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Cyclic subgroup separability of certain graph products of subgroup separable groups
Kok Bin Wong,Peng Choon Wong 대한수학회 2013 대한수학회보 Vol.50 No.5
In this paper, we show that tree products ofcertain subgroup separable groups amalgamating normal subgroups are cyclic subgroup separable. We then extend this result to certain graph product of certain subgroup separable groups amalgamating normal subgroups, that is we show that if the graph has exactly one cycle and the cycle is of length at least four, then the graph product is cyclic subgroup separable.
POLYGONAL PRODUCTS OF RESIDUALLY FINITE GROUPS
Wong, Kok-Bin,Wong, Peng-Choon Korean Mathematical Society 2007 대한수학회보 Vol.44 No.1
A group G is called cyclic subgroup separable for the cyclic subgroup H if for each $x\;{\in}\;G{\backslash}H$, there exists a normal subgroup N of finite index in G such that $x\;{\not\in}\;HN$. Clearly a cyclic subgroup separable group is residually finite. In this note we show that certain polygonal products of cyclic subgroup separable groups amalgamating normal subgroups are again cyclic subgroup separable. We then apply our results to polygonal products of polycyclic-by-finite groups and free-by-finite groups.
CYCLIC SUBGROUP SEPARABILITY OF CERTAIN GRAPH PRODUCTS OF SUBGROUP SEPARABLE GROUPS
Wong, Kok Bin,Wong, Peng Choon Korean Mathematical Society 2013 대한수학회보 Vol.50 No.5
In this paper, we show that tree products of certain subgroup separable groups amalgamating normal subgroups are cyclic subgroup separable. We then extend this result to certain graph product of certain subgroup separable groups amalgamating normal subgroups, that is we show that if the graph has exactly one cycle and the cycle is of length at least four, then the graph product is cyclic subgroup separable.
The residual finiteness of certain HNN extensions
Wong Peng Choon,Wong Kok Bin 대한수학회 2005 대한수학회보 Vol.42 No.3
In this note we give characterizations for certain HNNextensions with central associated subgroups to be residually nite.We then apply our results to HNN extensions of polycyclic-by-finite groups.
THE RESIDUAL FINITENESS OF CERTAIN HNN EXTENSIONS
Choon, Wong-Peng,Bin, Wong-Kok Korean Mathematical Society 2005 대한수학회보 Vol.42 No.3
In this note we give characterizations for certain HNN extensions with central associated subgroups to be residually finite. We then apply our results to HNN extensions of polycyclic-by-finite groups.
AN ANALOGUE OF THE HILTON-MILNER THEOREM FOR WEAK COMPOSITIONS
Ku, Cheng Yeaw,Wong, Kok Bin Korean Mathematical Society 2015 대한수학회보 Vol.52 No.3
Let $\mathbb{N}_0$ be the set of non-negative integers, and let P(n, l) denote the set of all weak compositions of n with l parts, i.e., $P(n,l)=\{(x_1,x_2,{\cdots},x_l){\in}\mathbb{N}^l_0\;:\;x_1+x_2+{\cdots}+x_l=n\}$. For any element $u=(u_1,u_2,{\cdots},u_l){\in}P(n,l)$, denote its ith-coordinate by u(i), i.e., $u(i)=u_i$. A family $A{\subseteq}P(n,l)$ is said to be t-intersecting if ${\mid}\{i:u(i)=v(i)\}{\mid}{\geq}t$ for all $u,v{\epsilon}A$. A family $A{\subseteq}P(n,l)$ is said to be trivially t-intersecting if there is a t-set T of $[l]=\{1,2,{\cdots},l\}$ and elements $y_s{\in}\mathbb{N}_0(s{\in}T)$ such that $A=\{u{\in}P(n,l):u(j)=yj\;for\;all\;j{\in}T\}$. We prove that given any positive integers l, t with $l{\geq}2t+3$, there exists a constant $n_0(l,t)$ depending only on l and t, such that for all $n{\geq}n_0(l,t)$, if $A{\subseteq}P(n,l)$ is non-trivially t-intersecting, then $${\mid}A{\mid}{\leq}(^{n+l-t-l}_{l-t-1})-(^{n-1}_{l-t-1})+t$$. Moreover, equality holds if and only if there is a t-set T of [l] such that $$A=\bigcup_{s{\in}[l]{\backslash}T}\;A_s{\cup}\{q_i:i{\in}T\}$$, where $$A_s=\{u{\in}P(n,l):u(j)=0\;for\;all\;j{\in}T\;and\;u(s)=0\}$$ and $$q_i{\in}P(n,l)\;with\;q_i(j)=0\;fo\;all\;j{\in}[l]{\backslash}\{i\}\;and\;q_i(i)=n$$.
