Is the only difference between functions just a change of basis?

In summary, the conversation discussed the concept of Hilbert spaces and functional analysis, specifically in relation to decomposing functions into power series or Fourier series. It was suggested that these processes involve a change of basis, with the "standard" basis for polynomials being {1, x, x^2, x^3, ...}. The conversation also touched on the challenges of defining a basis for function spaces, as the functions themselves make up the basis. It was mentioned that while infinite dimensional spaces can be difficult to understand, some basic principles can still be applied.
  • #1
Monocles
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I just started teaching myself a little bit about Hilbert spaces and functional analysis in general, and I had an idea that I don't think I've seen in my textbook, so I'm wondering if I have the right idea:

All functions "live in" the space of all functions. When you are doing something like decomposing a function as a power series, or a Fourier series, is the only thing you are doing a change of basis? So, is the only difference between, say, e^x and 1 + x + (x^2)/2! + (x^3)/3! + ... is the basis you have written the function in?

If so, in what basis are typical elementary equations usually written in? Does it have a name? How do you even define the basis the functions are in without "knowing" what the function is (since the basis itself is made up of functions)?

Any other additional tidbits on this topic are appreciated as well. Try to keep things simple though, I've only taken 1 proof based math class (though it was on vector spaces, but we never got to Hilbert spaces).
 
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  • #2
Infinite dimensional spaces can get kind of weird, but some are easy enough to understand.

The "standard" basis for polynomials would be the set {1, x, x^2, x^3, ...}. Then, you can represent each polynomial with an infinite vector with a finite number of nonzero entries. For example, x^2+1 would be (1, 0, 1, 0, 0, ...) and 3x^3 - x^2 would be (0, 0, -1, 3, 0, ...). Add them together, and you get (1, 0, 0, 3, 0, ...) which is x^3 + 1.

Similarly with Fourier functions, you have the basis {1, cos x, sin x, cos 2x, sin 2x, cos 3x, sin 3x, ...}. You can represent these guys in the same way.

You can also do a change of basis. With polynomials, for example, we can use an ugly basis like {1, x + x^2, x - x^2, x^3 - x^4, x^3 + x^4, ...} or something like that.

But keep in mind that functionspaces don't play nicely all the time like R^n does. You can define the norm or "length" of a function by taking the integral over its domain. But you can only do this if you can guarantee that this integral is always finite! So for polynomials, you must restrict the domain to some bounded subset of R.
 

FAQ: Is the only difference between functions just a change of basis?

What is a basis in function space?

A basis in function space is a set of functions that can be used to represent any other function in that space through a linear combination of those basis functions. This is similar to how a basis in linear algebra is a set of vectors that can be used to represent any other vector through a linear combination.

Why is a basis important in function space?

A basis is important in function space because it allows us to simplify complex functions by breaking them down into simpler components. It also helps us to analyze and manipulate functions more easily, as well as make predictions and solve problems in various fields such as engineering, physics, and mathematics.

How do you determine a basis in function space?

The process of determining a basis in function space involves finding a set of linearly independent functions that span the entire space. This can be done through various methods such as trial and error, or through the use of mathematical techniques like Gram-Schmidt orthogonalization.

Can a function space have multiple bases?

Yes, a function space can have multiple bases. This is because there are often many different sets of functions that can be used as a basis for the same function space. However, all of these bases will have the same number of elements, known as the dimension of the function space.

What is the relationship between a basis and a coordinate system in function space?

In function space, a basis is similar to a coordinate system in that it provides a way to represent functions in a simpler form. Just as a coordinate system uses axes to represent points in space, a basis uses functions to represent other functions in the space. The coefficients of the basis functions in a linear combination can be thought of as the coordinates of the function in that basis.

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