Pub Date : 2024-10-10DOI: 10.1016/j.jalgebra.2024.08.042
Francesco Fournier-Facio , Maria Gerasimova , Pieter Spaas
We introduce a notion of local Hilbert–Schmidt stability, motivated by the recent definition by Bradford of local permutation stability, and give examples of (non-residually finite) groups that are locally Hilbert–Schmidt stable but not Hilbert–Schmidt stable. For amenable groups, we provide a criterion for local Hilbert–Schmidt stability in terms of group characters, by analogy with the character criterion of Hadwin and Shulman for Hilbert–Schmidt stable amenable groups. Furthermore, we study the (very) flexible analogues of local Hilbert–Schmidt stability, and we prove several results analogous to the classical setting. Finally, we prove that infinite sofic, respectively hyperlinear, property (T) groups are never locally permutation stable, respectively locally Hilbert–Schmidt stable. This strengthens the result of Becker and Lubotzky for classical stability, and answers a question of Lubotzky.
{"title":"Local Hilbert–Schmidt stability","authors":"Francesco Fournier-Facio , Maria Gerasimova , Pieter Spaas","doi":"10.1016/j.jalgebra.2024.08.042","DOIUrl":"10.1016/j.jalgebra.2024.08.042","url":null,"abstract":"<div><div>We introduce a notion of local Hilbert–Schmidt stability, motivated by the recent definition by Bradford of local permutation stability, and give examples of (non-residually finite) groups that are locally Hilbert–Schmidt stable but not Hilbert–Schmidt stable. For amenable groups, we provide a criterion for local Hilbert–Schmidt stability in terms of group characters, by analogy with the character criterion of Hadwin and Shulman for Hilbert–Schmidt stable amenable groups. Furthermore, we study the (very) flexible analogues of local Hilbert–Schmidt stability, and we prove several results analogous to the classical setting. Finally, we prove that infinite sofic, respectively hyperlinear, property (T) groups are never locally permutation stable, respectively locally Hilbert–Schmidt stable. This strengthens the result of Becker and Lubotzky for classical stability, and answers a question of Lubotzky.</div></div>","PeriodicalId":14888,"journal":{"name":"Journal of Algebra","volume":null,"pages":null},"PeriodicalIF":0.8,"publicationDate":"2024-10-10","publicationTypes":"Journal Article","fieldsOfStudy":null,"isOpenAccess":false,"openAccessPdf":"","citationCount":null,"resultStr":null,"platform":"Semanticscholar","paperid":"142445112","PeriodicalName":null,"FirstCategoryId":null,"ListUrlMain":null,"RegionNum":2,"RegionCategory":"数学","ArticlePicture":[],"TitleCN":null,"AbstractTextCN":null,"PMCID":"OA","EPubDate":null,"PubModel":null,"JCR":null,"JCRName":null,"Score":null,"Total":0}
Pub Date : 2024-10-10DOI: 10.1016/j.jalgebra.2024.08.041
Tomasz Jędrzejak
Consider a two-parameter family of hyperelliptic curves defined over , and their Jacobians where q is an odd prime and without loss of generality b is a non-zero squarefree integer. The curve is a quadratic twist by b of (a generalized Mordell curve of degree q). First, we obtain a few upper bounds for the ranks e.g., if the class number of is odd, and any prime divisor of not equal to q is a primitive root modulo q then . Then we focus on and get the best possible bound (by 1) or even the exact value of rank (0). In particular, we found infinitely many b with any number of prime factors such that . We deduce as conclusions the complete list (or the bounds for the number) of rational points on in such cases. Finally, we found for any given q infinitely many non-isomorphic curves such that .
