1. Not finding help here? Sign up for a free 30min 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!

Absolute value of a function integrable?

  1. Nov 23, 2008 #1
    this is the question,
    Prove that if f is continuous on (a,b] and if |f| is bounded on [a,b] then f is integrable on [a,b]. (note: it is not assumed that f is continuous at a.)

    I know you have to use the upper and lower bounds to prove this statement but i don't know where to start?

    Thanks,
    Jonnah Song
     
  2. jcsd
  3. Nov 23, 2008 #2

    HallsofIvy

    User Avatar
    Staff Emeritus
    Science Advisor

    The simplest thing to do is to construct Riemann sums with base having n intervals taking one "rectangle" with base from a to a+ 1/n. Since f is bounded and continuous on (a, b], it is integrable from a+ 1/n to b and the area of the "leftmost rectangle", from a to a+ 1/n goes to 0 as 1/n goes to 0.

    By the way, this doesn't have anything at all to do with |f| being integrable. In fact, I don't know why you say "|f| bounded". That is exactly equivalent to f itself being bounded which is the important thing.
     
  4. Nov 23, 2008 #3
    but, that's what it says on the question. I didn't state that.
     
  5. Nov 23, 2008 #4
    My TA said " this function is integrable if and only if the upper and lower limits are the same. Function is continuous, so consider the intermediate value theorem." Do you guys have any idea what he's talking about?

    I'm very confused.
     
  6. Dec 15, 2008 #5
    I'm still not sure how to solve this.. because..

    I tried doing this...
    We set sk= inf U(f,P)
    tk= sup L(f,P)
    define step function s and t by letting their values equal
    sk and tk on (xk-1, xk)
    s(xk)= t(xk)= f(xk)
    s(x)[tex]\leq[/tex]f(x)[tex]\leq[/tex]t(x) for all x
    tk-sk<E
    [tex]\int[/tex](a+1/n, b) t - [tex]\int[/tex](a+1/n, b) = [tex]\int[/tex](a+1/n, b)(t-s) [tex]\leq[/tex] E(b-a)
    therefore f is integrable on [a+1/n, b]

    On the other hand,
    for [a, a+1/n]
    lim [tex]\int[/tex] (a, a+1/n) f[tex]\leq[/tex]E(a+1/n-a)
    n[tex]\rightarrow[/tex]0
    lim [tex]\int[/tex] (a, a+1/n) f[tex]\leq[/tex] E(1/n)
    therefore f is integrable on [a,a+1/n]
    therefore f is integrable on [a,b]

    what am i doing wrong?
    because i got this wrong.
     
Know someone interested in this topic? Share this thread via Reddit, Google+, Twitter, or Facebook

Have something to add?