Let $G$ be an unramified group over a $p$-adic field. This article introduces a base change homomorphism for Bernstein centers of depth-zero principal series blocks for $G$ and proves the corresponding base change fundamental lemma. This result is used in the approach to Shimura varieties with ${\Gamma }_1\left(p\right)$-level structure initiated by M. Rapoport and the author in [15].

Completely regular semigroups equipped with the unary operation of inversion within their maximal subgroups form a variety, denoted by $\mathrm{𝒞ℛ}$. The lattice of subvarieties of $\mathrm{𝒞ℛ}$ is denoted by $ℒ\left(\mathrm{𝒞ℛ}\right)$. For each variety in an $\bigcap$-subsemilattice $\Gamma$ of $ℒ\left(\mathrm{𝒞ℛ}\right)$, we construct at least one basis of identities, and for some important varieties, several. We single out certain remarkable types of bases of general interest. As an application for the local relation $L$, we construct $𝐋$-classes of all varieties in $\Gamma$. Two figures illustrate...

In the present paper, we will show that the set of minimal elements of a full affine semigroup $A\hookrightarrow {\mathbb{N}}_0^k$ contains a free basis of the group generated by $A$ in ${\mathbb{Z}}^k$. This will be applied to the study of the group ${\text{K}}_0\left(R\right)$ for a semilocal ring $R$.

These are Lecture Notes of a course given by the author at the French-Spanish School Tresses in Pau, held in Pau (France) in October 2009. It is basically an introduction to distinct approaches and techniques that can be used to show results in braid groups. Using these techniques we provide several proofs of well known results in braid groups, namely the correctness of Artin’s presentation, that the braid group is torsion free, or that its center is generated by the full twist. We also recall some...

Suppose $R$ is a commutative ring with identity of prime characteristic $p$ and $G$ is an arbitrary abelian $p$-group. In the present paper, a basic subgroup and a lower basic subgroup of the $p$-component $U_p\left(RG\right)$ and of the factor-group $U_p\left(RG\right)/G$ of the unit group $U\left(RG\right)$ in the modular group algebra $RG$ are established, in the case when $R$ is weakly perfect. Moreover, a lower basic subgroup and a basic subgroup of the normed $p$-component $S\left(RG\right)$ and of the quotient group $S\left(RG\right)/G_p$ are given when $R$ is perfect and $G$ is arbitrary whose $G/G_p$ is $p$-divisible....

Let $S\left(RG\right)$ be a normed Sylow $p$-subgroup in a group ring $RG$ of an abelian group $G$ with $p$-component $G_p$ and a $p$-basic subgroup $B$ over a commutative unitary ring $R$ with prime characteristic $p$. The first central result is that $1+I(RG;B_p)+I(R\left(p^i\right)G;G)$ is basic in $S\left(RG\right)$ and $B[1+I(RG;B_p)+I(R\left(p^i\right)G;G)]$ is $p$-basic in $V\left(RG\right)$, and $[1+I(RG;B_p)+I(R\left(p^i\right)G;G)]G_p/G_p$ is basic in $S\left(RG\right)/G_p$ and $[1+I(RG;B_p)+I(R\left(p^i\right)G;G)]G/G$ is $p$-basic in $V\left(RG\right)/G$, provided in both cases $G/G_p$ is $p$-divisible and $R$ is such that its maximal perfect subring $R^{p^i}$ has no nilpotents whenever $i$ is natural. The second major result is that $B(1+I(RG;B_p))$ is $p$-basic in $V\left(RG\right)$ and $(1+I(RG;B_p))G/G$ is $p$-basic in $V\left(RG\right)/G$,...

Suppose $F$ is a perfect field of $\mathrm{c}harF=p\ne 0$ and $G$ is an arbitrary abelian multiplicative group with a $p$-basic subgroup $B$ and $p$-component $G_p$. Let $FG$ be the group algebra with normed group of all units $V\left(FG\right)$ and its Sylow $p$-subgroup $S\left(FG\right)$, and let $I_p(FG;B)$ be the nilradical of the relative augmentation ideal $I(FG;B)$ of $FG$ with respect to $B$. The main results that motivate this article are that $1+I_p(FG;B)$ is basic in $S\left(FG\right)$, and $B(1+I_p(FG;B))$ is $p$-basic in $V\left(FG\right)$ provided $G$ is $p$-mixed. These achievements extend in some way a result of N. Nachev (1996) in Houston...