Proof of Continuity of f+g & f*g on R

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Homework Help Overview

The discussion revolves around the proof of continuity for the sum and product of two nowhere continuous functions, specifically the functions f and g defined using the Dirichlet function. The original poster seeks to demonstrate that while f and g are not continuous, their sum f+g and product f*g can be continuous on the real line.

Discussion Character

  • Exploratory, Conceptual clarification, Problem interpretation

Approaches and Questions Raised

  • The original poster attempts to define f and g using the Dirichlet function and explores the implications of their sum and product. Some participants question the equivalence of D(x) and D(x)^2 as n approaches infinity, while others suggest clarifying the definitions of f and g in terms of rational and irrational inputs.

Discussion Status

The discussion is ongoing, with participants exploring different interpretations of the functions involved. There is an emphasis on understanding the behavior of f and g based on the nature of the input (rational vs. irrational), but no consensus has been reached yet.

Contextual Notes

Participants are navigating the definitions and behaviors of the Dirichlet function, particularly in relation to continuity and the implications of its properties when combined through addition and multiplication.

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Homework Statement



Show that there exist nowhere continuous functions f and g whose sum f+g is continuous on R. Show that the same is true for their product.

Homework Equations



None

The Attempt at a Solution



Let f(x) = 1-D(x), where D(x) is the Dirichlet function
Let g(x) = D(x)

(f+g)(x) = 1

(f*g)(x) = D(x) - D(x)^2 <-- where I'm befuddled

I know that D(x) can be written as the limit of cos(m!*pi*x)^(2n) as n, m --> infinity and that D(x)^2 is then equal to cos(m!*pi*x)^(4n). Since n --> infinity, are D(x) and D(x)^2 equivalent?
 
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Write out the definitions of f(x) and g(x) in terms of x and look at their product.
 
LCKurtz said:
Write out the definitions of f(x) and g(x) in terms of x and look at their product.

You mean it will alternate between 0^2 and 1^2 then?
 
ƒ(x) said:
You mean it will alternate between 0^2 and 1^2 then?

No. I mean if x is rational what are f(x) and g(x) and their product. And what if x is irrational?
 

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