{"title":"Ranks of quadratic twists of Jacobians of generalized Mordell curves","authors":"Tomasz Jędrzejak","doi":"10.1016/j.jalgebra.2024.08.041","DOIUrl":"10.1016/j.jalgebra.2024.08.041","url":null,"abstract":"<div><div>Consider a two-parameter family of hyperelliptic curves <span><math><msub><mrow><mi>C</mi></mrow><mrow><mi>q</mi><mo>,</mo><mi>b</mi></mrow></msub><mo>:</mo><msup><mrow><mi>y</mi></mrow><mrow><mn>2</mn></mrow></msup><mo>=</mo><msup><mrow><mi>x</mi></mrow><mrow><mi>q</mi></mrow></msup><mo>−</mo><msup><mrow><mi>b</mi></mrow><mrow><mi>q</mi></mrow></msup></math></span> defined over <span><math><mi>Q</mi></math></span>, and their Jacobians <span><math><msub><mrow><mi>J</mi></mrow><mrow><mi>q</mi><mo>,</mo><mi>b</mi></mrow></msub></math></span> where <em>q</em> is an odd prime and without loss of generality <em>b</em> is a non-zero squarefree integer. The curve <span><math><msub><mrow><mi>C</mi></mrow><mrow><mi>q</mi><mo>,</mo><mi>b</mi></mrow></msub></math></span> is a quadratic twist by <em>b</em> of <span><math><msub><mrow><mi>C</mi></mrow><mrow><mi>q</mi><mo>,</mo><mn>1</mn></mrow></msub></math></span> (a generalized Mordell curve of degree <em>q</em>). First, we obtain a few upper bounds for the ranks e.g., if the class number of <span><math><mi>Q</mi><mrow><mo>(</mo><msub><mrow><mi>ζ</mi></mrow><mrow><mi>q</mi></mrow></msub><mo>)</mo></mrow></math></span> is odd, <span><math><mi>q</mi><mo>≡</mo><mn>1</mn><mrow><mo>(</mo><mi>mod</mi><mspace></mspace><mn>4</mn><mo>)</mo></mrow></math></span> and any prime divisor of <span><math><mspace></mspace><mn>2</mn><mi>b</mi></math></span> not equal to <em>q</em> is a primitive root modulo <em>q</em> then <span><math><mi>rank</mi><mspace></mspace><msub><mrow><mi>J</mi></mrow><mrow><mi>q</mi><mo>,</mo><mi>b</mi></mrow></msub><mrow><mo>(</mo><mi>Q</mi><mo>)</mo></mrow><mo>≤</mo><mrow><mo>(</mo><mi>q</mi><mo>−</mo><mn>1</mn><mo>)</mo></mrow><mo>/</mo><mn>2</mn></math></span>. Then we focus on <span><math><mi>q</mi><mo>=</mo><mn>5</mn></math></span> and get the best possible bound (by 1) or even the exact value of rank (0). In particular, we found infinitely many <em>b</em> with any number of prime factors such that <span><math><mi>rank</mi><mspace></mspace><msub><mrow><mi>J</mi></mrow><mrow><mn>5</mn><mo>,</mo><mi>b</mi></mrow></msub><mrow><mo>(</mo><mi>Q</mi><mo>)</mo></mrow><mo>=</mo><mn>0</mn></math></span>. We deduce as conclusions the complete list (or the bounds for the number) of rational points on <span><math><msub><mrow><mi>C</mi></mrow><mrow><mn>5</mn><mo>,</mo><mi>b</mi></mrow></msub></math></span> in such cases. Finally, we found for any given <em>q</em> infinitely many non-isomorphic curves <span><math><msub><mrow><mi>C</mi></mrow><mrow><mi>q</mi><mo>,</mo><mi>b</mi></mrow></msub></math></span> such that <span><math><mi>rank</mi><mspace></mspace><msub><mrow><mi>J</mi></mrow><mrow><mi>q</mi><mo>,</mo><mi>b</mi></mrow></msub><mrow><mo>(</mo><mi>Q</mi><mo>)</mo></mrow><mo>≥</mo><mn>1</mn></math></span>.</div></div>","PeriodicalId":14888,"journal":{"name":"Journal of Algebra","volume":null,"pages":null},"PeriodicalIF":0.8,"publicationDate":"2024-10-10","publicationTypes":"Journal Article","fieldsOfStudy":null,"isOpenAccess":false,"openAccessPdf":"","citationCount":null,"resultStr":null,"platform":"Semanticscholar","paperid":"142444920","PeriodicalName":null,"FirstCategoryId":null,"ListUrlMain":null,"RegionNum":2,"RegionCategory":"数学","ArticlePicture":[],"TitleCN":null,"AbstractTextCN":null,"PMCID":"","EPubDate":null,"PubModel":null,"JCR":null,"JCRName":null,"Score":null,"Total":0}
Pub Date : 2024-10-10DOI: 10.1016/j.jalgebra.2024.09.024
Geo Kam-Fai Tam
We classify what we call “typically almost symmetric” depth zero supercuspidal representations of a classical group over a local field of odd residual characteristic into L-packets. Our main results resolve an ambiguity in the paper of Lust-Stevens [24] in this case, where they could only classify these representations in two or four, if not one, L-packets. By assuming the expected numbers of supercuspidal representations in the L-packets, we employ only simple properties of the representations to prove the main results. In particular, we do not require any deep calculations of character values. With the same method, we also compute the parity of a (conjugate-)self-dual depth zero supercuspidal representation of a general linear group.