An Erdos-Ko-Rado theorem for minimal covers
Cheng Yeaw Ku,Kok Bin Wong 대한수학회 2017 대한수학회보 Vol.54 No.3
Let $[n]=\{1,2,\dots, n\}$. A set $\mathbf A=\{A_1,A_2,\dots ,A_l\}$ is a minimal cover of $[n]$ if $\bigcup_{1\leq i\leq l} A_i =[n]$ and \[ \bigcup_{\substack{1\leq i\leq l,\\ i\neq j_0}} A_i \neq [n] \quad\textnormal{for all $j_0\in [l]$}. \] Let $\mathcal{C}(n)$ denote the collection of all minimal covers of $[n]$, and write $C_{n} = \vert \mathcal{C}(n)\vert$. Let $\mathbf A \in \mathcal{C}(n)$. An element $u \in [n]$ is critical in $\mathbf A$ if it appears exactly once in $\mathbf A$. Two minimal covers $\mathbf A$, $\mathbf B \in \mathcal{C}(n)$ are said to be restricted $t$-intersecting if they share at least $t$ sets each containing an element which is critical in both $\mathbf A$ and $\mathbf B$. A family $\A \subseteq \mathcal{C}(n)$ is said to be restricted $t$-intersecting if every pair of distinct elements in $\A$ are restricted $t$-intersecting. In this paper, we prove that there exists a constant $n_{0}=n_{0}(t)$ depending on $t$, such that for all $n \ge n_{0}$, if $\A \subseteq \mathcal{C}(n)$ is restricted $t$-intersecting, then $|\A| \le C_{n-t}$. Moreover, the bound is attained if and only if $\A$ is isomorphic to the family $\mathcal{D}_{0}(t)$ consisting of all minimal covers which contain the singleton parts $\{1\}$, $\ldots$, $\{t\}$. A similar result also holds for restricted $r$-cross intersecting families of minimal covers.
ON DIVERSITY OF CERTAIN t-INTERSECTING FAMILIES
Ku, Cheng Yeaw,Wong, Kok Bin Korean Mathematical Society 2020 대한수학회보 Vol.57 No.4
Let [n] = {1, 2, …, n} and 2<sup>[n]</sup> be the set of all subsets of [n]. For a family 𝓕 ⊆ 2<sup>[n]</sup>, its diversity, denoted by div(𝓕), is defined to be $$div(\mathcal{F})=\min_{x{\in}[n]}\{{\mid}{\mathcal{F}}(\bar{x}){\mid}\}$$, where ${\mathcal{F}}(\bar{x})=\{F{\in}{\mathcal{F}}:x{\not\in}F\}$. Basically, div(𝓕) measures how far 𝓕 is from a trivial intersecting family, which is called a star. In this paper, we consider a generalization of diversity for t-intersecting family.
On diversity of certain t-intersecting families
Cheng Yeaw Ku,Kok Bin Wong 대한수학회 2020 대한수학회보 Vol.57 No.4
Let $[n]=\{1,2,\dots, n\}$ and $2^{[n]}$ be the set of all subsets of $[n]$. For a family $\F\subseteq 2^{[n]}$, its diversity, denoted by $\di(\F)$, is defined to be \begin{align*} \di(\F)=\min_{x\in [n]} \left\{ \left\vert \F(\overline x) \right\vert \right\}, \end{align*} where $\F(\overline x)=\left\{ F\in\F : x\notin F \right\}$. Basically, $\di(\F)$ measures how far $\F$ is from a trivial intersecting family, which is called a star. In this paper, we consider a generalization of diversity for $t$-intersecting family.