A new criterion for finite non-cyclic groups

Let $H$ be a subgroup of a group $G$. We say that $H$ satisfies the power condition with respect to $G$, or $H$ is a power subgroup of $G$, if there exists a non-negative integer $m$ such that $H=G^{m}=$. In this note, the following theorem is proved: Let $G$ be a group and $k$ the number of non-power subgroups of $G$. Then (1) $k=0$ if and only if $G$ is a cyclic group(theorem of F. Sz$\acute{a}$sz) ;(2) $0 < k <\infty$ if and only if $G$ is a finite non-cyclic group; (3) $k=\infty$ if and only if $G$ is a infinte non-cyclic group. Thus we get a new criterion for the finite non-cyclic groups.Comment: 6 page

Large 2-groups of automorphisms of curves with positive 2-rank

Let K be an algebraically closed field of characteristic 2, and let X be a curve over K of genus g>1 and 2-rank r>0. For 2-subgroups S of the K-automorphism group Aut(X) of X, the Nakajima bound is |S| < 4g-3. For every g=2^h+1>8, we construct a curve X attaining the Nakajima bound and determine its relevant properties: X is a bielliptic curve with r=g, and its K-automorphism group has a dihedral K-automorphism group of order 4(g-1) which fixes no point in X. Moreover, we provide a classification of 2-groups S of K-automorphisms not fixing a point of X and such that |S|> 2g-1

Gottlieb groups of spheres

This paper takes up the systematic study of the Gottlieb groups $G_{n+k}(\S^n)$ of spheres for $k\le 13$ by means of the classical homotopy theory methods. The groups $G_{n+k}(\S^n)$ for $k\le 7$ and $k=10,12,13$ are fully determined. Partial results on $G_{n+k}(\S^n)$ for $k=8,9,11$ are presented as well. We also show that $[\iota_n,\eta^2_n\sigma_{n+2}]=0$ if $n=2^i-7$ for $i\ge 4$.Comment: 38 page

Large 33-groups of automorphisms of algebraic curves in characteristic 33

Let $S$ be a $p$-subgroup of the \K-automorphism group \aut(\cX) of an algebraic curve \cX of genus $\gg\ge 2$ and $p$-rank $\gamma$ defined over an algebraically closed field $\mathbb{K}$ of characteristic $p\geq 3$.In this paper we prove that if $|S|>2(\gg-1)$ then one of the following cases occurs. \begin{itemize} \item[(i)] $\gamma=0$ and the extension \K(\cX)/\K(\cX)^S completely ramifies at a unique place, and does not ramify elsewhere. \item[(ii)] $\gamma>0$, $p=3$, \cX is a general curve, $S$ attains the Nakajima's upper bound $3(\gamma-1)$ and \K(\cX) is an unramified Galois extension of the function field of a general curve of genus $2$ with equation $Y^2=cX^6+X^4+X^2+1$ where c\in\K^*. \end{itemize} Case (i) was investigated by Stichtenoth, Lehr, Matignon, and Rocher

Arithmetic and geometry of the Hecke groups

We study the arithmetic and geometry properties of the Hecke group $G_q$. In particular, we prove that $G_q$ has a subgroup $X$ of index $d$, genus $g$ with $v_{\infty}$ cusps, and $\tau_2$ (resp. $v_{r_i}$) conjugacy classes of elements that are conjugates of $S$ (resp. $R^{q/r_i}$) if and only if (i) $2g-2 + \tau_2/2 +\sum_{i=1}^k v_{r_i}(1-1/r_i) + v_{\infty} = d(1/2-1/q)$, and (ii) $m _0= 4g-4 +\tau_2 + 2 v_{\infty} + \sum _{i=1}^k v_{r_i}(2-q/r_i)\ge 0$ is a multiple of $q-2$, (iii) $m \ge 0$. In the case $q$ is odd, (ii) is a consequence of (i)

Alternating groups as monodromy groups in positive characteristic

Let $X$ be a generic curve of genus $g$ defined over an algebraically closed field $k$ of characteristic $p\geq 0$. We show that for $n$ sufficiently large there exists a tame rational map f:X\to \PP^1_k with monodromy group $A_n$. This generalizes a result of Magaard--V\"olklein to positive characteristic.Comment: 15 pages, revised versio

