, and hence automatically splendid Morita equivalences. Finally, we compute the generalised decomposition numbers in each case.

亏群为Suzuki 2-群的块

Blocks whose defect groups are Suzuki $2$-groups

本研究完成了亏群为Suzuki 2-群的块的基本Morita等价分类，验证了Donovan猜想对这类群成立，并推导得到无非平凡奇阶正规子群的有限群的Suzuki Sylow 2-子群为平凡交的推论。

有限群块的函子等价类研究

On functorial equivalence classes of blocks of finite groups

本文在与Bouc共同提出有限群块函子等价概念的基础上，分类了特征0代数闭域上循环亏群块及亏数2、3的2-块的函子等价类，证明其仅由块的融合系统决定。

profinite 块与亏群为 $\mathbb Z_p^n$ 的块的有限性性质

Finiteness properties of profinite blocks and blocks with defect group $\mathbb Z_p^n$

研究profinite群块的有限性性质，证明拓扑有限生成亏群的块同构于virtually pro-p群的块，且亏群为Z_p^n的块Morita等价于某扭曲群环，从而验证Donovan猜想。

Towards Donovan's conjecture for abelian defect groups

We define a new invariant for a $p$-block, the strong Frobenius number, which we use to address the problem of reducing Donovan's conjecture to normal subgroups of index p. As an application we use the strong Frobenius number to complete the proof of Donovan's conjecture for 2-blocks with abelian defect groups of rank at most 4 and for 2-blocks with abelian defect groups of order at most 64.

具有二面体和广义四元数亏群的主2-块的 splendid Morita 等价群

The group of splendid Morita equivalences of principal $2$-blocks with dihedral and generalised quaternion defect groups

本研究针对特征2代数闭域上亏群为二面体或广义四元数的有限群主2-块，证明其splendid Morita自等价群同构于固定亏群的源代数外自同构群与稳定融合系的亏群外自同构群的半直积，明确了该类自等价群的结构。

Frobenius-Schur indicators of characters in blocks with cyclic defect

Let $p$ be an odd prime and let $B$ be a $p$-block of a finite group which has cyclic defect groups. We show that all exceptional characters in $B$ have the same Frobenius-Schur indicators. Moreover the common indicator can be computed, using the canonical character of $B$. We also investigate the Frobenius-Schur indicators of the non-exceptional characters in $B$. For a finite group which has cyclic Sylow $p$-subgroups, we show that the number of irreducible characters with Frobenius-Schur indicator $-1$ is greater than or equal to the number of conjugacy classes of weakly real $p$-elements i…

On blocks of defect two and one simple module, and Lie algebra structure of $HH^1$

Let $k$ be a field of odd prime characteristic $p$. We calculate the Lie algebra structure of the first Hochschild cohomology of a class of quantum complete intersections over $k$. As a consequence, we prove that if $B$ is a defect $2$-block of a finite group algebra $kG$ whose Brauer correspondent $C$ has a unique isomorphism class of simple modules, then a basic algebra of $B$ is a local algebra which can be generated by at most $2\sqrt I$ elements, where $I$ is the inertial index of $B$, and where we assume that $k$ is a splitting field for $B$ and $C$.

Kuelshammer ideals and the scalar problem for blocks with dihedral defect groups

In by now classical work, K. Erdmann classified blocks of finite groups with dihedral defect groups (and more generally algebras of dihedral type) up to Morita equivalence. In the explicit description by quivers and relations of such algebras with two simple modules, several subtle problems about scalars occurring in relations remained unresolved. In particular, for the dihedral case it is a longstanding open question whether blocks of finite groups can occur for both possible scalars 0 and 1. In this article, using Kuelshammer ideals (a.k.a. generalized Reynolds ideals), we provide the first …

关于具有循环缺陷群的块的源代数等价类，第三部分

On the source algebra equivalence class of blocks with cyclic defect groups, III

本文研究了具有循环缺陷群的p-块的源代数等价类，通过分类不变量W(B)来确定其等价类。

Blocks with normal abelian defect and abelian p' inertial quotient

Let $k$ be an algebraically closed field of characteristic $p$, and let $\mathcal{O}$ be either $k$ or its ring of Witt vectors $W(k)$. Let $G$ a finite group and $B$ a block of $\mathcal{O}G$ with normal abelian defect group and abelian $p'$ inertial quotient. We show that $B$ is isomorphic to its second Frobenius twist. This is motivated by the fact that bounding Frobenius numbers is one of the key steps towards Donovan's conjecture. For $\mathcal{O}=k$, we give an explicit description of the basic algebra of $B$ as a quiver with relations. It is a quantised version of the group algebra of t…

Morita Equivalence Classes of Tame Blocks of Finite Groups

We show that several Morita equivalence classes of tame algebras do not occur as blocks of finite groups. This refines classifications by Erdmann of classes of blocks with dihedral, semidihedral, and generalised quaternion defect groups. In particular we now have a complete classification of the Morita equivalence classes of blocks of finite groups with dihedral defect groups.

