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Joseph O'Rourke

@josephorourke.bsky.social
732 followers 483 following 97 posts

Mathematician and Computer Scientist, Smith College, USA. cs.smith.edu/~jorourke Polyhedron displayed in banner has max volume of all convex foldings from a square.

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Joseph O'Rourke @josephorourke.bsky.social · 27/06/2026
An unsolved problem. Given an m×n map formed of unit squares, with a given Mountain/Valley assignment for every crease, is there a subexponential algorithm to decide if it can be folded to a 1×1 stack of squares? Example: Yes. #MathSky #Mathematics #Geometry #Origami 🧪 cs.smith.edu/~jorourke/Ma...
2x4 map to fold to 1x1
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Joseph O'Rourke @josephorourke.bsky.social · 02/05/2026
A billiard knot is a path in a mirror polyhedron that realizes a knot by reflected lightrays. Not every knot is realizable in a cube, but every knot is a billiard knot in some convex right prism. arxiv.org/abs/1106.5600 mathoverflow.net/questions/38... #MathSky #Mathematics #Geometry #Knots 🧪
Lightray in mirror cube.
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Reposted by Joseph O'Rourke
Joseph O'Rourke @josephorourke.bsky.social · 23/03/2026
A 164 triangles version of the Stanford Bunny folded from a 4ft x 4ft thin aluminum sheet, following a crease pattern created by the *Origamizer* algorithm of E.Demaine & T.Tachi. Folded by an MIT group in 2011. cs.smith.edu/~jorourke/Ma... #MathSky #MathArt #Origami #Mathematics #Engineering 🧪
Aluminum Stanford Bunny.
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Joseph O'Rourke @josephorourke.bsky.social · 28/03/2026
There is research on fabricating micropolyhedra using lithographic techniques via self-assembly / self-folding of nets that fold to, e.g., a cube as illustrated. Note the tiny size: 0.2mm. The goal is to minimize mis-foldings. doi:10.1371/journal.pone.0004451 cs.smith.edu/~jorourke/Ma... #MathSky 🧪
Cube net folding.
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Joseph O'Rourke @josephorourke.bsky.social · 23/03/2026
A 164 triangles version of the Stanford Bunny folded from a 4ft x 4ft thin aluminum sheet, following a crease pattern created by the *Origamizer* algorithm of E.Demaine & T.Tachi. Folded by an MIT group in 2011. cs.smith.edu/~jorourke/Ma... #MathSky #MathArt #Origami #Mathematics #Engineering 🧪
Aluminum Stanford Bunny.
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Joseph O'Rourke @josephorourke.bsky.social · 16/03/2026
In rigid origami, the rigid faces hinge on creases. Much is unknown, but degree-4 vertices are understood. An example is the Miura Map fold. #MathSky #Mathematics #Geometry #Origami 🧪
Miura map fold is built from degree-4 vertices.
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Joseph O'Rourke @josephorourke.bsky.social · 31/12/2025
Robert Lang's origami *White-Tailed Deer*, Opus 550. Design based on his "uniaxial bases" and the "circle/river" and "tree methods." Chapter 6 in *The Mathematics of Origami*. cs.smith.edu/~jorourke/Ma... #MathSky #Mathematics #MathArt #SciArt #Origami 🧪
Lang's White-Tailed DeerCrease pattern.
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Joseph O'Rourke @josephorourke.bsky.social · 18/12/2025
Published today 18Dec2025: *The Mathematics of Origami.* Cambridge link: view.updates.cambridge.org?qs=99a0b7610... #MathSky #MathArt #Mathematics #Geometry #Science #Origami
Cover image.
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Joseph O'Rourke @josephorourke.bsky.social · 14/11/2025
Curved circular creases of annuli. A construction by Erik and Martin Demaine (all rights reserved). Several annuli intertwined. #MathSky #MathArt #Geometry #Origami More examples: erikdemaine.org/curved/)
Intertwined annuli.
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Joseph O'Rourke @josephorourke.bsky.social · 05/11/2025
*The Mathematics of Origami*. Expected online publication date: December 2025. Print publication: 31 December 2025. www.science.smith.edu/~jorourke/Ma... #MathSky #Mathematics 🧪 #Geometry #Origami #MathArt
Cover: The Mathematics of Origami
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Joseph O'Rourke @josephorourke.bsky.social · 02/11/2025
"Louvre robbery: Could a 50-year-old maths problem have kept the museum safe?" This is a BBC article by Kit Yates about the art gallery theorem. In the figure, four red vertex guards suffice to visually cover the whole polygon. #Mathematics #MathSky #GraphTheory www.bbc.com/future/artic...
