We show that for all $m,k,r\in \mathbb{N}$, there is an $n\in \mathbb{N}$ such that whenever $L$ is a Latin square of order $m$ and the Cartesian product $L^{n}$ of $n$ copies of $L$ is $r$-coloured, there is a monochrome Latin subsquare of $L^{n}$, isotopic to $L^{k}$. In particular, for every prime $p$ and for all $k,r\in \mathbb{N}$, there is an $n\in \mathbb{N}$ such that whenever the multiplication table $L({\mathbb{Z}_{p}}^{n})$ of the group ${\mathbb{Z}_{p}}^{n}$ is $r$-coloured, there is a monochrome Latin subsquare of order $p^{k}$. On the other hand, we show that for every group $G$ of order $\leq 15$, there is a 2-colouring of $L(G)$ without a nontrivial monochrome Latin subsquare.

Let G be a countably infinite discrete group, let βG be the Stone–Čech compactification of G, and let ${G^{\rm{*}}} = \beta G \setminus G$. An idempotent $p \in {G^{\rm{*}}}$ is left (right) maximal if for every idempotent $q \in {G^{\rm{*}}}$, pq = p (qp = P) implies qp = q (qp = q). An idempotent $p \in {G^{\rm{*}}}$ is strongly right maximal if the equation xp = p has the unique solution x = p in G*. We show that there is an idempotent $p \in {G^{\rm{*}}}$ which is both left maximal and strongly right maximal.

Let $X$ be a locally compact, $\unicode[STIX]{x1D70E}$-compact, non-compact Hausdorff space and let $\unicode[STIX]{x1D6FD}X$ be the Stone-Čech compactification of $X$. Let $G$ be a countably infinite discrete group continuously acting on $X$, and suppose that, for every $g\in G$, $\text{Fix}(g)=\{x\in X:gx=x\}$ is compact. The action of $G$ on $X$ induces the action on $\unicode[STIX]{x1D6FD}X$, and so on $X^{\ast }=\unicode[STIX]{x1D6FD}X\setminus X$. Let ${\mathcal{D}}$ denote the finest decomposition of $X^{\ast }$ into closed invariant subsets such that the corresponding quotient space of $X^{\ast }$ is Hausdorff. Such a decomposition can be defined for any action of $G$ on a compact Hausdorff space. Applying it to every member of ${\mathcal{D}}$ gives us a decomposition ${\mathcal{D}}^{2}$ of $X^{\ast }$, then ${\mathcal{D}}^{3}$, and so on. We show that (1) ${\mathcal{D}}^{\unicode[STIX]{x1D714}_{1}}$ is the coarsest decomposition of $X^{\ast }$ into closed invariant topologically transitive subsets, (2) there is a dense subset of points $p\in X^{\ast }$ such that $\overline{Gp}\in {\mathcal{D}}^{2}\setminus {\mathcal{D}}$, in particular, ${\mathcal{D}}$ is non-trivial and ${\mathcal{D}}^{2}$ is finer than ${\mathcal{D}}$, and (3) for every ordinal $\unicode[STIX]{x1D6FC}<\unicode[STIX]{x1D714}_{1}$, there is $p\in X^{\ast }$ such that $\overline{Gp}\in {\mathcal{D}}^{\unicode[STIX]{x1D6FC}+2}\setminus {\mathcal{D}}^{\unicode[STIX]{x1D6FC}+1}$, so all the decompositions ${\mathcal{D}}^{\unicode[STIX]{x1D6FC}}$, $\unicode[STIX]{x1D6FC}\leq \unicode[STIX]{x1D714}_{1}$, are distinct.

A colouring of the vertices of a regular polygon is symmetric if it is invariant under some reflection of the polygon. We count the number of symmetric $r$-colourings of the vertices of a regular $n$-gon.