Representations of some Hopf algebras associated to the symmetric group S_n

We study the representations and their Frobenius-Schur indicators of two semisimple Hopf algebras related to the symmetric group $S_n$, namely the bismash products H_n = k^{C_n}# kS_{n-1} and its dual J_n = k^{S_{n-1}}# kC_n = (H_n)^*, where $k$ is an algebraically closed field of characteristic 0. Both algebras are constructed using the standard representation of $S_n$ as a factorizable group, that is $S_n = S_{n-1}C_n = C_nS_{n-1}$. We prove that for $H_n$, the indicators of all simple modules are +1. For the dual Hopf algebra J_n = k^{S_{n_1}}# kC_n, the indicator can have values either 0 or 1. When $n = p$, a prime, we obtain a precise result as to which representations have indicator +1 and which ones have 0; in fact as $p \to \infty$, the proportion of simple modules with indicator 1 becomes arbitrarily small. We also prove a result about Frobenius-Schur indicators for more general bismash products H =k^G# kF, coming from any factorizable group of the form $L = FG$ such that $F\cong C_p.$ We use the definition of Frobenius-Schur indicators for Hopf algebras, as described in work of Linchenko and the second author, which extends the classical theorem of Frobenius and Schur, for a finite group $G.

Some Properties of Finite-Dimensional Semisimple Hopf Algebras

Kaplansky conjectured that if H is a finite-dimensional semisimple Hopf algebra over an algebraically closed field k of characteristic 0, then H is of Frobenius type (i.e. if V is an irreducible representation of H then dimV divides dimH). It was proved by Montgomery and Witherspoon that the conjecture is true for H of dimension p^n, p prime, and by Nichols and Richmond that if H has a 2-dimensional representation then dimH is even. In this paper we first prove that if V is an irreducible representation of D(H), the Drinfeld double of any finite-dimensional semisimple Hopf algebra H over k, then dimV divides dimH (not just dimD(H)=(dimH)^2). In doing this we use the theory of modular tensor categories (in particular Verlinde formula). We then use this statement to prove that Kaplansky's conjecture is true for finite-dimensional semisimple quasitriangular Hopf algebras over k. As a result we prove easily the result of Zhu that Kaplansky's conjecture on prime dimensional Hopf algebras over k is true, by passing to their Drinfeld doubles. Second, we use a theorem of Deligne on characterization of tannakian categories to prove that triangular semisimple Hopf algebras over k are equivalent to group algebras as quasi-Hopf algebras.Comment: 7 pages, late

On the depth of separating invariants for finite groups

Abstract. Consider a finite group G acting on a vector space V over a field k of characteristic p > 0. A separating algebra is a subalgebra A of the ring of invariants k[V]^G with the same point separation properties. In this article we compare the depth of an arbitrary separating algebra with that of the corresponding ring of invariants. We show that, in some special cases, the depth of A is bounded above by the depth of k[V]^G

Noether's problem for pp-groups with an abelian subgroup of index pp

Let $K$ be a field and $G$ be a finite group. Let $G$ act on the rational function field $K(x(g):g\in G)$ by $K$-automorphisms defined by $g\cdot x(h)=x(gh)$ for any $g,h\in G$. Denote by $K(G)$ the fixed field $K(x(g):g\in G)^G$. Noether's problem then asks whether $K(G)$ is rational over $K$. Let $p$ be an odd prime and let $G$ be a $p$-group of exponent $p^e$. Assume also that {\rm (i)} char $K = p>0$, or {\rm (ii)} char $K \ne p$ and $K$ contains a primitive $p^e$-th root of unity. In this paper we prove that $K(G)$ is rational over $K$ for the following two types of groups: {\rm (1)} $G$ is a finite $p$-group with an abelian normal subgroup $H$ of index $p$, such that $H$ is a direct product of normal subgroups of $G$ of the type $C_{p^b}\times (C_p)^c$ for some $b,c:1\leq b,0\leq c$; {\rm (2)} $G$ is any group of order $p^5$ from the isoclinic families with numbers $1,2,3,4,8$ and 9.Comment: I added a new theorem for groups of order p^5. The paper will appear in Algebra Colloquium. arXiv admin note: text overlap with arXiv:0911.1162 by other author