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!

Homework Help: Simple Proof From Loomis and Sternberg's Calculus

  1. Jun 23, 2014 #1
    Hello everyone,

    I am currently working through Loomis and Sternberg's Advanced Calculus, and am having difficulty with rather simple proofs. Despite this, I am NOT having much difficultly with proofs that require more sophisticated mathematics. For instance, I am trying to prove that if ab = 0, then either a = 0 or b = 0. I have tried to add zero in manifold ways, yet have not been successful in proving this claim.

    I have two questions: could someone provide me with some hints; and is this an early indication of my failure as a mathematician? To me, problems of this sort are not truly going to display your ability as a mathematician, as they require merely exhausting all possibilities. Does anyone else share this sentiment?

    Thank you.
    Last edited: Jun 23, 2014
  2. jcsd
  3. Jun 23, 2014 #2
    Not at all. I have often found such "easy" proofs to be the hardest.

    Anyway, what axioms can you use?
  4. Jun 23, 2014 #3


    User Avatar
    Homework Helper

    One axiom of the real numbers is that the non-zero reals are closed under multiplication; in other words, if you multiply any two non-zero real numbers together, you get a non-zero real number.
  5. Jun 23, 2014 #4


    User Avatar
    Education Advisor

    Have you tried to use the contraposition? For me, this makes the proof a bit more mangable to work with. Also, don't feel too bad about 'simple' proofs being difficult. I think a lot of times these 'simple' proofs require a subtleness that is often easy to overlook.
  6. Jun 23, 2014 #5
    micromass, that is heartening to know. I attached a picture containing the properties that I am permitted to use.

    Attached Files:

  7. Jun 23, 2014 #6
    OK, so you are working in a vector space. And you need to prove that if ##\alpha\boldsymbol{v}=\boldsymbol{0}##, then either ##\alpha=0## or ##\boldsymbol{v}=\boldsymbol{0}##.

    Have you tried multiplying with ##\alpha^{-1}##?
  8. Jun 23, 2014 #7
    But do I truly know what alpha inverse is? That was my first thought, to multiply by the inverse element, but according to the axioms, I do not know such a thing as of yet.
  9. Jun 23, 2014 #8


    User Avatar
    Homework Helper

    It took me a while to see it, look carefully at the domain of the scalar multiplication operation. This is rather tricky, I must admit.
  10. Jun 23, 2014 #9
    Is it R x V?
  11. Jun 23, 2014 #10
    But ##\alpha## is a real number. That structure has different axioms.

    This axiom system defines addition and scalar multiplication on the set ##V## to make a vector space. But in order to have a vector space, you need another structure first. You need ##\mathbb{R}## which already has a addition and multiplication. So you can use this structure.
  12. Jun 23, 2014 #11
    Oh, okay. In that case, if we use the properties of R, then it is relatively simple to show that the statement ax = 0 implies a = 0, but, in comparison, it is greatly difficult to show that v = 0.
  13. Jun 23, 2014 #12
    But this is false.
  14. Jun 23, 2014 #13
    Whoops, I got the two cases mixed up.
  15. Jun 23, 2014 #14
    Okay, so I understand how to show that av = 0 implies that v = 0, by multiplying by a^-1; but how might I show the statement av = 0 implies a = 0?
  16. Jun 23, 2014 #15


    User Avatar
    Homework Helper

    If the scalars were from ##\mathbb{Z}/6\mathbb{Z}##, this wouldn't be provable. We could have 3<2,2> = <0,0> but neither part is zero.
    Last edited: Jun 23, 2014
  17. Jun 23, 2014 #16
    No, this is not what you need to show and it isn't true.

    It doesn't and this is not what you need to show.
  18. Jun 23, 2014 #17
    Then what am I suppose to show?
  19. Jun 23, 2014 #18


    User Avatar
    Homework Helper

    Is this true? think more about it.

    edit: maybe you are just not explaining fully what you mean. But anyway, it is important to be precise in what you say.
  20. Jun 23, 2014 #19


    User Avatar
    Homework Helper

    Bashyboy, I saw in an older post what you were using Spivak before, but now you are using this book. Both of these are pretty difficult books. Why are you choosing these tough books? I mean this proof was difficult, it was almost like a review question. You may be biting off more than you can chew here, or that anyone can chew without some preparatory exposure.
  21. Jun 23, 2014 #20


    User Avatar
    Homework Helper

    we are allowed to assume the scalars are the reals. So it is not that difficult.

    edit: although, I don't know if the rest of the book will be too difficult. I have not read either of them.
  22. Jun 23, 2014 #21
    I am just trying to prove the claim that I gave in my first post, that if av = 0, then either a = 0 or v = 0
  23. Jun 23, 2014 #22
    Yes, I was studying Spivak for some time, but stopped because of some time constraints. I just started Loomis And Sternberg's because my professor recommended I study it.
  24. Jun 23, 2014 #23
    I still do not see why what I am saying is false. I am trying to show that, if av = 0, then either a = 0 or v = 0.
  25. Jun 23, 2014 #24


    User Avatar
    Science Advisor

    You want to prove that "of ab= 0 the either a= 0 or b=0.

    A little earlier you said "if av= 0 then a= 0" which is NOT true.

    Do this as two cases.

    Case 1: a= 0. Well, we are done!

    Case 2: [itex]a\ne 0[/itex]. Since a is not 0 you can divide both sides by it. What do you get when you divide both sides by a?
  26. Jun 23, 2014 #25


    User Avatar
    Homework Helper

    yep. What Halls said is right. You need to think of two cases. But I think Bashy has said that he has done case 2 already. So he still needs to do case 1 (using just the properties that were given in the picture he showed).

    edit: in other words, he still needs to show that ##0\vec{v} = \vec{0}## (I've just used the vector sign so that the vector zero can be distinguished from the scalar zero).
Share this great discussion with others via Reddit, Google+, Twitter, or Facebook

Have something to add?
Draft saved Draft deleted