我们把在奇数残差特征局部域上的经典群的 "典型几乎对称 "深度为零的超pidal 表示归类为 L 包。在这种情况下,我们的主要结果解决了 Lust-Stevens [24] 论文中的一个模糊之处,即他们只能把这些表示归入两个或四个 L 包,如果不是一个的话。通过假设 L-packets 中超pidal 表征的预期数量,我们只利用表征的简单性质来证明主要结果。特别是,我们不需要对字符值进行任何深入计算。用同样的方法,我们还计算了一般线性群的(共轭)自双深度为零的超pidal 表示的奇偶性。
{"title":"Depth zero supercuspidal representations of classical groups into L-packets: The typically almost symmetric case","authors":"Geo Kam-Fai Tam","doi":"10.1016/j.jalgebra.2024.09.024","DOIUrl":"10.1016/j.jalgebra.2024.09.024","url":null,"abstract":"<div><div>We classify what we call “typically almost symmetric” depth zero supercuspidal representations of a classical group over a local field of odd residual characteristic into L-packets. Our main results resolve an ambiguity in the paper of Lust-Stevens <span><span>[24]</span></span> in this case, where they could only classify these representations in two or four, if not one, L-packets. By assuming the expected numbers of supercuspidal representations in the L-packets, we employ only simple properties of the representations to prove the main results. In particular, we do not require any deep calculations of character values. With the same method, we also compute the parity of a (conjugate-)self-dual depth zero supercuspidal representation of a general linear group.</div></div>","PeriodicalId":14888,"journal":{"name":"Journal of Algebra","volume":null,"pages":null},"PeriodicalIF":0.8,"publicationDate":"2024-10-10","publicationTypes":"Journal Article","fieldsOfStudy":null,"isOpenAccess":false,"openAccessPdf":"","citationCount":null,"resultStr":null,"platform":"Semanticscholar","paperid":"142444748","PeriodicalName":null,"FirstCategoryId":null,"ListUrlMain":null,"RegionNum":2,"RegionCategory":"数学","ArticlePicture":[],"TitleCN":null,"AbstractTextCN":null,"PMCID":"","EPubDate":null,"PubModel":null,"JCR":null,"JCRName":null,"Score":null,"Total":0}
Pub Date : 2024-10-10DOI: 10.1016/j.jalgebra.2024.08.035
Boris Kruglikov, Andreu Llabrés
For every parabolic subgroup P of a Lie supergroup G the homogeneous superspace carries a G-invariant supergeometry. We address the problem whether is the maximal (local and global) symmetry of this supergeometry in the case of exceptional Lie superalgebras and . Our approach is to consider the negatively graded Lie superalgebras for every choice of parabolic, and to compute the Tanaka-Weisfeiler prolongations, with reduction of the structure group when required (2 resp 3 special cases), thus realizing and as symmetries of supergeometries. This gives 19 inequivalent -supergeometries and 55 inequivalent -supergeometries, in majority of cases (17 resp 52 cases) those being encoded as vector superdistributions. We describe those supergeometries and realize supersymmetry explicitly.
