What is the Taylor series expansion at ##a/2## for finding real functions?

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Homework Help Overview

The problem involves finding real functions \( f \) that satisfy the equation \( f' (x) + f (a - x) = e^x \), where \( a \) is a constant. The context suggests a focus on the Taylor series expansion at the point \( a/2 \) as a potential method for exploring solutions.

Discussion Character

  • Exploratory, Conceptual clarification, Problem interpretation

Approaches and Questions Raised

  • Participants discuss the use of the Abel Identity and question its appropriateness for the problem. There are attempts to clarify whether proposed solutions actually satisfy the original equation. Some suggest strategies for eliminating references to \( f(a-x) \) and its derivatives, while others propose expanding a solution in the form of a Taylor series at \( a/2 \).

Discussion Status

The discussion is ongoing, with participants exploring different approaches and questioning the validity of proposed solutions. Guidance has been offered regarding strategies to manipulate the equation, and there is an acknowledgment of the need for further clarification on the role of \( a \) in the solutions.

Contextual Notes

Participants note the requirement for showing effort in their attempts, and there is a mention of a potential delay in responses due to notification issues. The original poster's use of the Abel Identity is under scrutiny, and there is an emphasis on the need to express solutions without references to \( f(a-x) \).

L Navarro H
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Homework Statement
Find the real functions f so: f' (x) + f (a - x) = e^x
a is a constant
Relevant Equations
none
Mod note: Member warned that some effort must be shown.
 
Last edited by a moderator:
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L Navarro H said:
Homework Statement:: Find the real functions f so: f' (x) + f (a - x) = e^x
a is a constant
Relevant Equations:: none

.
Please post your attempt, per forum rules.
 
haruspex said:
Please post your attempt, per forum rules.

I use the Abel Identity to find the functions, but I'm no sure if it's correct to used it in this problem
my answer is {e^x , - 1/(3*e^(2x))
 
L Navarro H said:
I use the Abel Identity to find the functions, but I'm no sure if it's correct to used it in this problem
my answer is {e^x , - 1/(3*e^(2x))
Are you saying that each of those is a solution to the equation? Doesn’t look that way to me. Shouldn't 'a' figure in the answer?
Please post your working.

Hint: you need a strategy for eliminating references to f(a-x) and its derivatives so that all the references to f are to f(x) and its derivatives.

Btw, sorry for the delay.. for some reason I did not get the alert. Happens sometimes.

Still there @L Navarro H ? If you need a stronger hint, try two things...
1. Write the equation swapping x and a-x.
2. Differentiate the equation.
Can you see how get rid of references to f(a-x)?
 
Last edited:
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have you tried to expand a solution in the Taylor series at the point ##a/2##? That is to look for solutions in the form ##f(x)=\sum_{k=0}^\infty f_k(x-a/2)^k##

upd: but haruspex's idea is better
 
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