3-coloring if a triangulated polygon
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Joseph O'Rourke @josephorourke.bsky.social · 31/10/2025
Crescent Moon. Did you ever notice that the outer convex curve of the crescent is a semicircle, but the inner concave curve is (half of) an ellipse. An ellipse because we are viewing a circle at an angle; a circle projects to an ellipse. #MathSky #Mathematics #Geometry #Pumpkin #Moon
Crescent moon carved into pumpkin.
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Joseph O'Rourke @josephorourke.bsky.social · 18/10/2025
It is *still* unknown whether or not every triangle admits a periodic billiard trajectory. Every triangle with rational angles does. And so does every obtuse triangle of at most 112.4 deg. "112.5 appears to be a natural barrier." gwtokarsky.github.io. #MathSky #Mathematics #Geometry #Billiards
Triangle w complex periodic orbit.
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Joseph O'Rourke @josephorourke.bsky.social · 26/09/2025
Stoker's Conjecture settled by Cho & Kim positively: Every 3D polyhedron is uniquely determined by its dihedral angles and edge lengths, even if nonconvex or self-intersecting (subject to technical restrictions). doi.org/10.1007/s004... #MathSky #Mathematics #Geometry #Polyhedra
Vertex v mapped to sphere.
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Joseph O'Rourke @josephorourke.bsky.social · 23/09/2025
What is the probability that 4 points chosen uniformly at random on surface of a sphere form a tetrahedron whose four faces are each acute? Asked on MathOverflow (mathoverflow.net/q/498296/6094) with evidence that the answer is 1/12. But not yet resolved. #MathSky #Mathematics #Geometry #Probability
Tetrahedron in a sphere.
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Joseph O'Rourke @josephorourke.bsky.social · 17/09/2025
A monohedral tiling of the plane by "spandrelized" squares. Each unit square includes a circular arc of a 1/2-radius circle centered at each vertex. Adams, Colin. "Spandrelized Tilings." Amer. Math. Monthly 132, no. 3 (2025): 199-217. doi.org/10.1080/0002... #MathSky #Mathematics #Geometry #Tiling
Scalloped square tiling.
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Joseph O'Rourke @josephorourke.bsky.social · 14/09/2025
Archimedes: "Every cylinder whose base is the greatest circle in a sphere and whose height is equal to the diameter of the sphere has a volume equal to 3/2 the volume of the sphere." Cicero found Archimedes' tomb ~137 yrs later with his famous theorem represented. #Mathematics #MathSky #Geometry
Sphere/Cylinder
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Joseph O'Rourke @josephorourke.bsky.social · 11/09/2025
New tiling results on the arXiv, one of which says that determining whether or not two connected polycubes can together tile R^3 is undecidable (Cor. 5.5). A polycube is an object built by gluing cubes face-to-face. (Unrelated fig.) arxiv.org/abs/2509.07906 #MathSky #Mathematics #Geometry #Tiling
Fig. 22(b)
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Joseph O'Rourke @josephorourke.bsky.social · 30/08/2025
Believe it or not, origami stents have been explored: Kuribayashi et al., "Self-deployable origami stent grafts ..." (doi.org/10.1016/j.ms...) Here I show a hexagonal design built with origami waterbomb crease patterns. cs.smith.edu/~jorourke/Ma... #Mathematics #Geometry #MathSky
Hexagonal Waterbomb stent.
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Joseph O'Rourke @josephorourke.bsky.social · 27/08/2025
The conjecture that every convex polyhedron is Rupert is settled in the negative! The convex body in the image cannot pass straight through a hole inside itself. arxiv.org/abs/2508.18475 #Mathematics #Geometry #MathSky
Non-Rupert convex polyhedron.
