Geometry: Learn About Toruses & N-Balls

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In summary, the conversation discusses the study of algebraic topology and the objects that are commonly encountered in this field, such as toruses and n-balls. The conversation also mentions the difficulty in understanding these objects and asks if there is a specific subject or course that focuses on them. The conversation includes explanations and visual aids for understanding these objects and their properties, such as the torus being the product of two S1 spheres. It also mentions the concept of removing open disks from spheres and their homeomorphism to other shapes. Overall, the conversation highlights the complexity of these objects and the need for further study and understanding.
  • #1
Evilinside
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Right now, I am currently trying to study algebraic topology. In my geometrical studies I keep seeing objects such as toruses and n-balls. While an explanation of these objects is usually supplemented, I wanted to know if there was actually some subject or course out there from which I can learn about these objects specifically. Does anyone know of one?
 
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  • #2
Evilinside said:
Right now, I am currently trying to study algebraic topology. In my geometrical studies I keep seeing objects such as toruses and n-balls. While an explanation of these objects is usually supplemented, I wanted to know if there was actually some subject or course out there from which I can learn about these objects specifically. Does anyone know of one?

What you're talking about is shape, not necessarily geometry. If you're interested in n-tori (surfaces of genus n) or n-spheres, then you should consider trying to learn a bit about topology. Google for some introductory courses on topology and you should find what you're looking for.
 
  • #3
What else is there to know about them than the definition of what they are? Actually plenty, but you're studying algebraic topology to learn these facts about them. Beyond knowing what they are as (topologized) sets of points in R^n there is no other prerequisite knowledge required, I wouldn't have thought. Of course it'd be handy to understand that the torus and klein bottle are quotients of the plane for when you come to work out the fundamental groups.
 
  • #4
I don't mean to sound incompetent, but I'm having difficulty studying Allen Hatcher's "Algebraic Topology" based his explanations of these objects alone. For example, A torus is the product of two S1 spheres. Algebraically that's simple, but intuitively and visually that makes no sense to me. How a doughnut comes from two circles in different planes, I don't know. Also why is a sphere always embedded in Rn+1 space and then it's boundary be an Sn-1 sphere? I've read that removing a single open disk from the 2-sphere gives a space which is homeomorphic to the closed 2-disk and removing two open disks from the 2-sphere gives a space homeomorphic to a closed cylinder. What is an open disk again and how does removing two from a 2 sphere give you a cylinder? Questions like this go on and on- mobius bands, klein bottles and other objects. I can still continue studying algebraic topology algebraically, computing fundamental groups, but I still feel like I'm in the dark and like I'm missing half of everything that is being said since I can never keep up with geometrical intiution the author expects you to know at that point.

I've studied point set topology before, but I will try look for a different source to see if it discusses these objects.
 
  • #5
It is really easy to explain if I were able to draw it fofr you. Indeed, you should draw a picture yourself, or get a bicycle inner tube, or even a donut, for what follows.

Take a donut draw a circle round the 'crown' (i.e. on the top of the donut, where the chocolate frosting is) going round the hole. There's one S^1, now pick a point on the circle as a base point. How do I specify the other points on the donut? I go round this circle some way and then travel on a second circle that goes off at right angles and loops round the body of the donut. There, the Torus T^1 is the same as S^1xS^1

The torus is also the same as the unit square with opposite sides glued together, which also shows that it is like S^1xS^1, since S^1 is the same as the unit interval with the end points identified.

Take S^2, remove an open disc, then you can imagine spreading it out by putting your fingers into the hole and pulling, and you'll get a disc, necessarily closed if we are only thinking of homeomorphism (homeomorphisms must be 'undoable' in this sense, homotopies need not be, thus a sphere with any point, open disc, or closed disc, removed is homotopic to a point.

An open disk is something homeomorphic to the set of points {(x,y) : x^2+y^2<1}, or better, it is well, a disc without its boundary (thus making it open).

Removing an open disc is punching a hole in the sphere, but leaving the boundary of the hole there (so what remains is closed).

These thigns really are a lot easier with a pad of paper to draw them on.

here, try this

http://www.answers.com/topic/torus

(googles third hit on searching for 'torus homeomorphic product circles', by the way), about a 1/3 of the way down it draws a picture of the torus and the two embedded circles idea.
 
  • #6
Evilinside said:
Also why is a sphere always embedded in Rn+1 space and then it's boundary be an Sn-1 sphere?

It isn't always so embedded, but that embedding is the easiest way to give a description of it. Spheres don't have boundaries, so I don't get the next part. The set of lines through the origin in complex two space is homeomorphic to a sphere, for instance.
 

FAQ: Geometry: Learn About Toruses & N-Balls

What is a torus?

A torus is a three-dimensional shape that resembles a donut. It has a circular cross-section and a hole in the middle. A torus is a type of curved surface known as a "surface of revolution," meaning it is created by rotating a two-dimensional curve around an axis.

What are some real-world examples of toruses?

Some common real-world examples of toruses include donuts, car tires, and life preservers. Toruses can also be found in nature, such as in the shape of some fruits like apples or oranges.

How is a torus different from a sphere?

A torus has a hole in the middle, while a sphere is a completely closed shape. Additionally, a torus has two types of curvature - the inner and outer surfaces have different radii - while a sphere has the same curvature at all points. A torus also has a more complex topology, meaning it has a different number of holes and handles than a sphere.

Can a torus exist in higher dimensions?

Yes, a torus can exist in higher dimensions. In fact, a torus is an example of a shape that can be generalized to any number of dimensions. For example, a four-dimensional torus would be a shape with a hole in the middle and a curved surface that extends in four dimensions.

What is an N-ball?

An N-ball is a generalization of a sphere to higher dimensions. It is a shape with a certain radius that extends in N dimensions. For example, a 3-ball would be a sphere, a 4-ball would be a shape with a spherical surface in four dimensions, and so on. N-balls are used in mathematics and geometry to study higher-dimensional shapes and spaces.

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