An $r$-ary necklace (bracelet) of length $n$ is an equivalence class of $r$-colourings of vertices of a regular $n$-gon, taking all rotations (rotations and reflections) as equivalent. A necklace (bracelet) is symmetric if a corresponding colouring is invariant under some reflection. We show that the number of symmetric $r$-ary necklaces (bracelets) of length $n$ is $\frac{1}{2} (r+ 1){r}^{n/ 2} $ if $n$ is even, and ${r}^{(n+ 1)/ 2} $ if $n$ is odd.

Let $G$ be an infinite discrete group and let $\beta G$ be the Stone-Čech compactification of $G$. We take the points of $\beta G$ to be the ultrafilters on $G$, identifying the principal ultrafilters with the points of $G$. The set $U\left( G \right)$ of uniform ultrafilters on $G$ is a closed two-sided ideal of $\beta G$. For every $p\,\in \,U\left( G \right)$, define ${{I}_{p}}\,\subseteq \,\beta G$ by ${{I}_{p}}\,=\,{{\bigcap }_{A\in p}}\text{cl}\left( GU\left( A \right) \right)$, where $U\left( A \right)\,=\,\left\{ p\,\in \,U\left( G \right)\,:\,A\in \,p \right\}$. We show that if $\left| G \right|$ is a regular cardinal, then $\left\{ {{I}_{p}}\,:\,p\,\in \,U\left( G \right) \right\}$ is the finest decomposition of $U\left( G \right)$ into closed left ideals of $\beta G$ such that the corresponding quotient space of $U\left( G \right)$ is Hausdorff.

We present a structure theorem for a broad class of homeomorphisms of finite order on countable zero dimensional spaces. As applications we show the following.

1. (a) Every countable nondiscrete topological group not containing an open Boolean subgroup can be partitioned into infinitely many dense subsets.

2. (b) If $G$ is a countably infinite Abelian group with finitely many elements of order 2 and $\beta G$ is the Stone–Čech compactification of $G$ as a discrete semigroup, then for every idempotent $p\,\,\in \,\,\beta G\backslash \{0\}$, the subset $\{p,-p\}\subset \beta G$ generates algebraically the free product of one-element semigroups $\{p\}$ and $\{-p\}$.

Let $G$ be a compact topological group and let $f:\,G\,\to \,G$ be a continuous transformation of $G$. Define ${{f}^{*}}:G\to G$ by ${{f}^{*}}\left( x \right)\,=\,f\left( {{x}^{-1}} \right)x$ and let $\mu \,=\,{{\mu }_{G}}$ be Haar measure on $G$. Assume that $H\,=\,\text{IM}\,{{f}^{*}}$ is a subgroup of $G$ and for every measurable $C\,\subseteq \,H,\,{{\mu }_{G}}{{\left( \left( {{f}^{*}} \right) \right)}^{-1}}\left( \left( C \right) \right)\,=\,\mu H\left( C \right)$. Then for every measurable $C\,\subseteq \,G$, there exist $S\subseteq C$ and $g\,\in \,G$ such that $f\left( S{{g}^{-1}} \right)\,\subseteq \,C{{g}^{-1}}$ and $\mu \left( S \right)\,\ge \,{{\left( \mu \left( C \right) \right)}^{2}}$.

A mapping f : G → s from a left topological group G into a semigroup S is a local homomorphism if for every x є G \ {e}, there is a neighborhood Ux of e such that f (xy) = f (x)f (y) for all y є Ux \ {e}. A local homomorphism f : G → S is onto if for every neighborhood U of e, f(U \ {e}) = S. We show that

(1) every countable regular left topological group containing a discrete subset with exactly one accumulation point admits a local homomorphism onto N,

(2) it is consistent that every countable topological group containing a discrete subset with exactly one accumulation point admits a local homomorphism onto any countable semigroup,

(3) it is consistent that every countable nondiscrete maximally almost periodic topological group admits a local homomorphism onto the countably infinite right zero semigroup.