TECH-MUSINGS

Thoughts On Algorithms, Geometry etc...

Thursday, July 13, 2023

Enclosing and Embedded isosceles triangles for a triangle - orientations

In this paper: https://arxiv.org/pdf/2205.11637.pdf, the following questions are answered:

- Given a triangle, how to find the smallest area(perimeter) isosceles triangle that contains it?

- Given a triangle, how to find the largest area(perimeter) isosceles triangle contained in it?

Let me just record a pair of associated questions for which the answer might be readily obtained from the above paper:

Which is the triangle for which the smallest area (perimeter) isosceles container and largest area (perimeter) isosceles containee have angular difference between their orientations is maximum? The orientation of an isosceles triangle is given treating the triangle as an arrowhead - from the midpoint of its base towards the apex.

A further pair of questions:
Which is the convex polygon for which the smallest area (perimeter) rectangle that contains it and the largest area (perimeter) rectangle contained in it have the angular difference between their orientations a maximum? The orientation of a rectangle is naturally its length or width - for a square, one could take either as orientation.

The answers for the above should be available soon. I shall update this post when they reach me.

Monday, July 10, 2023

Wrapping a 2D lamina with paper

Basic question: to wrap a given planar region with a convex sheet (such that every point on both sides of the lamina has at least one layer of paper covering it) with the wrapping convex sheet being of least area/perimeter. An n-fold wrap is a wrap such that to reach any point on the lamina from outside, a 'needle' will have to cut through at least n layers of paper.

It is not difficult to cook up convex 2D regions such that the least area wrapper and least perimeter wrapper are different. Here is a pentagon for which the least area wrapper is a 7-gon and the least perimeter wrapper appears to be a rectangle.



Note that above pic is a rough one. The x length of the pentagon being wrapped ought to be considerably more than the y height (say, twice). the slope of the near horizontal tilted edges should be much less than the pic indicates. Indeed, the pentagonal least area wrapper is only very marginally different from a rectangle.
Overall, the 7-gon wrapper (least area) is almost a rectangle with dimensions say 20 X 5 and the rectangular wrapper (least perimeter) has dimensions 10X11 approximately.

Questions:
- Which planar convex region of unit area has the least area wrapper having max area? Is it a disk?
- Which planar convex region of unit perimeter has least perimeter wrapper with max perimeter?
- Same questions as above with the wrapping generalized to an n-fold one.