Need some hints on my HW about Linear functionals

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Discussion Overview

The discussion revolves around homework questions related to linear functionals, specifically focusing on their properties, definitions, and applications in polynomial spaces. Participants seek clarification on the concepts and notation involved in the problems presented.

Discussion Character

  • Homework-related
  • Technical explanation
  • Conceptual clarification

Main Points Raised

  • One participant expresses uncertainty about how to start the first question and considers writing out a specific case for the linear functional.
  • Another participant suggests defining notations clearly, including the distinction between uppercase and lowercase letters in the context of polynomials.
  • There is a discussion about the definition of the field $$F$$ and the polynomial space $$P_m(F)$$, with one participant questioning the phrasing used in the textbook.
  • Participants propose that the linear functional can be expressed in terms of constants and derivatives evaluated at zero, but express uncertainty about how to proceed with the proof.
  • There is confusion regarding the mapping of polynomials, with one participant suggesting that it maps a polynomial to its value at zero, while another believes it maps to the square of that value.
  • A question is raised about the third derivative of a polynomial of degree at most 2, indicating a need for clarification on derivatives in the context of the problems.
  • One participant mentions extending a vector to a basis of a space as part of the homework questions.

Areas of Agreement / Disagreement

Participants do not appear to reach consensus on several points, including the definitions and properties of linear functionals, the specific notation used, and the interpretation of the mapping of polynomials. The discussion remains unresolved with multiple competing views and uncertainties present.

Contextual Notes

Limitations include unclear notation, potential inconsistencies in the professor's handwriting, and unresolved questions regarding the definitions of terms and the properties of linear functionals.

BaconInDistress
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I not very good at using the LaTex editor, so I took a photo of my HW questions.
View attachment 8831
For the first question, I'm not really sure how to get started, should I write out a specific case? Like what would $$\varphi (P)$$ be when m=1?

For the second question, I know that a linear functional have two properties, one being $$\varphi(u +v) = \varphi(u) + \varphi(v)$$ and the other one being $$\varphi(\lambda u) = \lambda\varphi(u)$$. I'm a bit confused about the mapping, is it saying that when we have a polynomial P(z), the map makes it goes to p(0) for all z?
 

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Perhaps you should start by defining notations, including $\mathbb{F}$, $P_m(\mathbb{F})$ and $p^{(k)}(x)$. Also, it is not very clear which of $p$'s are uppercase and which are lowercase letters. For example, in problem 1 in the left-hand side $\varphi(P)$ the letter $P$ seems to be uppercase, but in the right-hand side it is probably lowercase.
 
According to the textbook we use, $$F$$ is a field over $$\Bbb{R}$$ or $$\Bbb{C}$$. $${P}_{m}(F)$$ is the polynomial space of degree m.
One of the many problem with my homework is indeed the handwriting, my professor is not very consistent with his upper and lower case letters. I just copied down what he wrote.
I think for question one, we need to prove the existence of constants $${a}_{0},...,{a}_{m}$$ such that the linear functional $$\varphi (P) = {a}_{0}p(0) + \sum_{k=1}^{m} {p}^{(k)}(0)$$
Not really sure how to go from there
 
BaconInDistress said:
According to the textbook we use, $$F$$ is a field over $$\Bbb{R}$$ or $$\Bbb{C}$$.
"A field over $\mathbb{R}$" sounds strange. There are vector spaces and algebras over fields. Perhaps $\mathbb{F}$ is simply a field.

BaconInDistress said:
I think for question one, we need to prove the existence of constants $${a}_{0},...,{a}_{m}$$ such that the linear functional $$\varphi (P) = {a}_{0}p(0) + \sum_{k=1}^{m} {p}^{(k)}(0)$$
Not really sure how to go from there
I think $p$ should be lowercase in the left-hand side as well.

Every functional on $\mathbb{F}^m$ has the form $\varphi((b_0,\ldots,b_m))=a_0b_0+\dots a_mb_m$ for some constants $a_0,\ldots,a_m$. Now suppose $p(t)=b_0+b_1x+\dots b_mx^m$. What are $p(0), p'(0), \ldots, p^{(m)}(0)$?

I'm a bit confused about the mapping, is it saying that when we have a polynomial P(z), the map makes it goes to p(0) for all z?
Yes, I believe the function maps a polynomial $p(z)$ to $(p(0))^2$. The question is whether it is a linear functional.

In problem 3, what is the third derivative of a polynomial whose degree is at most 2?

For problem 4, extend $v$ to a basis of the space $V$.
 

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