Klein polyhedron

In the geometry of numbers, the Klein polyhedron, named after Felix Klein, is used to generalize the concept of continued fractions to higher dimensions.

Definition

Let $\textstyle C$ be a closed simplicial cone in Euclidean space $\textstyle {\mathbb {R}}^{n}$. The Klein polyhedron of $\textstyle C$ is the convex hull of the non-zero points of $\textstyle C\cap {\mathbb {Z}}^{n}$.

Relation to continued fractions

Suppose $\textstyle \alpha >0$ is an irrational number. In $\textstyle \mathbb {R} ^{2}$, the cones generated by $\textstyle \{(1,\alpha ),(1,0)\}$ and by $\textstyle \{(1,\alpha ),(0,1)\}$ give rise to two Klein polyhedra, each of which is bounded by a sequence of adjoining line segments. Define the integer length of a line segment to be one less than the size of its intersection with $\textstyle {\mathbb {Z}}^{n}$. Then the integer lengths of the edges of these two Klein polyhedra encode the continued-fraction expansion of $\textstyle \alpha$, one matching the even terms and the other matching the odd terms.

Graphs associated with the Klein polyhedron

Suppose $\textstyle C$ is generated by a basis $\textstyle (a_{i})$ of $\textstyle {\mathbb {R}}^{n}$ (so that $\textstyle C=\{\sum _{i}\lambda _{i}a_{i}:(\forall i)\;\lambda _{i}\geq 0\}$), and let $\textstyle (w_{i})$ be the dual basis (so that $\textstyle C=\{x:(\forall i)\;\langle w_{i},x\rangle \geq 0\}$). Write $\textstyle D(x)$ for the line generated by the vector $\textstyle x$, and $\textstyle H(x)$ for the hyperplane orthogonal to $\textstyle x$.

Call the vector $\textstyle x\in {\mathbb {R}}^{n}$ irrational if $\textstyle H(x)\cap {\mathbb {Q}}^{n}=\{0\}$; and call the cone $\textstyle C$ irrational if all the vectors $\textstyle a_{i}$ and $\textstyle w_{i}$ are irrational.

The boundary $\textstyle V$ of a Klein polyhedron is called a sail. Associated with the sail $\textstyle V$ of an irrational cone are two graphs:

• the graph $\textstyle \Gamma _{{{\mathrm e}}}(V)$ whose vertices are vertices of $\textstyle V$, two vertices being joined if they are endpoints of a (one-dimensional) edge of $\textstyle V$;
• the graph $\textstyle \Gamma _{{{\mathrm f}}}(V)$ whose vertices are $\textstyle (n-1)$-dimensional faces (chambers) of $\textstyle V$, two chambers being joined if they share an $\textstyle (n-2)$-dimensional face.

Both of these graphs are structurally related to the directed graph $\textstyle \Upsilon _{n}$ whose set of vertices is $\textstyle {\mathrm {GL}}_{n}({\mathbb {Q}})$, where vertex $\textstyle A$ is joined to vertex $\textstyle B$ if and only if $\textstyle A^{{-1}}B$ is of the form $\textstyle UW$ where

$U=\left({\begin{array}{cccc}1&\cdots &0&c_{1}\\\vdots &\ddots &\vdots &\vdots \\0&\cdots &1&c_{{n-1}}\\0&\cdots &0&c_{n}\end{array}}\right)$

(with $\textstyle c_{i}\in {\mathbb {Q}}$, $\textstyle c_{n}\neq 0$) and $\textstyle W$ is a permutation matrix. Assuming that $\textstyle V$ has been triangulated, the vertices of each of the graphs $\textstyle \Gamma _{{{\mathrm e}}}(V)$ and $\textstyle \Gamma _{{{\mathrm f}}}(V)$ can be described in terms of the graph $\textstyle \Upsilon _{n}$:

• Given any path $\textstyle (x_{0},x_{1},\ldots )$ in $\textstyle \Gamma _{{{\mathrm e}}}(V)$, one can find a path $\textstyle (A_{0},A_{1},\ldots )$ in $\textstyle \Upsilon _{n}$ such that $\textstyle x_{k}=A_{k}(e)$, where $\textstyle e$ is the vector $\textstyle (1,\ldots ,1)\in {\mathbb {R}}^{n}$.
• Given any path $\textstyle (\sigma _{0},\sigma _{1},\ldots )$ in $\textstyle \Gamma _{{{\mathrm f}}}(V)$, one can find a path $\textstyle (A_{0},A_{1},\ldots )$ in $\textstyle \Upsilon _{n}$ such that $\textstyle \sigma _{k}=A_{k}(\Delta )$, where $\textstyle \Delta$ is the $\textstyle (n-1)$-dimensional standard simplex in $\textstyle {\mathbb {R}}^{n}$.

Generalization of Lagrange's theorem

Lagrange proved that for an irrational real number $\textstyle \alpha$, the continued-fraction expansion of $\textstyle \alpha$ is periodic if and only if $\textstyle \alpha$ is a quadratic irrational. Klein polyhedra allow us to generalize this result.

