# [Math] Classes of functions with specific product integration property

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Does anyone of you know if there are any known sets of real-valued continous functions that for any two memebers g(x) and h(x) satisfy the following? That is, I'm looking for different sets of functions which, when integrated in product, yield the same result as when integrated separately and then multiplied. I'm curious since in one of my color algorithms that are under development I have found that if I could approximate my current functions with functions of such a class the calculations would be simplified tenfold. What are your thoughts on this?

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If the functions g and h are normalized, then replacing the = with <= gives you a special case of Holder's inequality with p = q = 1. Before moving onto this it might be helpful to look at the p = q = 2 case, known as the Cauchy-Schwartz inequality, which is easier to think about because it is just a continuous version of the familiar inequality involving the dot product |a.b| <= |a||b|.

The Cauchy-Schwartz inequality is an equality whenever a and b are parallel, or linearly dependent, and this can be extended to a similar concept for continuous functions.

For the p = q = 1 case, something similar holds except using a different norm, and unfortunately I believe it's only an equality when g and h are constant.

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Looks remarkably similar to Fubini's theorem but possibly different...

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@sQuid
Thanks for your reply. I'm not sure if this will help me but I will certainly look into it. I've only had a crach course in L2 spaces and I'm not familiar with the general properties of Lp spaces. I'll look into this before I get back.

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@etothex
Yes, I've seen the quite elegant solution to the Gaussian integral a few times but I didn't know about this Fubini's theorem. On the other hand I always thought the part where the integral is squared was a bit lofty but now I see that it is well founded. [smile]

So basically it states that the product of two integrals is equal to the product of the integrands integrated with respect to the product of their measures. This under the assumption that the product integral is absolutely convergent. Does this always hold? Thanks for the reply.

[Edited by - staaf on March 3, 2006 2:21:19 AM]

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Another approach:
All continuous functions on a closed interval can be approximated by uniform-sized step functions under the L^n (0 < n < infinity) norms.

Integrals of uniform step functions look a hell of alot like sums.

If a_n are your steps, and C the width of your steps, then
int f =~ C sum a_n

so
int fg = C^2 sum (a_nb_n)
while
int f int g = C^2 sum a_n sum b_n
= C^2 sum a_n b_n + 2 C^2 sum a_j b_i with j!=i
= int fg + 2 C^2 sum a_j b_i with j!=i
(note: this looks a hell of alot like varience from probability.)
or even better:
= 2 int fg - int fg + 2 C^2 sum a_j b_i with j!=i
= - int fg + 2 C^2 sum a_j b_i

so int f int g - int fg =~
- 2 int fg + 2 C^2 sum a_j b_i

Lets look at
2 C^2 sum a_j b_i
= 2 C sum (a_j C sum b_i)
C sum b_i =~ int g
= 2 C sum (a_j int g)
= 2 int g C sum a_j
=~ 2 int g int f

so int f int g - int fg =~
- 2 int fg + 2 int g int f
= 2 int g int f - 2 int fg
=> int f int g - int fg = 0
=> int f int g = int fg

strange. Didn't expect that. Probably made a mistake.

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@NotAYakk
What exactly are you doing in the first part of your derivation?
This is what I arrive at:

int f int g = C2(sum an)(sum bn) = C2 sum aibj

Compare to:

(a1 + a2 + a3)(b1 + b2 + b3) = a1b1 + a1b2 + a1b3 + a2b1 + a2b2 + a2b3 + a3b1 + a3b2 + a3b3

You have a 2 sneaking in there that I can't see where it's coming from.

I hope you are aware that if this would hold then every continous function defined on a closed interval that can be approximated by such step functions would satisfy the equation at the top. [smile] This is very strong and my intuition tells me it can't be so since it is easy to find functions of the given type which does not satisfy the equation.

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@staaf
I believe you meant int{g(x)h(x)dx} = int{g(x)dx}int{h(x)dx}?

Let f(x) and g(x) be polynomials (you can always use Taylor's). In this case rhs doesn't have x^1 term while lhs has (assuming that first term is x^0). Obviously we could set on the coeffs corresponding to x^0 term to zero on both f and g, but this would cause the same situation to propagate to the next term. Therefore I think it's probable that there does not exist such functions f and g.

This reasoning has atleast one loophole: a polynomial having terms from -inf to +inf (this would restrict our domain to [0,inf)). I didn't bother to think this further, but it is easy to get rid of the integrals by expressing the pol. as sum_n=-inf^inf{c_n*x^n} and integrate. From there it is easy to form the relations between the coefficients.

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Quote:
 Original post by staaf@NotAYakkWhat exactly are you doing in the first part of your derivation?This is what I arrive at:int f int g = C2(sum an)(sum bn) = C2 sum aibj

What I was doing was utter crap. =) Did a square instead of a multiply. Which also explains why it looks like varience. ~_~

Quote:
 I hope you are aware that if this would hold then every continous function defined on a closed interval that can be approximated by such step functions would satisfy the equation at the top. [smile] This is very strong and my intuition tells me it can't be so since it is easy to find functions of the given type which does not satisfy the equation.

Going to go and fix the math.

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Quote:
 Original post by WinogradI believe you meant int{g(x)h(x)dx} = int{g(x)dx}int{h(x)dx}?

Yes. My mistake. I didn't think about that it wouldn't be the same thing so I used y to distinguish the two integrals.

Anyway, you are right about polynomials not being eligible as g(x) or h(x), but there are lots of other kinds of continous functions that behave differently when integrated. I think it is wrong to rule out the existence of such functions just based on the result from polynomials. Probably the functions that actually has this property are quite few in number, but nevertheless I'm curious about them.

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