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2023-04-30 20:27:45 -07:00
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\theorem{Blichfeldt's theorem}
Let $X$ be a finite connected region. If the volume of $X$ is greater than $1$, $X$ must contain two distinct points that differ by an element of $\mathbb{Z}^n$. In other words, there exist distinct $x, y \in X$ so that $x - y \in \mathbb{Z}^n$.
\vfill{4mm}
Intuitively, this means that you can translate $X$ to cover two lattice points at the same time.
\problem{}
Draw a region in $\mathbb{R}^2$ with volume greater than 1 that contains no lattice points. Find two points in that region which differ by an integer vector.
\hint{Area is two-dimensional volume.}
@ -33,6 +38,10 @@ The following picture gives the idea for the proof of Blichfeldt's theorem. Expl
\problem{}
Let $X$ be a region $\in \mathbb{R}^2$ of volume $k$. How many integral points must $X$ contain after a translation?
\begin{solution}
$\lceil k \rceil$
\end{solution}
\vfill
\definition{}
@ -75,7 +84,7 @@ Let $K$ be a region in $\mathbb{R}^2$ satisfying \ref{mink}. Scale this region b
\begin{itemize}
\item How does the volume of $K'$ compare to $K$?
\item Show that the sum of any two points in $K'$ lies in $K$
\item Show that the sum of any two points in $K'$ lies in $K$ \hint{Use convexity.}
\item Apply Blichfeldt's theorem to $K'$ to prove Minkowski's theorem in $\mathbb{R}^2$.
\end{itemize}