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Subspace. Help please.

  1. Apr 3, 2010 #1
    1. Let U be a subspace of Rn and let
    U⊥ = {w ∈ Rn : w is orthogonal to U} .
    Prove that
    (i) U⊥ is a subspace of Rn,
    (ii) dimU + dimU⊥ = n.


    Attempt.


    i)
    U. ( U⊥)T=0
    If U⊥ does not passes the origin , the above equation cannot be satisfied.
    Therefore U⊥ passes the origin.
    U.( U⊥+ U⊥)T=U. ( U⊥)T+U.( U⊥)T=0+0=0
    U.(k U⊥)T=k[U. (U⊥)T]=k.0=0

    Therefore U⊥ is a subspace in Rn

    ii) let U have rank r.
    U⊥ is the nullspace of the U transpose.
    and if U is a matrix of mxn , UT is nxm.
    Therefore , the sum of dim of the two matrices is exactly n.
     
  2. jcsd
  3. Apr 3, 2010 #2

    HallsofIvy

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    I don't understand your notation. You use "T" which I guess is "transpose" and talk about U being a matrix. U is given as a subspace, not a matrix.
     
  4. Apr 3, 2010 #3
    yup you are right.. it is transpose and subspace..
     
  5. Apr 4, 2010 #4

    Let u,v be in U⊥. Let k be a constant in R.
    Now <u,w>=0 for all w in U and <v,w>=0 for all w in U.
    Thus by linearity of inner product we have <u+kv,w>=0 for all w in U. Thus, u+kv is also in U⊥ for all u,v,k. Thus U⊥ is a subspace.

    For the second part, DIY but here are some hints:
    - We know that U [tex]\cap[/tex] U⊥ = {0}. Easy to check (if a vector is in U and perpendicular to U then it has to be the zero vector).
    - Take x in Rn. Prove that x = u+u' for u in U and u' in U⊥.
    - This is unique representation. If x = u+u' = v+v' with u,v in U and u',v' in U⊥ then 0 = (u-v)+(u'-v') implies u-v = v'-u' implies u-v = v'-u' = 0 since U [tex]\cap[/tex] U⊥ = {0}. Thus u=v and u'=v'.
    - Thus projection map [tex]\pi[/tex]: Rn -->> U defined by [tex]\pi[/tex](x)=u where x = u+u' with u in U and u' in U⊥.
    - Use Isomorphism theorem corollary (that dim(range)+dim(kernel)=dim(Rn)=n)
     
  6. Apr 4, 2010 #5

    Hmm.. i think for part 2 i can explain by stating that dim(U+U⊥)=dim(U)+dim(U⊥)+dim(U[tex]\cap[/tex] U⊥)
    Rn=dim(U)+dim(U⊥)
    since dim(U[tex]\cap[/tex] U⊥) ought to be atleast {0} to satisfy the condition of subspace , and all the more they are orthogonal to one another , the can't be parallel , thus they have to be {0} itself.
    Hence proving part 2.
     
    Last edited: Apr 4, 2010
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