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Joseph O'Rourke @josephorourke.bsky.social · 23/06/2025
A surprising result: 3-space can be filled with disjoint geometric unit-radius circles. So each point of R^3 lies on exactly one circle. The circles may even be chosen to be unlinked. M. Jonsson and J. Wästlund: www.jstor.org/stable/24493.... #MathSky #Geometry #Mathematics
jstor.org
PARTITIONS OF R 3 INTO CURVES on JSTOR
M. JONSSON, J. WÄSTLUND, PARTITIONS OF R 3 INTO CURVES, Mathematica Scandinavica, Vol. 83, No. 2 (1998), pp. 192-204
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Joseph O'Rourke @josephorourke.bsky.social · 10/06/2025
You might guess that the maximal volume 8-vertex polyhedron inscribed in a unit sphere is the cube. But it's not even close : cube 1.54; 8-vertex max 1.82. Proved by Berman and Hanes in 1970. V=8, E=16, F=10. #MathSky #Geometry #Mathematics
Max volume 8-vertex inscribed polyhedron.
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Joseph O'Rourke @josephorourke.bsky.social · 31/05/2025
Angel-wing net (edge-unfolding) of a nearly flat prismoid, top & bottom two 40-vertex regular polygons. No mathematical significance, just an attractive image. (The two red edges are not cut.) #MathSky #Geometry #Mathematics #MathArt
Net for 40-vertex top & bottom prismoid.
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Joseph O'Rourke @josephorourke.bsky.social · 26/05/2025
The Eye-Ball Theorem: Two disjoint spheres S1 and S2. Form cone C1 tangent to S1 with apex at the center of S2, and form cone C2 similarly. Then the radii of the circles of cone/sphere intersections (red) are equal. #MathSky #Geometry
Two spheres, two tangent cones.
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Joseph O'Rourke @josephorourke.bsky.social · 11/05/2025
Among every set of six points in 3-space (in general position) are two linked triangles: The Conway-Gordon-Sachs theorem. General position excludes three points collinear and four points coplanar. #MathSky #Geometry
Two linked triangles.
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Joseph O'Rourke @josephorourke.bsky.social · 07/05/2025
Saturn's North pole hexagon. Still not thoroughly understood. Multiple Earths could fit inside. en.wikipedia.org/wiki/Saturn%... #MathSky #Geometry #Astronomy #Planets
North pole hexagon.
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Joseph O'Rourke @josephorourke.bsky.social · 20/04/2025
Happy Easter from the Stanford Bunny! (en.wikipedia.org/wiki/Stanfor...) Developed by Stanford researchers in 1994 as a test bed model for computer graphics algorithms. This version: 2,503 vertices. #MathSky #Geometry #Graphics
Stanford Bunny
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Joseph O'Rourke @josephorourke.bsky.social · 17/04/2025
A cube can be reoriented so that it can pass through a hole carved in a congreunt cube: Prince Rupert's cube (1693!) "It is unknown whether this is true for all convex polyhedra"! (en.wikipedia.org/wiki/Prince_...) #MathSky #Geometry
Prince Ruperts Cube
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Joseph O'Rourke @josephorourke.bsky.social · 16/04/2025
Correcting an earlier post (thanks Sophie Huiberts) to show off this attractive image. It shows the shortest paths from the white dot to each of 260 points on the genus-6 surface: 56 vertices, and regularly spaced points on edges. With student Biliana Kaneva. #MathSky #Geometry
Shortest paths on hollowed cube.
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Joseph O'Rourke @josephorourke.bsky.social · 06/04/2025
This open question was resolved ~20 yrs ago: Can one tie a knot with one foot of one-inch diameter rope? The answer is No: Diao, Yuanan. "The lower bounds of the lengths of thick knots." *Journal of Knot Theory and Its Ramifications* 12, no. 01 (2003): 1-16. #MathSky #Geometry #Topology
Thick Trefoil.
