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\begin_body

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TODO 1,
 3,
 6 (2pp.,
 0:46)
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\backslash
rexerc1[M10]
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The text refers to a set 
\begin_inset Formula $S$
\end_inset

 containing finite sequences of positive integers,
 and states that this set is 
\begin_inset Quotes eld
\end_inset

essentially an oriented tree.
\begin_inset Quotes erd
\end_inset

 What is the root of this oriented tree,
 and what are the arcs?
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\backslash
answer 
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The vertices are the elements of 
\begin_inset Formula $S$
\end_inset

,
 the root is 
\begin_inset Formula $\emptyset$
\end_inset

,
 and the arcs go from 
\begin_inset Formula $(x_{1},\dots,x_{n})$
\end_inset

 to 
\begin_inset Formula $(x_{1},\dots,x_{n-1})$
\end_inset

,
 for every 
\begin_inset Formula $(x_{1},\dots,x_{n})\in S\setminus\{\emptyset\}$
\end_inset

.
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\backslash
rexerc3[M23]
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If it is possible to tile the upper right quadrant of the plane when given an 
\emph on
infinite
\emph default
 set of tetrad types,
 is it always possible to tile the whole plane?
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answer 
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No.
 For example,
 our tiles might be of the form
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\backslash
begin{center}
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\backslash
begin{tikzpicture}
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\backslash
draw (0,0) -- (0,2) -- (2,2) -- (2,0) -- (0,0) -- (2,2)
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      (0,2) -- (2,0)
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      (0.5,1) node{$n$} (1,0.5) node{$n$}
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\begin_layout Plain Layout

      (1.5,1) node{$n+1$} (1,1.5) node{$n+1$};
\end_layout

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\backslash
end{tikzpicture}
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\backslash
end{center}
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for 
\begin_inset Formula $n\in\mathbb{N}$
\end_inset

.
 Then in the 
\begin_inset Formula $(x,y)$
\end_inset

 square we might place the tile with 
\begin_inset Formula $n=\max\{x,y\}$
\end_inset

 and this would tile the upper right quadrant,
 but there's obviously no way to tile the whole plane.
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\backslash
rexerc6[M23]
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(Otto Schreier.) In a famous paper,
 B.
 L.
 van der Waerden proved the following theorem:
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\begin_layout Quote

\shape slanted
If 
\begin_inset Formula $k$
\end_inset

 and 
\begin_inset Formula $m$
\end_inset

 are positive integers,
 and if we have 
\begin_inset Formula $k$
\end_inset

 sets 
\begin_inset Formula $S_{1},\dots,S_{k}$
\end_inset

 of positive integers with every positive integer included in at least one of these sets,
 then at least of the sets 
\begin_inset Formula $S_{j}$
\end_inset

 contains an arithmetic progression of length 
\begin_inset Formula $m$
\end_inset

.
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(The latter statement means there exist integers 
\begin_inset Formula $a$
\end_inset

 and 
\begin_inset Formula $\delta>0$
\end_inset

 such that 
\begin_inset Formula $a+\delta$
\end_inset

,
 
\begin_inset Formula $a+2\delta$
\end_inset

,
 ...,
 
\begin_inset Formula $a+m\delta$
\end_inset

 are all in 
\begin_inset Formula $S_{j}$
\end_inset

.) If possible,
 use this result and the infinity lemma to prove the following stronger statement:
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\begin_layout Quote

\shape slanted
If 
\begin_inset Formula $k$
\end_inset

 and 
\begin_inset Formula $m$
\end_inset

 are positive integers,
 there is a number 
\begin_inset Formula $N$
\end_inset

 such that if we have 
\begin_inset Formula $k$
\end_inset

 sets 
\begin_inset Formula $S_{1},\dots,S_{k}$
\end_inset

 of integers with every integer between 1 and 
\begin_inset Formula $N$
\end_inset

 included in at least one of these sets,
 then at least one of the sets 
\begin_inset Formula $S_{j}$
\end_inset

 contains an arithmetic progression of length 
\begin_inset Formula $m$
\end_inset

.
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\backslash
answer 
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\end_inset

It is enough to prove it when the set that contains every integer between 1 and 
\begin_inset Formula $N$
\end_inset

 is 
\begin_inset Formula $S_{1}$
\end_inset

.
 We define a tree whose vertices are the tuples 
\begin_inset Formula $(S_{1},\dots,S_{k})$
\end_inset

 of finite sets of integers such that,
 if 
\begin_inset Formula $n=\max\bigcup_{i=1}^{k}S_{i}$
\end_inset

,
 every positive integer up to 
\begin_inset Formula $n$
\end_inset

 is in one of the sets,
 and such that no set contains an arithmetic progression of length 
\begin_inset Formula $m$
\end_inset

.
 The edges go from one such tuple to 
\begin_inset Formula $(S_{1}\setminus\{n\},\dots,S_{k}\setminus\{n\})$
\end_inset

,
 so the root is 
\begin_inset Formula $(\emptyset,\dots,\emptyset)$
\end_inset

 and 
\begin_inset Formula $n$
\end_inset

 is the height of the node in the tree.
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\begin_layout Standard
Since each vertex contains a finite number of children (
\begin_inset Formula $2^{k}-1$
\end_inset

,
 corresponding to the ways of adding the next number to one or more of the sets),
 it follows that if this tree were infinite,
 there would be an infinite path.
 By van der Waerden theorem,
 the component-wise union of this path would give us a tuple 
\begin_inset Formula $(S_{1},\dots,S_{k})$
\end_inset

 such that some 
\begin_inset Formula $S_{j}$
\end_inset

 contains an arithmetic progression of length 
\begin_inset Formula $m$
\end_inset

,
 so by construction one of the nodes in the path would follow this condition.
\begin_inset Formula $\#$
\end_inset


\end_layout

\begin_layout Standard
Thus the tree is finite and we just need to take 
\begin_inset Formula $N$
\end_inset

 as one plus the depth of the tree.
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