Let $\textstyle K\subseteq {\mathbb {R}}$ be a totally real algebraic number field of degree $\textstyle n$, and let $\textstyle \alpha _{i}:K\to {\mathbb {R}}$ be the $\textstyle n$ real embeddings of $\textstyle K$. The simplicial cone $\textstyle C$ is said to be split over $\textstyle K$ if $\textstyle C=\{x\in {\mathbb {R}}^{n}:(\forall i)\;\alpha _{i}(\omega _{1})x_{1}+\ldots +\alpha _{i}(\omega _{n})x_{n}\geq 0\}$ where $\textstyle \omega _{1},\ldots ,\omega _{n}$ is a basis for $\textstyle K$ over $\textstyle {\mathbb {Q}}$.

Given a path $\textstyle (A_{0},A_{1},\ldots )$ in $\textstyle \Upsilon _{n}$, let $\textstyle R_{k}=A_{{k+1}}A_{k}^{{-1}}$. The path is called periodic, with period $\textstyle m$, if $\textstyle R_{{k+qm}}=R_{k}$ for all $\textstyle k,q\geq 0$. The period matrix of such a path is defined to be $\textstyle A_{m}A_{0}^{{-1}}$. A path in $\textstyle \Gamma _{{{\mathrm e}}}(V)$ or $\textstyle \Gamma _{{{\mathrm f}}}(V)$ associated with such a path is also said to be periodic, with the same period matrix.

The generalized Lagrange theorem states that for an irrational simplicial cone $\textstyle C\subseteq {\mathbb {R}}^{n}$, with generators $\textstyle (a_{i})$ and $\textstyle (w_{i})$ as above and with sail $\textstyle V$, the following three conditions are equivalent:

• $\textstyle C$ is split over some totally real algebraic number field of degree $\textstyle n$.
• For each of the $\textstyle a_{i}$ there is periodic path of vertices $\textstyle x_{0},x_{1},\ldots$ in $\textstyle \Gamma _{{{\mathrm e}}}(V)$ such that the $\textstyle x_{k}$ asymptotically approach the line $\textstyle D(a_{i})$; and the period matrices of these paths all commute.
• For each of the $\textstyle w_{i}$ there is periodic path of chambers $\textstyle \sigma _{0},\sigma _{1},\ldots$ in $\textstyle \Gamma _{{{\mathrm f}}}(V)$ such that the $\textstyle \sigma _{k}$ asymptotically approach the hyperplane $\textstyle H(w_{i})$; and the period matrices of these paths all commute.

Example

Take $\textstyle n=2$ and $\textstyle K={\mathbb {Q}}({\sqrt {2}})$. Then the simplicial cone $\textstyle \{(x,y):x\geq 0,\vert y\vert \leq x/{\sqrt {2}}\}$ is split over $\textstyle K$. The vertices of the sail are the points $\textstyle (p_{k},\pm q_{k})$ corresponding to the even convergents $\textstyle p_{k}/q_{k}$ of the continued fraction for $\textstyle {\sqrt {2}}$. The path of vertices $\textstyle (x_{k})$ in the positive quadrant starting at $\textstyle (1,0)$ and proceeding in a positive direction is $\textstyle ((1,0),(3,2),(17,12),(99,70),\ldots )$. Let $\textstyle \sigma _{k}$ be the line segment joining $\textstyle x_{k}$ to $\textstyle x_{{k+1}}$. Write $\textstyle {\bar {x}}_{k}$ and $\textstyle {\bar {\sigma }}_{k}$ for the reflections of $\textstyle x_{k}$ and $\textstyle \sigma _{k}$ in the $\textstyle x$-axis. Let $\textstyle T=\left({\begin{array}{cc}3&4\\2&3\end{array}}\right)$, so that $\textstyle x_{{k+1}}=Tx_{k}$, and let $\textstyle R=\left({\begin{array}{cc}6&1\\-1&0\end{array}}\right)=\left({\begin{array}{cc}1&6\\0&-1\end{array}}\right)\left({\begin{array}{cc}0&1\\1&0\end{array}}\right)$.

Let $\textstyle M_{{{\mathrm e}}}=\left({\begin{array}{cc}{\frac 12}&{\frac 12}\\{\frac 14}&-{\frac 14}\end{array}}\right)$, $\textstyle {\bar {M}}_{{{\mathrm e}}}=\left({\begin{array}{cc}{\frac 12}&{\frac 12}\\-{\frac 14}&{\frac 14}\end{array}}\right)$, $\textstyle M_{{{\mathrm f}}}=\left({\begin{array}{cc}3&1\\2&0\end{array}}\right)$, and $\textstyle {\bar {M}}_{{{\mathrm f}}}=\left({\begin{array}{cc}3&1\\-2&0\end{array}}\right)$.

