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: Linear Algebra - Linear Spaces

  1. Sep 29, 2010 #1
    Hello there,
    I'm having some problems with a proof-like exercise on linear algebra. Here's what I'm supposed to do:

    1. The problem statement, all variables and given/known data
    Determine whether each of the given sets is a real linear space, if addition and multiplication by real scalars are defined in the usual way. For those that are not, tell which axioms fail to hold.

    2. Relevant equations
    All increasing functions.
    (there are others, like rational functions, all Taylor polynomials of degree [tex]\leq[/tex] n, etc. but I guess my doubts are related to all of them).

    3. The attempt at a solution
    Well, after a couple of attempts my teacher told me that once I prove that increasing functions are a subset of the functions linear space, I just need to prove the closure axioms. This can be done just by saying "the increasing fuctions are contained in the set of functions". Ok, then I started to prove the closure under addtion.

    Let f be an increasing funcion, if x1 < x2, then f(x1) < f(x2). The same applies to a increasing function g.
    So, adding g(x1) to both sides of the f(x) equation and f(x2) to both sides of the g(x) equation, we have:

    (1) f(x1) + g(x1) < f(x2) + g(x1)
    (2) g(x1) + f(x2) < g(x2) + f(x2)
    [tex]\Rightarrow[/tex] f(x1) + g(x1) < g(x2) + f(x2)
    [tex]\Rightarrow[/tex] (f+g)(x1) < (f+g)(x2)

    And I guess it's proven that it's closed under addition. Is that right?
    Now, I know it's not closed under multiplication because if I multiplicate f by -1 it won't be an increasing function anymore. But can I just say that and it's ok or is there another way to show it's not closed under multiplication?

    I have tried this (let a be a real number):
    f(x1) < f(x2) [tex]\Rightarrow[/tex] af(x1) < af(x2) [tex]\Rightarrow[/tex] (af)(x1) < (af)(x2). But I don't think I got anything useful.

    I'd be very grateful if anyone could help.
    Thanks in advance.
  2. jcsd
  3. Sep 29, 2010 #2


    Staff: Mentor

    Since g(x1) < g(x2), you can continue the inequality above with < f(x2) + g(x2).

    This shows that f(x1) + g(x1) < f(x2) + g(x2), or
    (f + g)(x1) < (f + g)(x2), and you're done with that part.
    This is enough to show that the set of increasing functions is not closed under scalar multiplication.
  4. Sep 29, 2010 #3
    Thanks a lot for the fast reply!
Share this great discussion with others via Reddit, Google+, Twitter, or Facebook