1. Limited time only! Sign up for a free 30min personal tutor trial with Chegg Tutors
    Dismiss Notice
Dismiss Notice
Join Physics Forums Today!
The friendliest, high quality science and math community on the planet! Everyone who loves science is here!

If f''(a) exists then f''(a) = ...

  1. Jun 5, 2015 #1
    1. The problem statement, all variables and given/known data
    Let ##I \subset \mathbb{R}## be an open interval and let ##f:I \rightarrow \mathbb{R}## be differentiable on I. Show that if ##f''(a)## exists, then ##f''(a) = \lim_{h \rightarrow 0}\frac{f(a + h) - 2f(a) + f(a - h)}{h^2}##


    2. Relevant equations


    3. The attempt at a solution

    I tried tackling this by using limit definition of the derivative for ##f''(a)##.

    $$f''(a) = \lim_{h \rightarrow 0}\frac{f'(a + h) - f'(a)}{h}$$


    and then maybe use limit definition again for the terms in the numerator above? maybe use L'hopitals's rule? Not sure
     
  2. jcsd
  3. Jun 5, 2015 #2

    DEvens

    User Avatar
    Education Advisor
    Gold Member

    Can you then write down the definition in terms of limits of ## f'(a + h) ## and ## f'(a)## ?
     
  4. Jun 5, 2015 #3

    pasmith

    User Avatar
    Homework Helper

    Use Taylor's theorem: If [itex]f[/itex] is twice differentiable at [itex]a[/itex] then [tex]
    f(a + h) = f(a) + hf'(a) + \frac12 h^2f''(a) + \epsilon(h)[/tex] where [tex]
    \lim_{h \to 0} \frac{\epsilon(h)}{h^2} = 0.[/tex]
     
  5. Jun 5, 2015 #4
    Using the limit definition of differentiation in the numerator and sone manipulation will give you the answer.
     
  6. Jun 5, 2015 #5

    Ok so the limit def of diff for the following

    $$f'(a) = \lim_{h \rightarrow 0} \frac{f(a + h) - f(a)}{h}$$

    $$f'(a + h) = \lim_{h \rightarrow 0} \frac{f(a + 2h) - f(a + h)}{h}$$

    The def of f'(a + h) I am not too sure about.
     
    Last edited: Jun 5, 2015
  7. Jun 5, 2015 #6

    pasmith

    User Avatar
    Homework Helper

    [tex]
    f'(a + h) = \lim_{k \to 0} \frac{f(a + h + k) - f(a + h)}{k}.
    [/tex]
     
  8. Jun 5, 2015 #7
    pasmith, How do you deal with that index of k when we are trying to simplify the numerator of


    $$f''(a) = \lim_{h \rightarrow 0}\frac{f'(a + h) - f'(a)}{h}$$

     
  9. Jun 5, 2015 #8

    pasmith

    User Avatar
    Homework Helper

    I don't; I would instead use the Taylor theorem approach I suggested in my earlier post.
     
  10. Jun 5, 2015 #9
    No need to use taylor, just "fuse" limits, using your notation, it is clear that f'(a+h) = lim h->0 f(a+2h)-f(a+h) / h and the same for f'(a), and write all the limits as one, show me you attempt so far !!
     
  11. Jun 5, 2015 #10

    pasmith

    User Avatar
    Homework Helper

    False: [tex]\lim_{h \to 0} \frac{f(a + 2h) - f(a + h)}h = \lim_{h \to 0} \frac{f(a) + 2hf'(a) - f(a) - hf'(a)}h = f'(a).[/tex]
     
  12. Jun 5, 2015 #11

    SammyS

    User Avatar
    Staff Emeritus
    Science Advisor
    Homework Helper
    Gold Member

    What happens if you put these into your expression for ##\ f''(a)\ ?##

    You get something pretty close to what you're looking for.

    Since the limit exists, it is the same as each of its one-sided limits. So, isn't it true that ##\displaystyle \ f''(a) = \lim_{h\to 0}\frac{\,f'(a - h) - f'(a)\,}{-h}\ ?##
     
  13. Jun 5, 2015 #12
    Not false, of course lim h-> 0 f'(a+h) = f'(a), f is continous and free if corners, you haven't prooved or denied anything !
     
  14. Jun 5, 2015 #13
    When i put those into my expression for f''(a) i get this...

    $$\lim_{h\rightarrow 0} \frac{\lim_{h\rightarrow 0} \frac{f(a + 2h) - 2f(a + h) + f(a)}{h}}{h}$$

    what on earth am I suppose to do with that monster?
     
  15. Jun 5, 2015 #14

    Dick

    User Avatar
    Science Advisor
    Homework Helper

    You are given that ##f## is differentiable on an open interval. That gives you the right to apply l'Hopital once if you can show it's a 0/0 form. THEN apply definitions of differentiability.
     
    Last edited: Jun 5, 2015
  16. Jun 6, 2015 #15

    vela

    User Avatar
    Staff Emeritus
    Science Advisor
    Homework Helper
    Education Advisor

    You really should be using two different variables for the limits.
    $$\lim_{h_1\to 0} \frac{\lim_{h_2\to 0} \frac{[f(a + h_1 + h_2) - f(a + h_1)] - [f(a+h_2)-f(a)]}{h_2}}{h_1}.$$ The question you've run into now about how to combine the two limits into one is probably why pasmith suggested the other method using Taylor's theorem. I don't offhand see how to justify combining the limits.
     
  17. Jun 7, 2015 #16
    it seems even with the taylor method you get into the same problem.

    $$f(a + h) = f(a) + hf'(a) + \frac{1}{2} h^2f''(a) + \epsilon (h)$$

    solve for f''(a) and get

    $$f''(a) = \frac{2f(a + h) - 2f(a) - 2hf'(a) - 2\epsilon (h)}{h^2}$$
    so i guess at this point you would take limit of both sides, but what do you do about the 2hf'(a) in the numerator?
     
  18. Jun 8, 2015 #17

    DEvens

    User Avatar
    Education Advisor
    Gold Member

    How do you know Taylor's theorem is correct? Hint: You need the limit that the original post requires to be proved.
     
  19. Jun 8, 2015 #18
    DEvens, would you use the limit definition for differentiation to prove this? If so how would you resolve the different limit variables?
     
  20. Jun 8, 2015 #19

    vela

    User Avatar
    Staff Emeritus
    Science Advisor
    Homework Helper
    Education Advisor

    Hint: Consider both f(a+h) and f(a-h).
     
  21. Jun 8, 2015 #20

    pasmith

    User Avatar
    Homework Helper

    I only need that [tex]\lim_{h \to 0} \frac{\epsilon(h)}{h^2} = \lim_{h \to 0} \frac{f(a + h) - f(a) - hf'(a) - \frac12h^2f''(a)}{h^2} \\
    = \lim_{h \to 0} \frac{f'(a + h) - f'(a) - hf''(a)}{2h} = \frac{f''(a) - f''(a)}{2} = 0[/tex] using l'Hopital's Theorem and the definition of the second derivative. The OP's limit then follows. :smile:

    (l'Hopital's Theorem would also work to establish the OP's limit directly, once one has proven that [tex]
    \lim_{h \to 0} \frac{f(x + h) - f(x - h)}{2h} = f'(x).)[/tex]
     
Know someone interested in this topic? Share this thread via Reddit, Google+, Twitter, or Facebook

Have something to add?
Draft saved Draft deleted