After I published the last post, which I did not originally intend to have two parts, this comment was left by one of my blog’s followers. My answer is also shown.

A torus can be viewed as a flexible rectangle rolled into a donut shape, and I had used 24 small rectangles by 24 small rectangles as the settings for *Stella 4* for the torus, and its dual, in the last post — which, due to the nature of that program, are actually rendered as toroidal polyhedra. To investigate my new question, I increased 24×24 to 90×90, and these three images show the results. The first shows a 90×90 torus, the second shows its dual, and the third shows the compound of the two.

When I compare these images to those in the previous post, it is clear that these figures are approaching a limit as n, in the expression “nxn rectangle,” increases. What’s more, I recognize the dual now, of the true torus, at the limit, as n approaches infinity — it’s a cone. It’s not a finite-volume cone, but the infinite-volume cone one obtains by rotating a line around an axis which intersects that line. This figure, not a finite-volume cone, is the cone used to define the conic sections: the circle, ellipse, parabola, and hyperbola.

What’s more, I smell calculus afoot here. I do not yet know enough calculus.

“Learn a lot more about calculus” is definitely on my agenda for the coming Summer, for several reasons, not the least of which is that I plainly need it to make more headway in my understanding of geometry.

[Note: *Stella 4d*, the program used to make these images, may be found at http://www.software3d.com/Stella.php.]

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## About RobertLovesPi

I go by RobertLovesPi on-line, and am interested in many things, the majority of which are geometrical. Welcome to my little slice of the Internet.
The viewpoints and opinions expressed on this website are my own. They should not be confused with the views of my employer, nor any other organization, nor institution, of any kind.

Thanks Robert. You’ll need a lot of calculus for this one, it looks like differential geometry to me (a topic I know almost nothing about!)

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There exist precise definition (as mathematical operation) of several kinds of duality. Some of them lead to different shapes in the same space, others lead to different spaces. Even the same duality generally used in polyhedron analysis gives different results depending on whether you consider a figure as its bounds or also its interior. Consider a pentagon. Its dual is also a pentagon (vertex edge, 5 of each). However the same pentagon viewed as a facet of dodecahedron gives icosahedron, wich is completely other story.

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Thank you for the info!

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I smell topology!

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Very interesting! I too smell topology!

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