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Homework Help: Ordered basis and linear independence

  1. Feb 14, 2010 #1
    1. The problem statement, all variables and given/known data

    I have a set of Vector [tex]v_1,v_2,v_3,v_4[/tex] in [tex]\mathbb{R}^4[/tex] and need to show that [tex]E = v_1,v_2,v_3,v_4[/tex] is an ordered basis for [tex]\mathbb{R}^4[/tex]

    3. The attempt at a solution

    I know that for this being the case

    [tex] v = c_1 \cdot v_1 + \cdots + c_4\cdot v_4 [/tex] where [tex]v \in \mathbb{R}^4[/tex] but if the vectors are linear independent if there doesn't exist any set of unique scalars that makes the linear combination above true other than [tex]c_1 = c_2 = c_3 = c_4 = 0[/tex] and thus v = {0}. Then these vectors are linear independent and an ordered basis for [tex]\mathbb{R}^4[/tex]. right?

    Best regards
  2. jcsd
  3. Feb 14, 2010 #2
    Four vectors v1, v2, v3, v4 are a basis for R^4 if the equation

    0 = c_1 \cdot v_1 + \cdots + c_4\cdot v_4

    has the unique solution [itex]
    c_1 = c_2 = c_3 = c_4 = 0

    More generally, if v1, v2, v3, v4 are a basis, then the equation
    v = c_1 \cdot v_1 + \cdots + c_4\cdot v_4
    has a unique solution for any v, but you only need to check it for v=0 to make sure that you have a basis.
  4. Feb 14, 2010 #3
    so what you are saying owlpride is that if I can show that the linear-combination has the where they are linear independent then they form not just a basis but an ordered basis for R4?
  5. Feb 14, 2010 #4
    The difference between an ordered basis and a basis is just an explicit order of the basis vectors. For example, the set {(1,1), (0,1)} is a basis for R^2. It becomes an ordered basis when I declare that v1 = (1,1) and v2 = (0,1). Once you have an ordered basis, you can work with coordinates. For example, relative to this ordered basis the coordinates (1,3) refer to the vector 1*v1 + 3*v2 = (1,4) (relative to the standard basis (1,0) and (0,1)).

    You cannot work with coordinates until you specify an order on the basis vectors. Without an order, you would not be able to tell if the coordinates (1,3) refer to 1*(1,1) + 3*(0,1) or 1*(0,1) + 3*(1,1).

    Don't get caught up on the word "ordered". All you need to prove is that your vectors form a basis.
  6. Feb 14, 2010 #5

    So basicly then professor says "show that the the vectors v1,v2,v3,v4 is an ordered basis for R4". Then I use the standard definition of a the the basis: and since here there c4 elements(vectors) then the basis has the dimension 4, and if the matrix A which contains the vectors as columns are linear independent then they form a basis for R4?
    Last edited: Feb 14, 2010
  7. Feb 14, 2010 #6
    That's it!
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