• The paths $\textstyle (M_{{{\mathrm e}}}R^{k})$ and $\textstyle ({\bar {M}}_{{{\mathrm e}}}R^{k})$ are periodic (with period one) in $\textstyle \Upsilon _{2}$, with period matrices $\textstyle M_{{{\mathrm e}}}RM_{{{\mathrm e}}}^{{-1}}=T$ and $\textstyle {\bar {M}}_{{{\mathrm e}}}R{\bar {M}}_{{{\mathrm e}}}^{{-1}}=T^{{-1}}$. We have $\textstyle x_{k}=M_{{{\mathrm e}}}R^{k}(e)$ and $\textstyle {\bar {x}}_{k}={\bar {M}}_{{{\mathrm e}}}R^{k}(e)$.
• The paths $\textstyle (M_{{{\mathrm f}}}R^{k})$ and $\textstyle ({\bar {M}}_{{{\mathrm f}}}R^{k})$ are periodic (with period one) in $\textstyle \Upsilon _{2}$, with period matrices $\textstyle M_{{{\mathrm f}}}RM_{{{\mathrm f}}}^{{-1}}=T$ and $\textstyle {\bar {M}}_{{{\mathrm f}}}R{\bar {M}}_{{{\mathrm f}}}^{{-1}}=T^{{-1}}$. We have $\textstyle \sigma _{k}=M_{{{\mathrm f}}}R^{k}(\Delta )$ and $\textstyle {\bar {\sigma }}_{k}={\bar {M}}_{{{\mathrm f}}}R^{k}(\Delta )$.

Generalization of approximability

A real number $\textstyle \alpha >0$ is called badly approximable if $\textstyle \{q(p\alpha -q):p,q\in {\mathbb {Z}},q>0\}$ is bounded away from zero. An irrational number is badly approximable if and only if the partial quotients of its continued fraction are bounded.^[1] This fact admits of a generalization in terms of Klein polyhedra.

Given a simplicial cone $\textstyle C=\{x:(\forall i)\;\langle w_{i},x\rangle \geq 0\}$ in $\textstyle {\mathbb {R}}^{n}$, where $\textstyle \langle w_{i},w_{i}\rangle =1$, define the norm minimum of $\textstyle C$ as $\textstyle N(C)=\inf\{\prod _{i}\langle w_{i},x\rangle :x\in {\mathbb {Z}}^{n}\cap C\setminus \{0\}\}$.

Given vectors $\textstyle {\mathbf {v}}_{1},\ldots ,{\mathbf {v}}_{m}\in {\mathbb {Z}}^{n}$, let $\textstyle [{\mathbf {v}}_{1},\ldots ,{\mathbf {v}}_{m}]=\sum _{{i_{1}<\cdots <i_{n}}}\vert \det({\mathbf {v}}_{{i_{1}}}\cdots {\mathbf {v}}_{{i_{n}}})\vert$. This is the Euclidean volume of $\textstyle \{\sum _{i}\lambda _{i}{\mathbf {v}}_{i}:(\forall i)\;0\leq \lambda _{i}\leq 1\}$.

Let $\textstyle V$ be the sail of an irrational simplicial cone $\textstyle C$.

• For a vertex $\textstyle x$ of $\textstyle \Gamma _{{{\mathrm e}}}(V)$, define $\textstyle [x]=[{\mathbf {v}}_{1},\ldots ,{\mathbf {v}}_{m}]$ where $\textstyle {\mathbf {v}}_{1},\ldots ,{\mathbf {v}}_{m}$ are primitive vectors in $\textstyle {\mathbb {Z}}^{n}$ generating the edges emanating from $\textstyle x$.
• For a vertex $\textstyle \sigma$ of $\textstyle \Gamma _{{{\mathrm f}}}(V)$, define $\textstyle [\sigma ]=[{\mathbf {v}}_{1},\ldots ,{\mathbf {v}}_{m}]$ where $\textstyle {\mathbf {v}}_{1},\ldots ,{\mathbf {v}}_{m}$ are the extreme points of $\textstyle \sigma$.

Then $\textstyle N(C)>0$ if and only if $\textstyle \{[x]:x\in \Gamma _{{{\mathrm e}}}(V)\}$ and $\textstyle \{[\sigma ]:\sigma \in \Gamma _{{{\mathrm f}}}(V)\}$ are both bounded.

The quantities $\textstyle [x]$ and $\textstyle [\sigma ]$ are called determinants. In two dimensions, with the cone generated by $\textstyle \{(1,\alpha ),(1,0)\}$, they are just the partial quotients of the continued fraction of $\textstyle \alpha$.

See also

• Building (mathematics)

References

1. Bugeaud, Yann (2012). Distribution modulo one and Diophantine approximation. Cambridge Tracts in Mathematics. 193. Cambridge: Cambridge University Press. p. 245. ISBN 978-0-521-11169-0. Zbl pre06066616.

• O. N. German, 2007, "Klein polyhedra and lattices with positive norm minima". Journal de théorie des nombres de Bordeaux 19: 175–190.
• E. I. Korkina, 1995, "Two-dimensional continued fractions. The simplest examples". Proc. Steklov Institute of Mathematics 209: 124–144.
• G. Lachaud, 1998, "Sails and Klein polyhedra" in Contemporary Mathematics 210. American Mathematical Society: 373–385.