Dismiss Notice
Join Physics Forums Today!
The friendliest, high quality science and math community on the planet! Everyone who loves science is here!

Homework Help: Help with question interpretation

  1. Apr 23, 2008 #1
    1. The problem statement, all variables and given/known data

    Give an example of a function f for which the following assertion is false:

    If [tex]|f(x)-l|<\epsilon[/tex] when [tex]0<|x-a|<\delta[/tex], then [tex] |f(x)-l|<\epsilon/2[/tex], when [tex] 0<|x-a|<\delta/2[/tex]

    3. The attempt at a solution

    I am really not quite sure what I am looking for here. I think i want a function for which [tex]\delta[/tex] gets smaller much more quickly than epsilon does, any input as to what I am actually looking for would be great.
     
  2. jcsd
  3. Apr 23, 2008 #2

    HallsofIvy

    User Avatar
    Science Advisor

    Since the problem only asks for "an example", I would go for the simplest. And it looks like a linear function, f(x)= mx+ b, should work. Draw an arbitrary straight line on an xy coordinate system, draw a rectangle at a point on that line, so the line is its diagonal, with [itex]\delta[/itex] as the length of the horizontal side and [itex]\epsilon[/itex] as the length of the vertical side. Now, imagine making [itex]\epsilon[/itex] smaller. How does [itex]\delta[/itex] change? What does the slope have to be so that [itex]\delta[/itex] decreases faster than [itex]\epsilon[/itex]?
     
  4. Apr 23, 2008 #3
    If i let b =1, and if m < 1, then [tex]\delta[/tex] gets smaller more quickly than [tex]\epsilon[/tex]. Is f(x) = 0.25x + 1 a suitable answer to this question?
     
  5. Apr 23, 2008 #4

    HallsofIvy

    User Avatar
    Science Advisor

    I may have answered too quickly and lead you astray. Yes, if m< 1, [itex]\delta[/itex] gets smaller more quickly than [itex]\epsilon[/itex]- but [itex]\delta[/itex] will reach half its original size exactly when [itex]\epsilon[/itex] reaches half its original size- so linear equations will not work here. Okay, then, what about y= x2? Take (0,0) as your initial point and [itex]\epsilon= 1[/itex]. What does [itex]\delta[/itex] have to be? Now take [itex]\epsilon= 1/2[/itex]. What does [itex]\delta[/itex] have to be.
     
  6. Apr 23, 2008 #5
    [tex]\delta[/tex] has to be sqrt(2), which is still greater than [tex]\delta/2[/tex]. Further investigation revealed that this seemed to be the case for x^3 etc too. I tried it with 1/(x^2), as a=1, l=1, that seemed to work quite nicely. Is this a useful candidate?
     
    Last edited: Apr 23, 2008
Share this great discussion with others via Reddit, Google+, Twitter, or Facebook