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Joseph O'Rourke @josephorourke.bsky.social · 29/03/2025
There are three simple closed geodesics on a cube. Two are planar: an equatorial band and the hexagon slice. The 3rd is a bit more unusual. It angles off the front bottom edge at arctan(2) = ~63 deg, forming a nonplanar hexagon of length 2 sqrt{5}. #MathSky #Geometry
Geodesic on cube
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Joseph O'Rourke @josephorourke.bsky.social · 21/03/2025
A Collatz-like function f(n) that bifurcates on the primes, I posed a decade ago. It remains unknown if it always falls into a 2/4 cycle. For n=229, f(n) shoots off to 10^{376} before returning to that cycle after 6309 iterations. mathoverflow.net/questions/20... #MathSky
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Joseph O'Rourke @josephorourke.bsky.social · 13/03/2025
Unfoldings of the hypercube. There are 261 unfoldings of the 4D hypercube into 3D. It is known since 2021 that none self-overlap (doi.org/10.37236/9796) and each tiles 3-space (Moritz Firsching). The most famous is the Dali cross, immortalized in his painting *Corpus Hypercubus*. #MathSky #Geometry
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Joseph O'Rourke @josephorourke.bsky.social · 08/03/2025
The attractively intricate optimal tiling of an equilateral triangle by the trapezoid polyiamond (pentiamond), found in 2019 by an anonymous user ('theonetruepath') on MathOverflow. #MathSky #Geometry
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Joseph O'Rourke @josephorourke.bsky.social · 23/02/2025
There exists a minimum area convex k-gon Q circumscribing a convex n-gon P (n>k) that has at least (k-1) edges flush with P and includes the midpoint on the k-th edge. A theorem of Adlai DePano. In the example, k=4 and 3 edges are flush, with the 4th balanced on a midpoint. #geometry #mathsky
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Joseph O'Rourke @josephorourke.bsky.social · 15/02/2025
On a 𝘡𝘰𝘭𝘭 𝘴𝘶𝘳𝘧𝘢𝘤𝘦, every geodesic is simple (non-self-intersecting) and closed. Aside from the sphere, remarkable that such surfaces exist! Zoll was a 1903 student of David Hilbert. Image by Konrad Polthier and Markus Schmies. #Geometry #MathSky
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Joseph O'Rourke @josephorourke.bsky.social · 06/02/2025
The alpha-complex of ~1500 points on the surfaces of two linked tori. #Geometry #MathSky
Two linked tori.
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Joseph O'Rourke @josephorourke.bsky.social · 31/01/2025
An n=20 polar zonohedron and its edge-unfolding to a net. #geometry #mathsky
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Joseph O'Rourke @josephorourke.bsky.social · 27/01/2025
The traditional art gallery problem asks for the fewest guards to guard a polygonal art gallery. Through the work of Abrahamsen, Adamaszek, Miltzow, it is now known that if the polygon coordinates are rational, the guards may need irrational coordinates. [Wikipedia image] #geometry #mathsky
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Joseph O'Rourke @josephorourke.bsky.social · 19/01/2025
A nice theorem: Isosceles tetrahedra are the only convex polyhedra that admit arbitrarily long simple, closed geodesics. See V. Protasov, "Simple Closed Geodesics on a Polyhedron." *The Mathematical Intelligencer* (2024): 347-354. (Isosceles: Opposite edges have same length.) #geometry #mathsky
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Joseph O'Rourke @josephorourke.bsky.social · 16/01/2025
"Dudeney’s Dissection is Optimal." It took 120 yrs. I posed the question 13 yrs ago here: mathoverflow.net/q/80101/6094 #mathsky #geometry
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Joseph O'Rourke @josephorourke.bsky.social · 01/01/2025
A tetrahedron for 2025 Joe Malkevitch suggested exploring tetrahedra with edge lengths 3,3,3,3,5,5, in light of 3^4 5^2 = 2025. #geometry #mathsky #math
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Joseph O'Rourke @josephorourke.bsky.social · 26/12/2024
Curved crease origami from Vesica Piscis Image by Klara Mundilova, from "On mathematical folding of curved crease origami..." *Computer-Aided Design*, Vol. 115, Oct. 2019, pp. 34-41. #geometry #origami
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Joseph O'Rourke @josephorourke.bsky.social · 05/12/2024
"All Polyhedral Manifolds are Connected by a 2-Step Refolding": Any polyhedral manifold A can be unfolded to B, which can be refolded differently to manifold C. So the space is connected via unfold/refold operations, answering a 2007 question *Geom. Folding Algs*. arxiv.org/abs/2412.02174 #geometry
arxiv.org
All Polyhedral Manifolds are Connected by a 2-Step Refolding
We prove that, for any two polyhedral manifolds P, Q, there is a polyhedral manifold I such that P, I share a common unfolding and I, Q share a common unfolding. In other words, we can unfold P, refol...
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Joseph O'Rourke @josephorourke.bsky.social · 02/12/2024
Finally settled: "Optimality of Gerver's Sofa": arxiv.org/abs/2411.19826
arxiv.org
Optimality of Gerver's Sofa
We resolve the moving sofa problem by showing that Gerver's construction with 18 curve sections attains the maximum area $2.2195\cdots$.
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