Polynomials and the Inner Product

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Discussion Overview

The discussion revolves around whether certain formulae involving polynomials satisfy the properties of an inner product in the context of real polynomials. Participants explore the definitions and conditions necessary for a valid inner product, including specific examples and counterexamples.

Discussion Character

  • Technical explanation
  • Debate/contested
  • Mathematical reasoning

Main Points Raised

  • One participant suggests that both the formulae \( f(1)g(1) \) and \( \left(\int_{0}^{1}f(t)dt\right)\left(\int_{0}^{1}g(t)dt\right) \) satisfy the properties of an inner product, while the book disagrees.
  • Another participant emphasizes that the focus should be on the formula \( (f,g)=f(1)g(1) \) and questions the relevance of the integrals.
  • Several participants discuss the properties of an inner product, including commutativity, distributivity, associativity, and positivity (or positive definiteness).
  • Examples are provided to illustrate cases where the positive definiteness condition may not hold, such as with the polynomial \( f(x)=\frac{1}{2}-x \).
  • There is a clarification that the inner product \( \) can be zero even if \( f(x) \) is not the zero polynomial, which raises questions about the validity of the inner product under certain conditions.
  • Participants express confusion regarding the definitions and properties of inner products, particularly around the concept of positivity versus positive definiteness.

Areas of Agreement / Disagreement

Participants do not reach a consensus on whether the proposed formulae satisfy the inner product properties. There is ongoing debate about the definitions and implications of the properties involved.

Contextual Notes

Some participants express uncertainty about the definitions of inner product properties and their implications, leading to potential misunderstandings regarding the conditions for positive definiteness.

neutrino
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The question requires me to check whether the following formulae satisfy the properties of an inner product given the linear space of all real polynomials.
<br /> f(1)g(1)

\left(\int_{0}^{1}f(t)dt\right)\left(\int_{0}^{1}g(t)dt\right)

The properties are satisfied in both cases (at least, that's my answer), but the book says 'No'. How could this be?
 
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Eeh, wherever did you get those integrals from?
You have been GIVEN that the formula is (f,g)=f(1)*g(1).
THAT is the formula you are asked to check whether is correct or not!
 
arildno said:
Eeh, wherever did you get those integrals from?
You have been GIVEN that the formula is (f,g)=f(1)*g(1).
THAT is the formula you are asked to check whether is correct or not!

I think these might be two different questions, since the integrals don't follow from the first expression!
 
Sorry...cristo's right. Edited the post.
 
Show some work. What makes you think that the properties are satisfied?
 
Okay, for the first one...
Under the usual rules of multiplication...

f(1)g(1) = g(1)f(1)
(cf)(1)g(1) = c(f(1)g(1))
f(1)(g+h)(1) = f(1)g(1) + f(1)h(1)
f(1)f(1) = (f(1))2 > 0 for f(1) != 0, and of course, it's zero when f(1) = 0.

Second...
int(f)int(g) = int(g)int(f)
int(f)int(g+h) = int(f)( int(g)+int(h)) = int(f)(int(g)+int(f)int(h)
int(cf)int(g) = c int(f)int(g)
int(f)int(f) = (int(f))^2 >= 0
 
whatb ARE the properties of an inner product? MAYBE ONE OF THEM IS SUPPOSED TO BE positive definiteness.
 
mathwonk said:
whatb ARE the properties of an inner product?
Commutativity, Distributivity, Associativity and Positivity(that's the name given in the book for the last property).

MAYBE ONE OF THEM IS SUPPOSED TO BE positive definiteness.
I'm afraid I don't understand. :rolleyes:
 
What happens to the 4th condition if:
f(x)=\frac{1}{2}-x
 
  • #10
What is positivity?
 
  • #11
NateTG said:
What happens to the 4th condition if:
f(x)=\frac{1}{2}-x

f(1)f(1) = -0.5x(-0.5) = 0.25>0 ?
 
  • #12
neutrino said:
f(1)f(1) = -0.5x(-0.5) = 0.25>0 ?
He meant in relation to the integral inner product.
 
  • #13
AKG said:
He meant in relation to the integral inner product.
Ah! Okay, I get it. f(x) is not zero, but the inner product <f,f> is. Therefore it does not satisfy the last condition.

Similarly f(x) = x-1 is an example where the last condition is not satisfied (for the first problem), right?
 
  • #14
neutrino said:
Ah! Okay, I get it. f(x) is not zero, but the inner product <f,f> is. Therefore it does not satisfy the last condition.

Similarly f(x) = x-1 is an example where the last condition is not satisfied (for the first problem), right?
Right. When mathwonk mentioned "positive definiteness", he was referring to the following property:

<f,f> > 0 if f is not the zero polynomial, and <f,f> = 0 if f is the zero polynomial.

It is standard to call this property "positive definiteness", although you appear to be calling it "positivity".
 
  • #15
Thanks for the help, guys. :)

AKG said:
It is standard to call this property "positive definiteness", although you appear to be calling it "positivity".

Not me, but Tom M.Apostol. :-p
 

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