{"title":"Realization of Lie superalgebras G(3) and F(4) as symmetries of supergeometries","authors":"Boris Kruglikov, Andreu Llabrés","doi":"10.1016/j.jalgebra.2024.08.035","DOIUrl":"10.1016/j.jalgebra.2024.08.035","url":null,"abstract":"<div><div>For every parabolic subgroup <em>P</em> of a Lie supergroup <em>G</em> the homogeneous superspace <span><math><mi>G</mi><mo>/</mo><mi>P</mi></math></span> carries a <em>G</em>-invariant supergeometry. We address the problem whether <span><math><mi>g</mi><mo>=</mo><mrow><mi>Lie</mi></mrow><mo>(</mo><mi>G</mi><mo>)</mo></math></span> is the maximal (local and global) symmetry of this supergeometry in the case of exceptional Lie superalgebras <span><math><mi>G</mi><mo>(</mo><mn>3</mn><mo>)</mo></math></span> and <span><math><mi>F</mi><mo>(</mo><mn>4</mn><mo>)</mo></math></span>. Our approach is to consider the negatively graded Lie superalgebras for every choice of parabolic, and to compute the Tanaka-Weisfeiler prolongations, with reduction of the structure group when required (2 resp 3 special cases), thus realizing <span><math><mi>G</mi><mo>(</mo><mn>3</mn><mo>)</mo></math></span> and <span><math><mi>F</mi><mo>(</mo><mn>4</mn><mo>)</mo></math></span> as symmetries of supergeometries. This gives 19 inequivalent <span><math><mi>G</mi><mo>(</mo><mn>3</mn><mo>)</mo></math></span>-supergeometries and 55 inequivalent <span><math><mi>F</mi><mo>(</mo><mn>4</mn><mo>)</mo></math></span>-supergeometries, in majority of cases (17 resp 52 cases) those being encoded as vector superdistributions. We describe those supergeometries and realize supersymmetry explicitly.</div></div>","PeriodicalId":14888,"journal":{"name":"Journal of Algebra","volume":null,"pages":null},"PeriodicalIF":0.8,"publicationDate":"2024-10-10","publicationTypes":"Journal Article","fieldsOfStudy":null,"isOpenAccess":false,"openAccessPdf":"","citationCount":null,"resultStr":null,"platform":"Semanticscholar","paperid":"142529845","PeriodicalName":null,"FirstCategoryId":null,"ListUrlMain":null,"RegionNum":2,"RegionCategory":"数学","ArticlePicture":[],"TitleCN":null,"AbstractTextCN":null,"PMCID":"OA","EPubDate":null,"PubModel":null,"JCR":null,"JCRName":null,"Score":null,"Total":0}
Pub Date : 2024-10-10DOI: 10.1016/j.jalgebra.2024.09.012
C. Bagiński , G. Gromadzki , R.A. Hidalgo
A continuous action of a finite group G on a closed orientable surface X is said to be gpnf (Gilman purely non-free) if every element of G has a fixed point on X. We prove that the biggest order , of a gpnf-action on a surface of even genus , is bounded below by 8g and that this bound is sharp for infinitely many even g as well. This provides, for even genera, a gpnf-action analog of the celebrated Accola-Maclachlan bound for arbitrary finite continuous actions. We also describe the asymptotic behavior of μ. We define as the set of values of the function and its subsets and corresponding to even and odd genera g. We show that the set , of accumulation points of , consists of a single number 8. If g is odd, then we prove that . We conjecture that this lower bound is sharp for infinitely many odd g. Finally, we prove that this conjecture implies that 4 is the only element of , leading to .
