How do you prove this statement in geometry?

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Discussion Overview

The discussion revolves around the conditions under which geometric shapes can be formed, specifically focusing on polygons, polyhedra, and hyperspaces. Participants explore the minimum number of points required to define these shapes with nonnegative area, volume, or measure, and the implications of these definitions in geometry and linear algebra.

Discussion Character

  • Debate/contested
  • Conceptual clarification
  • Mathematical reasoning

Main Points Raised

  • Some participants assert that a polygon with nonnegative area cannot be formed with fewer than 3 points, while others challenge this by stating that a single point can be considered a polygon of zero area, which is nonnegative.
  • There is a distinction made between proving the statement using geometry versus using linear algebra, with some suggesting that the latter is easier.
  • One participant elaborates on the concept of constructing a prism from n-1 points and discusses the relationship between the area of the polygon and the volume of the prism, indicating that the volume tends to zero as the height approaches zero, leading to a conclusion about Lebesgue measure.
  • A later reply corrects the terminology from "nonnegative measure" to "positive measure," indicating a refinement in the discussion.

Areas of Agreement / Disagreement

Participants express differing views on the definition of polygons and the minimum number of points required, leading to an unresolved debate regarding the validity of the initial claims and the nature of the proofs involved.

Contextual Notes

Participants note the importance of definitions and the implications of dimensionality on the existence of volume and measure, suggesting that the discussion is limited by these factors.

phoenixthoth
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A polygon with nonnegative area cannot be formed with fewer than 3 points.
A polyhedra with nonnegative volume cannot be formed with fewer than 4 points.
A hyperspace with nonnegative measure cannot be formed with fewer than n points.

What I mean by "3 points" is that the cardinality of the set of vertices is 3.
 
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phoenixthoth said:
A polygon with nonnegative area cannot be formed with fewer than 3 points.

Actually a single point is a polygon of zero area which is nonnegative.

When you say "prove this statement in geometry" are you saying "prove this statement, which has to do with geometry" (because a proof using linear algebra is pretty easy) or are you asking "prove this statement using geometry" which I think would be significantly harder
 
phoenixthoth said:
A polygon with nonnegative area cannot be formed with fewer than 3 points.
A polyhedra with nonnegative volume cannot be formed with fewer than 4 points.
A hyperspace with nonnegative measure cannot be formed with fewer than n points.

What I mean by "3 points" is that the cardinality of the set of vertices is 3.

Take any n-1 points and build a small prism based on the polygon they create in a n-1-dimensional hyperplane (the polygon has to be completely inside the prism). Prism's volume is not more that S*h, where S is the "area"(n-1-dimensional) of the polygon and h is the length of the edge not coplanar with the hyperplane. Now it's clear that S*h tends to 0 as h tends to 0. By definition it means that the set of points of the polygon has "zero Lebesgue measure".
I hope you mentioned the word "cardinality" for no reason, because in an infinite-dimensional space volume as we're used to it (Lebesgue measure) doesn't exist.
 
Sorry, I meant positive measure.
 

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