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In the present popular science paper we determine when a square can be dissected into rectangles similar to a given rectangle. The approach to the question is based on a physical interpretation using electrical networks. Only secondary school background is assumed in the paper.
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We indicate that Herons formula (which relates the square of the area of a triangle to a quartic function of its edge lengths) can be interpreted as a scissors congruence in 4-dimensional space. In the process of demonstrating this, we examine a number of decompositions of hypercubes, hyper-parallelograms, and other elementary 4-dimensional solids.
In this paper, we consider the following geometric puzzle whose origin was traced to Allan Freedman cite{croft91,tutte69} in the 1960s by Dumitrescu and T{o}th cite{adriancasaba2011}. The puzzle has been popularized of late by Peter Winkler cite{Winkler2007}. Let $P_{n}$ be a set of $n$ points, including the origin, in the unit square $U = [0,1]^2$. The problem is to construct $n$ axis-parallel and mutually disjoint rectangles inside $U$ such that the bottom-left corner of each rectangle coincides with a point in $P_{n}$ and the total area covered by the rectangles is maximized. We would term the above rectangles as emph{anchored rectangles}. The longstanding conjecture has been that at least half of $U$ can be covered when anchored rectangles are properly placed. Dumitrescu and T{o}th cite{Dumitrescu2012} have shown a construction method that can cover at least $0.09121$, i.e., roughly $9%$ of the area.
We consider connections between similar sublattices and coincidence site lattices (CSLs), and more generally between similar submodules and coincidence site modules of general (free) $mathbb{Z}$-modules in $mathbb{R}^d$. In particular, we generalise results obtained by S. Glied and M. Baake [1,2] on similarity and coincidence isometries of lattices and certain lattice-like modules called $mathcal{S}$-modules. An important result is that the factor group $mathrm{OS}(M)/mathrm{OC}(M)$ is Abelian for arbitrary $mathbb{Z}$-modules $M$, where $mathrm{OS}(M)$ and $mathrm{OC}(M)$ are the groups of similar and coincidence isometries, respectively. In addition, we derive various relations between the indices of CSLs and their corresponding similar sublattices. [1] S. Glied, M. Baake, Similarity versus coincidence rotations of lattices, Z. Krist. 223, 770--772 (2008). DOI: 10.1524/zkri.2008.1054 [2] S. Glied, Similarity and coincidence isometries for modules, Can. Math. Bull. 55, 98--107 (2011). DOI: 10.4153/CMB-2011-076-x
The article contains a construction of a self-similar dendryte which cannot be the attractor of any self-similar zipper.
Using the geodesic distance on the $n$-dimensional sphere, we study the expected radius function of the Delaunay mosaic of a random set of points. Specifically, we consider the partition of the mosaic into intervals of the radius function and determine the expected number of intervals whose radii are less than or equal to a given threshold. Assuming the points are not contained in a hemisphere, the Delaunay mosaic is isomorphic to the boundary complex of the convex hull in $mathbb{R}^{n+1}$, so we also get the expected number of faces of a random inscribed polytope. We find that the expectations are essentially the same as for the Poisson-Delaunay mosaic in $n$-dimensional Euclidean space. As proved by Antonelli and collaborators, an orthant section of the $n$-sphere is isometric to the standard $n$-simplex equipped with the Fisher information metric. It follows that the latter space has similar stochastic properties as the $n$-dimensional Euclidean space. Our results are therefore relevant in information geometry and in population genetics.
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