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authorJuan Marín Noguera <juan@mnpi.eu>2025-05-16 22:18:44 +0200
committerJuan Marín Noguera <juan@mnpi.eu>2025-05-16 22:18:44 +0200
commit4f670b750af5c11e1eac16d9cd8556455f89f46a (patch)
treee0f8d7b33df2727d89150f799ee8628821fda80a /2.3.4.3.lyx
parent16ccda6c459c0fd7ca2081e9d541124c28b0c556 (diff)
Changed layout for more manageable volumes
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-#LyX 2.4 created this file. For more info see https://www.lyx.org/
-\lyxformat 620
-\begin_document
-\begin_header
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-TODO 1,
- 3,
- 6 (2pp.,
- 0:46)
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-
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-
-
-\end_layout
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-\begin_layout Standard
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-status open
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-\begin_layout Plain Layout
-
-
-\backslash
-rexerc1[M10]
-\end_layout
-
-\end_inset
-
-The text refers to a set
-\begin_inset Formula $S$
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-
- containing finite sequences of positive integers,
- and states that this set is
-\begin_inset Quotes eld
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-
-essentially an oriented tree.
-\begin_inset Quotes erd
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- What is the root of this oriented tree,
- and what are the arcs?
-\end_layout
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-\begin_layout Standard
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-\begin_layout Plain Layout
-
-
-\backslash
-answer
-\end_layout
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-\end_inset
-
-The vertices are the elements of
-\begin_inset Formula $S$
-\end_inset
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-,
- the root is
-\begin_inset Formula $\emptyset$
-\end_inset
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-,
- and the arcs go from
-\begin_inset Formula $(x_{1},\dots,x_{n})$
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-
- to
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-,
- for every
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-.
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-\begin_layout Standard
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-
-
-\backslash
-rexerc3[M23]
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-
-\end_inset
-
-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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-
-\backslash
-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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-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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-for
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-.
- Then in the
-\begin_inset Formula $(x,y)$
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- square we might place the tile with
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- and this would tile the upper right quadrant,
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-\begin_layout Standard
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-
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-\backslash
-rexerc6[M23]
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-
-\end_inset
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-(Otto Schreier.) In a famous paper,
- B.
- L.
- van der Waerden proved the following theorem:
-\end_layout
-
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-
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- and
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- are positive integers,
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- sets
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- of positive integers with every positive integer included in at least one of these sets,
- then at least of the sets
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- contains an arithmetic progression of length
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-.
-\end_layout
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-\begin_layout Standard
-(The latter statement means there exist integers
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- and
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- such that
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-,
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-,
- ...,
-
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-
- are all in
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-
-.) If possible,
- use this result and the infinity lemma to prove the following stronger statement:
-\end_layout
-
-\begin_layout Quote
-
-\shape slanted
-If
-\begin_inset Formula $k$
-\end_inset
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- and
-\begin_inset Formula $m$
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- are positive integers,
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- such that if we have
-\begin_inset Formula $k$
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- sets
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- of integers with every integer between 1 and
-\begin_inset Formula $N$
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-
- included in at least one of these sets,
- then at least one of the sets
-\begin_inset Formula $S_{j}$
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-
- contains an arithmetic progression of length
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-.
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-
-\backslash
-answer
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-
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-It is enough to prove it when the set that contains every integer between 1 and
-\begin_inset Formula $N$
-\end_inset
-
- is
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-\end_inset
-
-.
- We define a tree whose vertices are the tuples
-\begin_inset Formula $(S_{1},\dots,S_{k})$
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-
- 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 $m$
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-
-.
- The edges go from one such tuple to
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-,
- so the root is
-\begin_inset Formula $(\emptyset,\dots,\emptyset)$
-\end_inset
-
- and
-\begin_inset Formula $n$
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-
- is the height of the node in the tree.
-\end_layout
-
-\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 $(S_{1},\dots,S_{k})$
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- such that some
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- contains an arithmetic progression of length
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-,
- 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
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- as one plus the depth of the tree.
-\end_layout
-
-\end_body
-\end_document