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June 30th, 2010, 02:06 AM  #1 
Member Joined: May 2010 Posts: 47 Thanks: 0  Infinite dimensional
Hello everyone I read today that an equation is infinitedimensional. Since I have to study maths quite a few years .. could you please help me what these two words mean and suggest me some resources to start reading? Best Regards Alex. 
June 30th, 2010, 07:14 AM  #2 
Global Moderator Joined: Nov 2006 From: UTC 5 Posts: 16,046 Thanks: 938 Math Focus: Number theory, computational mathematics, combinatorics, FOM, symbolic logic, TCS, algorithms  Re: Infinite dimensional
I'm not sure what you heard or what you're asking. But dimensions are a simple concept in mathematics. We say that a plane (or figures that 'live in' a plane like squares or triangles) is twodimensional because you need precisely 2 numbers to specify its location, relative to a given origin. We say the same about the defining relation of a twodimensional object: describes a circle, and it uses two variables and hence is twodimensional. (There are technical points here  you could embed a circle in three or 47dimensional space  but that's not relevant here.) Example: The power radiated from an object (e.g. the sun) is where P is the power, A is the area, e is the emissivity, and T is the temperature (on an absolute scale, like Kelvins). This is (not counting the constant sigma) a fourdimensional relation. An infinitedimensional one is just one with infinitely many variables. It's usually expressed in terms of an infinite matrix. 
June 30th, 2010, 11:01 AM  #3 
Newbie Joined: Jun 2010 Posts: 14 Thanks: 0  Re: Infinite dimensional
If you provide a bit more context for your question, then you're likely to get a better answer. Dimension is a fundamnetal mathematical concept, but absolutely not a simple concept, despite the previous reply to this thread. It's an inherent property of the object in question, but it can be difficult to define exactly what that means. There is a concept of topological dimension for manifolds (like a circle, which is onedimensional), and an algebraic concept of dimension for vector spaces over a field. These are two different, but interrelated ideas of dimension. Since you are asking about infinite dimensionality, the most likely applicable reference is functional analysis. This is the general subject where people most often address infinite dimensional spaces, but is pretty fancy and it takes a lot of work to get there. 
July 1st, 2010, 01:36 AM  #4 
Member Joined: May 2010 Posts: 47 Thanks: 0  Re: Infinite dimensional
I would like to thank you for your answers Actually there is a function that models the spatiotemporal evolution Integral of w(x,u)*s(u,t1)du According to the author this is an infinite dimensional and a common approach to reduce this dimensionallity is to emply a basisexpansion representation. Then it proceeds by defining a basisexpansion representation. I hope this helps somehow. Best Regards Alex 
July 1st, 2010, 09:53 AM  #5 
Newbie Joined: Jun 2010 Posts: 14 Thanks: 0  Re: Infinite dimensional
I'm not sure at all what the term "basis expansion representation" means in this context, but given the explicit reference to infinite dimensionality, I have a guess. I imagine the integral in question is probably a functional integral, where the space that you are integrating over is a function space, like all paths connecting two points or some such. This idea started in two places (that I know of):  the study of brownian motion and the corresponding integral, called the Weiner integral or process, has its basis in rigorous mathematics.  quantum mechanics and the corresponding integral, called the Feynman path integral (wikipedia suspect here), DOES NOT (and cannot) have its basis in rigorous mathematics and is a purely formal idea. This is pretty fancy math, basically in the realm of functional analysis; depending on your level of sophisification, this may be a bit much. I don't feel like I'm helping you much, but I hopefully it does some good. 
July 15th, 2010, 04:17 AM  #6 
Member Joined: May 2010 Posts: 47 Thanks: 0  Re: Infinite dimensional
I would like to thank you for your reply. I am sorry I didn't reply for so long period. Best Regards Alex 
January 17th, 2011, 03:29 AM  #7  
Member Joined: May 2010 Posts: 47 Thanks: 0  Re: Infinite dimensional Quote:
I would like to ask you in your example T^4 is a fourdimensional relation. According to your simplified definition (which helped me a lot) I need two numbers(or variableS) to describe the (x,y) plane. Why T^4 depends on four variables and thus is a four dimensional plane? I would like to thank you in advance for your help Best Regards Alex  
January 17th, 2011, 09:38 AM  #8  
Global Moderator Joined: Nov 2006 From: UTC 5 Posts: 16,046 Thanks: 938 Math Focus: Number theory, computational mathematics, combinatorics, FOM, symbolic logic, TCS, algorithms  Re: Infinite dimensional Quote:
 
January 18th, 2011, 02:18 AM  #9 
Member Joined: May 2010 Posts: 47 Thanks: 0  Re: Infinite dimensional
No thanks, I could not see your equation correctly so I misinterpreted it. Thanks again Alex 

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