$2$-Blocks whose defect group is homocyclic and whose inertial quotient contains a Singer cycle

We consider a block $B$ of a finite group with defect group $D \cong (C_{2^m})^n$ and inertial quotient $\mathbb{E}$ containing a Singer cycle (an element of order $2^n-1$). This implies $\mathbb{E} = E \rtimes F$, where $E \cong C_{2^n-1}$, $F \leq C_n$, and $E$ acts transitively on the elements in $D$ of order $2$, and freely on $D \backslash \{1\}$. We classify the basic Morita equivalence classes of $B$ over a complete discrete valuation ring $\mathcal{O}$: when $m=1$, $B$ is basic Morita equivalent to the principal block of one of $SL_2(2^n) \rtimes F$, $D \rtimes \mathbb{E}$, or $J_1$ (w…

On the number of simple modules in a block of a finite group

We prove that if $B$ is a $p$-block with non-trivial defect group $D$ of a finite $p$-solvable group $G$, then $\ell(B) < p^r$, where $r$ is the sectional rank of $D$. We remark that there are infinitely many $p$-blocks $B$ with non-Abelian defect groups and $\ell(B) = p^r - 1$. We conjecture that the inequality $\ell(B) \leq p^r$ holds for an arbitrary $p$-block with defect group of sectional rank $r$. We show this to hold for a large class of $p$-blocks of various families of quasi-simple and nearly simple groups.

具有圈积亏群的主2-块在splendid Morita等价下的分类

Principal 2-blocks with wreathed defect groups up to splendid Morita equivalence

本文在Morita等价和splendid Morita等价下分类了Sylow 2-子群同构于圈积2-群的主2-块，证明了Puig的Finiteness Conjecture对此成立，并给出了相关群的纯群论分类。

On Picard groups of blocks of finite groups

We show that the subgroup of the Picard group of a $p$-block of a finite group given by bimodules with endopermutation sources modulo the automorphism group of a source algebra is determined locally in terms of the fusion system on a defect group. We show that the Picard group of a block over the a complete discrete valuation ring ${\mathcal O}$ of characteristic zero with an algebraic closure $k$ of ${\mathbb F}_p$ as residue field is a colimit of finite Picard groups of blocks over $p$-adic subrings of ${\mathcal O}$. We apply the results to blocks with an abelian defect group and Frobenius …

Real characters in blocks

We consider real versions of Brauer's k(B) conjecture, Olsson's conjecture and Eaton's conjecture. We prove the real version of Eaton's conjecture for 2-blocks of groups with cyclic defect group and for the principal 2-blocks of groups with trivial real core. We also characterize G-classes, real and rational G-classes of the defect group of a block B.

关于具有循环缺陷群的块的源代数等价类，II

On the source algebra equivalence class of blocks with cyclic defect groups, II

本文研究了循环p-群的端排列模块在拟单群p-块下的分类，通过将问题转化为一般线性群和单位ary群的分类来完成。

The blocks with four irreducible characters

Suppose that $B$ is a Brauer $p$-block with defect group $D$. If $B$ exactly contains 4 irreducible characters, then we show that $D$ has order 4 or 5, assuming the Alperin--McKay conjecture.

关于不可约布劳尔特征标次数平方和在块中的研究

On the Sum of Squares of Irreducible Brauer Character Degrees in Blocks

本文研究了霍姆-威尔姆斯局部猜想的弱化版本，假设缺陷群为阿贝尔群时将问题简化为准单群，并在p=2时给出了完整证明。

Units in Blocks of Defect 1 and the Zassenhaus Conjecture

Building on previous work by Caicedo and the second author, we develop a method that decides the existence of units of finite order in blocks of $\mathbb{Z}_p G$ of defect 1. This allows us to prove that if $p$ is a prime and $G$ is a finite group whose Sylow $p$-subgroup has order $p$, then any unit $u\in \mathbb{Z} G$ of order $p$ is conjugate to an element of $\pm G$. This is a special case of the Zassenhaus conjecture. We also prove some new results on units of finite order in $\mathbb{Z} \mathrm{PSL}(2,q)$ for certain $q$, and construct a unit of order $15$ in $V(\mathbb{Z}_{(3,5)}\mathrm…

仅有一个不可约 Brauer 特征标轨道的块

Blocks with a single automorphism orbit of irreducible Brauer characters

本文证明了当块覆盖一个 p-幂指数的正规子群的块且仅有一个不可约 Brauer 特征标轨道时，具有阿贝尔缺陷群的块是惰性的。

最小高度与具有两个特征标度的亏群

Minimal heights and defect groups with two character degrees

本文证明了当亏群具有两个特征标度时，主块中最小正高度等于亏群中最小正高度，从而推广了 Brauer 零高度猜想。

关于极大亏标符与素数2的特征三元组猜想

The Character Triple Conjecture for maximal defect characters and the prime 2

本文将极大亏格情形归约为归纳 Alperin–McKay 条件，证明 Späth 特征三元组猜想在素数2对所有有限群成立，并推出阿贝尔亏群2-块及任意素数无块版本的结果。