如果 G 的每个元素在 X 上都有一个定点,那么有限群 G 在封闭可定向曲面 X 上的连续作用被称为 gpnf(吉尔曼纯非自由)作用。我们证明,偶数属 g≥2 的曲面上的 gpnf 作用的最大阶 μ(g),其下限为 8g,并且这个下限对于无穷多个偶数属 g 也是尖锐的。这就为偶数属提供了著名的任意有限连续作用的阿克拉-麦克拉克伦界 8g+8 的 gpnf 作用类似物。我们还描述了 μ 的渐近行为。我们将 M 定义为函数 μ˜(g)=μ(g)/(g+1)的值集及其对应于偶数和奇数属 g 的子集 M+ 和 M-。如果 g 是奇数,那么我们证明 4g≤μ(g)<8g。最后,我们证明这一猜想意味着 4 是 M-d 的唯一元素,从而得出 Md={4,8}。
{"title":"On the biggest purely non-free conformal actions on compact Riemann surfaces and their asymptotic properties","authors":"C. Bagiński , G. Gromadzki , R.A. Hidalgo","doi":"10.1016/j.jalgebra.2024.09.012","DOIUrl":"10.1016/j.jalgebra.2024.09.012","url":null,"abstract":"<div><div>A continuous action of a finite group <em>G</em> on a closed orientable surface <em>X</em> is said to be gpnf (Gilman purely non-free) if every element of <em>G</em> has a fixed point on <em>X</em>. We prove that the biggest order <span><math><mi>μ</mi><mo>(</mo><mi>g</mi><mo>)</mo></math></span>, of a gpnf-action on a surface of even genus <span><math><mi>g</mi><mo>≥</mo><mn>2</mn></math></span>, is bounded below by 8<em>g</em> and that this bound is sharp for infinitely many even <em>g</em> as well. This provides, for even genera, a gpnf-action analog of the celebrated Accola-Maclachlan bound <span><math><mn>8</mn><mi>g</mi><mo>+</mo><mn>8</mn></math></span> for arbitrary finite continuous actions. We also describe the asymptotic behavior of <em>μ</em>. We define <span><math><mi>M</mi></math></span> as the set of values of the function <span><math><mover><mrow><mi>μ</mi></mrow><mrow><mo>˜</mo></mrow></mover><mo>(</mo><mi>g</mi><mo>)</mo><mo>=</mo><mi>μ</mi><mo>(</mo><mi>g</mi><mo>)</mo><mo>/</mo><mo>(</mo><mi>g</mi><mo>+</mo><mn>1</mn><mo>)</mo></math></span> and its subsets <span><math><msub><mrow><mi>M</mi></mrow><mrow><mo>+</mo></mrow></msub></math></span> and <span><math><msub><mrow><mi>M</mi></mrow><mrow><mo>−</mo></mrow></msub></math></span> corresponding to even and odd genera <em>g</em>. We show that the set <span><math><msubsup><mrow><mi>M</mi></mrow><mrow><mo>+</mo></mrow><mrow><mi>d</mi></mrow></msubsup></math></span>, of accumulation points of <span><math><msub><mrow><mi>M</mi></mrow><mrow><mo>+</mo></mrow></msub></math></span>, consists of a single number 8. If <em>g</em> is odd, then we prove that <span><math><mn>4</mn><mi>g</mi><mo>≤</mo><mi>μ</mi><mo>(</mo><mi>g</mi><mo>)</mo><mo><</mo><mn>8</mn><mi>g</mi></math></span>. We conjecture that this lower bound is sharp for infinitely many odd <em>g</em>. Finally, we prove that this conjecture implies that 4 is the only element of <span><math><msubsup><mrow><mi>M</mi></mrow><mrow><mo>−</mo></mrow><mrow><mi>d</mi></mrow></msubsup></math></span>, leading to <span><math><msup><mrow><mi>M</mi></mrow><mrow><mi>d</mi></mrow></msup><mo>=</mo><mo>{</mo><mn>4</mn><mo>,</mo><mn>8</mn><mo>}</mo></math></span>.</div></div>","PeriodicalId":14888,"journal":{"name":"Journal of Algebra","volume":null,"pages":null},"PeriodicalIF":0.8,"publicationDate":"2024-10-10","publicationTypes":"Journal Article","fieldsOfStudy":null,"isOpenAccess":false,"openAccessPdf":"","citationCount":null,"resultStr":null,"platform":"Semanticscholar","paperid":"142444921","PeriodicalName":null,"FirstCategoryId":null,"ListUrlMain":null,"RegionNum":2,"RegionCategory":"数学","ArticlePicture":[],"TitleCN":null,"AbstractTextCN":null,"PMCID":"","EPubDate":null,"PubModel":null,"JCR":null,"JCRName":null,"Score":null,"Total":0}
Pub Date : 2024-10-09DOI: 10.1016/j.jalgebra.2024.10.001
K. Robin McLean
We describe the characteristic subgroups of a transitive abelian 2-group, thus completing Kaplansky's study in section 18 of his classic monograph Infinite abelian groups. This appears to be a new result even for finite abelian 2-groups.
