
Arctriangle tilings
Every triangle tiles the plane, by 180° rotations around the midpoints of each side; some triangles have other tilings as well. But if we generalize from triangles to arctriangles (shapes bounded by three circular arcs), it is no longer true that everything tiles. Within any large region of the plane, the lengths of bulgingoutward arcs of each radius must be balanced by equal lengths of bulginginward arcs of each radius, and the only way to achieve this with a single tile shape is to keep that same balance between convex and concave length on each tile. Counting line segments as degenerate cases of circular arcs, this gives us three possibilities:

Congratulations, Dr. Matias!
Pedro Ascensao Ferreira Matias, one of the students working with Mike Goodrich in the UC Irvine Center for Algorithms and Theory of Computation, passed his Ph.D. defense today.

Linkage
 The EFF on FLoC (\(\mathbb{M}\)), Google’s plan for browsers to aggregate your browsing habits and make them public for adpersonalization. Short summary: it’s a bad idea and if you care about privacy you should switch to a nonChrome browser. Technical summary: it’s based on kanonymity, known as inadequate at protecting individual privacy in social networks. If you use Chrome, assume all bad guys on the web can see all your browsing.

How good is greed for the nothreeinline problem?
The 37th European Workshop on Computational Geometry (EuroCG 2021) was earlier this month, but its book of abstracts remains online. This has an odd position in the world of academic publishing: the “abstracts” are really short papers, so it looks a lot like a published conference proceedings. However, it declares that you should really pretend that it’s not a proceedings, in order to allow the same work to go on to another conference with a published proceedings, getting around the usual prohibitions on double publication. Instead, its papers “should be considered a preprint rather than a formally reviewed paper”. But I think that doesn’t preclude citing them, with care, just as you might occasionally cite arXiv preprints. The workshop’s lack of peer review and selectivity is actually a useful feature, allowing it to act as an outlet for works that are too small or preliminary for publication elsewhere. In North America, the Canadian Conference on Computational Geometry performs much the same role, but does publish a proceedings; its submission deadline is rapidly approaching.

Pick's shoelaces
Two important methods for computing area of polygons in the plane are Pick’s theorem and the shoelace formula. For a simple lattice polygon (a polygon with a single noncrossing boundary cycle, all of whose vertex coordinates are integers) with \(i\) integer points in its interior and \(b\) on the boundary, Pick’s theorem computes the area as

Linkage
 Keller’s conjecture (\(\mathbb{M}\)), another new Good Article on Wikipedia. The conjecture was falsified in 1992 with all remaining cases solved by 2019, but the name stuck. It’s about tilings of \(n\)space by unit cubes, and pairs of cubes that share \((n1)\)faces. In 2d, all squares share an edge with a neighbor, but a 3d tiling derived from tetrastix has many cubes with no facetoface neighbor. Up to 7d, some cubes must be facetoface, but tilings in eight or more dimensions can have no facetoface pair.

Islands
In the neighborhood where I live, fire safety regulations require the streets to be superwide (so wide that two fire trucks can pass even with cars parked along both sides of the street), and to have even wider turnarounds at the ends of the culsdesac. To break up the resulting vast expanses of pavement, we have occasional islands of green, public gardens too small to name as a park. They come in several different types: medians to separate the incoming and outgoing lanes at junctions with larger roads,

Linkage
As is often the case, not all of these are links; they’re copies of posts that I made over on my Mastodon account because they were too short to make full posts here.

Linkage for the Ides of March
 I am quite amused by the description of “some unimpressed Wikipedia editor” (\(\mathbb{M}\)) who didn’t think the evidence for or against the Kummer–Vandiver conjecture in algebraic number theory is particularly strong, but I am unable to explain why it is so amusing. (No, it wasn’t me.)

More mathematics books by women
A year ago, for International Women’s Day, I made a list of mathematics books by women covered by thennew Wikipedia articles. I thought it would be worthwhile to revisit the same topic and list several more mathematics books with at least one female author, at many different levels of audience, and again covered by new Wikipedia articles. They are (alphabetical by title):
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