# Lecture 17: Alexandrov's Theorem | Summary and Q&A

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August 26, 2014
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MIT OpenCourseWare
Lecture 17: Alexandrov's Theorem

## TL;DR

Folding polygons into polyhedra involves gluing the boundary to make a convex polyhedron, with various mathematical and algorithmic challenges.

## Key Insights

• 🙏 Zipping is a crucial step in folding polygons into polyhedra.
• 🤣 The gluing tree provides a different perspective on gluings and allows for the characterization of rolling belts.
• 🤣 Four rolling belts are impossible in a given example.

## Transcript

PROFESSOR: Let's get started. Today we're really going to take on folding polygons into polyhedra in a big way. We got started last time. Today we're going to do the real thing with some tape and scissors. Folding polyhedra. It's pretty much the last kind of folding that we're going to be talking about in this class. We see linkage folding, paper f... Read More

### Q: What is the main problem in folding polygons into polyhedra?

The main problem is determining whether a given polygon can be folded into a convex polyhedron.

### Q: How many rolling belts can you have in a single example?

In general, you can have more than one rolling belt, but four rolling belts is the maximum.

### Q: What does Alexandrov's Theorem state?

Alexandrov's Theorem states that given a convex polyhedral metric, there is a unique convex polyhedron that can be realized.

## Summary & Key Takeaways

• Folding polygons into polyhedra involves finding ways to glue the boundary of a given polygon to form a convex polyhedron.

• There are two main problems: the decision problem and the enumeration problem.

• Alexandrov's Theorem states that given a convex polyhedral metric, there is a unique convex polyhedra that can be realized.