{"title":"The characteristic subgroups of an abelian p-group","authors":"K. Robin McLean","doi":"10.1016/j.jalgebra.2024.10.001","DOIUrl":"10.1016/j.jalgebra.2024.10.001","url":null,"abstract":"<div><div>We describe the characteristic subgroups of a transitive abelian 2-group, thus completing Kaplansky's study in section 18 of his classic monograph <em>Infinite abelian groups</em>. This appears to be a new result even for finite abelian 2-groups.</div></div>","PeriodicalId":14888,"journal":{"name":"Journal of Algebra","volume":null,"pages":null},"PeriodicalIF":0.8,"publicationDate":"2024-10-09","publicationTypes":"Journal Article","fieldsOfStudy":null,"isOpenAccess":false,"openAccessPdf":"","citationCount":null,"resultStr":null,"platform":"Semanticscholar","paperid":"142444917","PeriodicalName":null,"FirstCategoryId":null,"ListUrlMain":null,"RegionNum":2,"RegionCategory":"数学","ArticlePicture":[],"TitleCN":null,"AbstractTextCN":null,"PMCID":"","EPubDate":null,"PubModel":null,"JCR":null,"JCRName":null,"Score":null,"Total":0}
Pub Date : 2024-10-05DOI: 10.1016/j.jalgebra.2024.09.010
Sean Sanford
Several complications arise when attempting to work with fusion categories over arbitrary fields. Here we describe some of the new phenomena that occur when the field is not algebraically closed, and we adapt tools such as the Frobenius-Perron dimension in order to accommodate these new effects.
{"title":"Fusion categories over non-algebraically closed fields","authors":"Sean Sanford","doi":"10.1016/j.jalgebra.2024.09.010","DOIUrl":"10.1016/j.jalgebra.2024.09.010","url":null,"abstract":"<div><div>Several complications arise when attempting to work with fusion categories over arbitrary fields. Here we describe some of the new phenomena that occur when the field is not algebraically closed, and we adapt tools such as the Frobenius-Perron dimension in order to accommodate these new effects.</div></div>","PeriodicalId":14888,"journal":{"name":"Journal of Algebra","volume":null,"pages":null},"PeriodicalIF":0.8,"publicationDate":"2024-10-05","publicationTypes":"Journal Article","fieldsOfStudy":null,"isOpenAccess":false,"openAccessPdf":"","citationCount":null,"resultStr":null,"platform":"Semanticscholar","paperid":"142427062","PeriodicalName":null,"FirstCategoryId":null,"ListUrlMain":null,"RegionNum":2,"RegionCategory":"数学","ArticlePicture":[],"TitleCN":null,"AbstractTextCN":null,"PMCID":"","EPubDate":null,"PubModel":null,"JCR":null,"JCRName":null,"Score":null,"Total":0}
Pub Date : 2024-10-02DOI: 10.1016/j.jalgebra.2024.09.009
Changlong Zhong
We generalize the property of small-torus equivariant K-homology of the affine Grassmannian to general oriented (co)homology theory in the sense of Levine and Morel. The main tool we use is the formal affine Demazure algebra associated to the affine root system. More precisely, we prove that the small-torus equivariant oriented cohomology of the affine Grassmannian satisfies the Goresky-Kottwitz-MacPherson (GKM) condition. We also show that its dual, the small-torus equivariant homology, is isomorphic to the centralizer of the equivariant oriented cohomology of a point in the formal affine Demazure algebra.
