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Span of an infinite set S

  1. Sep 8, 2013 #1
    1. The problem statement, all variables and given/known data
    Give S = {(x,|x|,2|x|) | x [itex]\in[/itex] R} [itex]\bigcup[/itex] {(0,2,4),(-1,3,6)}, find span(S)



    2. Relevant equations
    I know that span of a finite set of vectors is given by <a(0,2,4) + b(-1,3,6)+c(x,|x|,2|x|)>, where a,b,c are any real numbers. Can i use that same way to find the span of this infinite set.


    3. The attempt at a solution
    Is the solution same as the vector span for a finite set like span(S) = <a(0,2,4) + b(-1,3,6)+c(x,|x|,2|x|)>, or is it something else?
     
  2. jcsd
  3. Sep 8, 2013 #2

    LCKurtz

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    Lots of those x vectors are multiples of each other. I would start by looking at what S looks like for x > 0 and x < 0.
     
  4. Sep 8, 2013 #3
    So for any values of x I pick. the x vectors will be linearly dependent and they cannot form my span? So would that mean the span(S) = span of linearly independent independent vectors in S. So span(S) = <a(0,2,4)+b(-1,3,6)>?
     
  5. Sep 8, 2013 #4

    LCKurtz

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    I don't know what you are trying to say here. I will say it again: what do the x vectors look like if ##x > 0## versus ##x<0##? You might start by actually answering that question.
     
  6. Sep 8, 2013 #5
    So for x<0 The x vectors look like(x,-x,-2x) and for x>0 the x vectors look like (x,x,2x).
     
  7. Sep 8, 2013 #6

    LCKurtz

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    And if you factor an x out of each what happens? And why do you say they are linearly dependent?
     
  8. Sep 8, 2013 #7
    OH my fault, I see they are linearly independent. So could I generalize this and write span(S) = {a(1,1,2) + b(1,-1,2) + c(0,0,0) + d(0,2,4) + e(-1,3,6)} given a>0, b<0, c,d and e are any real numbers?
     
  9. Sep 8, 2013 #8

    LCKurtz

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    Is that b vector correct?

    Usually when you are asked to describe a span you wouldn't include extra vectors that don't add anything. Since these are 3D vectors you would expect at most to need 3 vectors and maybe fewer to get an independent spanning set. And remember that a span automatically doesn't restrict the multiplying constants to positive or negative.
     
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