A semigroup $S$ is *moving* if $S$ is infinite, and for all finite 
$F\subseteq S$ and infinite $A\subseteq S$, there are $a\_{1},\dots,a\_{k}\in A$ such that,
for all but finitely many $s\in S$,
$$
\{a\_{1}s,\dots,a\_{k}s\} \nsubseteq F.
$$
A function $f\colon X\to Y$ is *finite to one* if for each $y\in Y$, the set
$\{x\in X : f(x)=y\}$ of preimages of $y$ is finite.


The following implications hold among the properties listed below:
$$
(1)\Rightarrow(2)\Rightarrow(4)\Rightarrow(5);
(1)\Rightarrow(3)\Rightarrow(5).
$$
(1) $S$ is a group.


(2) $S$ is *left cancellative*: for all $a,b,c\in S$, if $ca=cb$ then $a=b$.


(3) $S$ is *right cancellative*: for all $a,b,c\in S$, if $ac=bc$
then $a=b$.


(4) Left multiplication in $S$ is finite-to-one: for all $a\in S$, the function $x\mapsto ax$ is finite-to-one.


(5) $S$ is moving.


**Question.**
Let $S$ be a smigroup such that *right multiplication* in $S$
is finite-to-one (that is, for all $a\in S$, the function $x\mapsto xa$ is finite-to-one).
Is $S$ necessarily moving?


**Remarks:** Moving semigroups are interesting since a genrealization of Hindman's Finite Sums coloring theorem applies to them. This is so because a semigroup $S$ is moving if and only if the Stone-Cech remainder $\beta S\setminus S$ is a subsemigroup of $\beta S$.


**Theorem.** Let $S$ be a moving semigroup. 
For each coloring of the elements of $S$ in finitely many colors,
there are distinct elements $a\_{1},a\_{2},\dots\in S$
such that all products $a\_{i\_1}a\_{i\_2}\cdots a\_{i\_n}$ with $i\_1<i\_2<\cdots<i\_n$ ($n$ arbitrary) have the same color.


Update: Shevrin's classification of semigroups (*On the theory of periodic semigroups*,
Izvestija Vys\v{s}ih U\v{c}ebnyh Zavedeni\u{\i} Matematika, 1974)
implies that, if right multiplication in $S$
is finite-to-one, then $S$ has a moving subsemigroup. It follows that the above coloring theorem holds true for semigroups with finite-to-one right multiplication.



Are semigroups with finite-to-one right multiplication "moving"?
The answer is no. Here is a monoid where right multiplication is finite-to-one but is not moving. 


Let $M$ be the monoid with presentation 


$\langle t,x\_0,x\_1,\ldots,\mid x\_0t=x\_0,x\_it=x\_{i-1}, i>0\rangle$.


Then each element of $M$ can be written uniquely in the form $t^nw$ with $n\geq 0$ and $w$ a word over the $x\_i$ possibly empty. It is easy to check multiplication on the right is finite-to-one. Each $x\_i$ acts injectively on the right. The element $t$ is at most two-to-one: $t^nw$ has 2 preimages under right multiplication by $t$ iff $w$ ends in $x\_0$. Compositions of finite-to-one maps are finite-to-one giving the general case. 


Take $F=\{x\_0\}$ and $A=\{x\_0,x\_1,\ldots\}$. Then infinitely many powers of $t$ right multiply any finite subset of $A$ into $F$. So $M$ is not moving. 


**Added May 2, 2014.** A finitely generated counterexample is the monoid $N$ with presentation 


$\langle a,b,t\mid at=a,ab^nt=ab^{n-1}, n>1\rangle$


If you set $x\_i=ab^i$, then the $x\_i$ and $t$ generate a submonoid isomorphic to $M$ above and hence $N$ is not moving. Since this presentation is Church-Rosser it is easy to check that right multiplication by $a,b$ is injective and right multiplication by $t$ is at most 2-to-1. The normal forms are words of the form $u$ and $uw$ where $u$ is a word in $b,t$ and $w$ is a word in $a,b$ with at least one $a$. 