{"title":"Equivariant oriented homology of the affine Grassmannian","authors":"Changlong Zhong","doi":"10.1016/j.jalgebra.2024.09.009","DOIUrl":"10.1016/j.jalgebra.2024.09.009","url":null,"abstract":"<div><div>We generalize the property of small-torus equivariant K-homology of the affine Grassmannian to general oriented (co)homology theory in the sense of Levine and Morel. The main tool we use is the formal affine Demazure algebra associated to the affine root system. More precisely, we prove that the small-torus equivariant oriented cohomology of the affine Grassmannian satisfies the Goresky-Kottwitz-MacPherson (GKM) condition. We also show that its dual, the small-torus equivariant homology, is isomorphic to the centralizer of the equivariant oriented cohomology of a point in the formal affine Demazure algebra.</div></div>","PeriodicalId":14888,"journal":{"name":"Journal of Algebra","volume":null,"pages":null},"PeriodicalIF":0.8,"publicationDate":"2024-10-02","publicationTypes":"Journal Article","fieldsOfStudy":null,"isOpenAccess":false,"openAccessPdf":"","citationCount":null,"resultStr":null,"platform":"Semanticscholar","paperid":"142427063","PeriodicalName":null,"FirstCategoryId":null,"ListUrlMain":null,"RegionNum":2,"RegionCategory":"数学","ArticlePicture":[],"TitleCN":null,"AbstractTextCN":null,"PMCID":"","EPubDate":null,"PubModel":null,"JCR":null,"JCRName":null,"Score":null,"Total":0}
Pub Date : 2024-09-26DOI: 10.1016/j.jalgebra.2024.09.007
Jianing Yang
Given a Galois cover of curves f over a field of characteristic p, the lifting problem asks whether there exists a Galois cover over a complete mixed characteristic discrete valuation ring whose reduction is f. In this paper, we consider the case where the Galois groups are elementary abelian p-groups. We prove a combinatorial criterion for lifting an elementary abelian p-cover, dependent on the branch loci of lifts of its p-cyclic subcovers. We also study how branch points of a lift coalesce on the special fiber. Finally, for , we analyze lifts for several families of -covers of various conductor types, both with equidistant branch locus geometry and non-equidistant branch locus geometry.
给定特性 p 域上曲线 f 的伽罗瓦盖,提升问题问是否存在一个完整混合特性离散估值环上的伽罗瓦盖,其还原为 f。我们证明了一个提升基本无常 p 盖的组合准则,它取决于其 p 循环子盖的提升支点位置。我们还研究了提升的分支点如何在特殊纤维上凝聚。最后,对于 p=2,我们分析了不同导体类型的 (Z/2)3 覆盖的几个族的提升,既有等距支点几何,也有非等距支点几何。
{"title":"Lifting elementary Abelian covers of curves","authors":"Jianing Yang","doi":"10.1016/j.jalgebra.2024.09.007","DOIUrl":"10.1016/j.jalgebra.2024.09.007","url":null,"abstract":"<div><div>Given a Galois cover of curves <em>f</em> over a field of characteristic <em>p</em>, the lifting problem asks whether there exists a Galois cover over a complete mixed characteristic discrete valuation ring whose reduction is <em>f</em>. In this paper, we consider the case where the Galois groups are elementary abelian <em>p</em>-groups. We prove a combinatorial criterion for lifting an elementary abelian <em>p</em>-cover, dependent on the branch loci of lifts of its <em>p</em>-cyclic subcovers. We also study how branch points of a lift coalesce on the special fiber. Finally, for <span><math><mi>p</mi><mo>=</mo><mn>2</mn></math></span>, we analyze lifts for several families of <span><math><msup><mrow><mo>(</mo><mi>Z</mi><mo>/</mo><mn>2</mn><mo>)</mo></mrow><mrow><mn>3</mn></mrow></msup></math></span>-covers of various conductor types, both with equidistant branch locus geometry and non-equidistant branch locus geometry.</div></div>","PeriodicalId":14888,"journal":{"name":"Journal of Algebra","volume":null,"pages":null},"PeriodicalIF":0.8,"publicationDate":"2024-09-26","publicationTypes":"Journal Article","fieldsOfStudy":null,"isOpenAccess":false,"openAccessPdf":"","citationCount":null,"resultStr":null,"platform":"Semanticscholar","paperid":"142427061","PeriodicalName":null,"FirstCategoryId":null,"ListUrlMain":null,"RegionNum":2,"RegionCategory":"数学","ArticlePicture":[],"TitleCN":null,"AbstractTextCN":null,"PMCID":"","EPubDate":null,"PubModel":null,"JCR":null,"JCRName":null,"Score":null,"Total":0}
Pub Date : 2024-09-26DOI: 10.1016/j.jalgebra.2024.09.008
Ari Markowitz
We present an algorithm which takes as input a finite set X of automorphisms of a simplicial tree, and outputs a generating set of such that either is purely hyperbolic and is a free basis of , or contains a non-trivial elliptic element. As a special case, the algorithm decides whether a finitely generated group acting on a locally finite tree is discrete and free. This algorithm, which is based on Nielsen's reduction method, works by repeatedly applying Nielsen transformations to X to minimise the generators of with respect to a given pre-well-ordering. We use this algorithm to solve the constructive membership problem for finitely generated purely hyperbolic automorphism groups of trees. We provide a Magma implementation of these algorithms, and report its performance.
我们提出一种算法,输入简单树的有限自变集 X,然后输出〈X〉的生成集 X′,这样,要么〈X〉是纯双曲的,且 X′是〈X〉的自由基,要么 X′包含一个非三维的椭圆元素。作为特例,该算法还能判定作用于局部有限树的有限生成群是否离散和自由。该算法基于尼尔森还原法,通过对 X 重复应用尼尔森变换,使 X′ 的生成数最小化,并与给定的前韦尔排序相关。我们使用这种算法来解决有限生成的纯双曲树自形群的构造成员问题。我们提供了这些算法的 Magma 实现,并报告了其性能。
{"title":"Recognition and constructive membership for purely hyperbolic groups acting on trees","authors":"Ari Markowitz","doi":"10.1016/j.jalgebra.2024.09.008","DOIUrl":"10.1016/j.jalgebra.2024.09.008","url":null,"abstract":"<div><div>We present an algorithm which takes as input a finite set <em>X</em> of automorphisms of a simplicial tree, and outputs a generating set <span><math><msup><mrow><mi>X</mi></mrow><mrow><mo>′</mo></mrow></msup></math></span> of <span><math><mo>〈</mo><mi>X</mi><mo>〉</mo></math></span> such that either <span><math><mo>〈</mo><mi>X</mi><mo>〉</mo></math></span> is purely hyperbolic and <span><math><msup><mrow><mi>X</mi></mrow><mrow><mo>′</mo></mrow></msup></math></span> is a free basis of <span><math><mo>〈</mo><mi>X</mi><mo>〉</mo></math></span>, or <span><math><msup><mrow><mi>X</mi></mrow><mrow><mo>′</mo></mrow></msup></math></span> contains a non-trivial elliptic element. As a special case, the algorithm decides whether a finitely generated group acting on a locally finite tree is discrete and free. This algorithm, which is based on Nielsen's reduction method, works by repeatedly applying Nielsen transformations to <em>X</em> to minimise the generators of <span><math><msup><mrow><mi>X</mi></mrow><mrow><mo>′</mo></mrow></msup></math></span> with respect to a given pre-well-ordering. We use this algorithm to solve the constructive membership problem for finitely generated purely hyperbolic automorphism groups of trees. We provide a <span>Magma</span> implementation of these algorithms, and report its performance.</div></div>","PeriodicalId":14888,"journal":{"name":"Journal of Algebra","volume":null,"pages":null},"PeriodicalIF":0.8,"publicationDate":"2024-09-26","publicationTypes":"Journal Article","fieldsOfStudy":null,"isOpenAccess":false,"openAccessPdf":"","citationCount":null,"resultStr":null,"platform":"Semanticscholar","paperid":"142437718","PeriodicalName":null,"FirstCategoryId":null,"ListUrlMain":null,"RegionNum":2,"RegionCategory":"数学","ArticlePicture":[],"TitleCN":null,"AbstractTextCN":null,"PMCID":"OA","EPubDate":null,"PubModel":null,"JCR":null,"JCRName":null,"Score":null,"Total":0}