Results 1 to 5 of 5

Math Help - Determining Subspace

  1. #1
    Member
    Joined
    May 2006
    Posts
    148
    Thanks
    1

    Determining Subspace

    In each of the following, carefully justify whether the set H is a subpace of vector space V. If H is infact a subspace, give a basis for H.

    1.) V = R^4, H = {(a,b,c,d) "is an element of" R^4 | a + b + c + d = 0}

    2.) V = R^3, H = {(a,b,c) "is an element of" R^3 | abc = 0}

    I think there are 3 properties to check? Seeing if 0 "is an element of" H; Seeing if u + v "is an element of" H, and if u "is an element of H" then is cu "an element of H" for c "an element of" R.
    Follow Math Help Forum on Facebook and Google+

  2. #2
    TD!
    TD! is offline
    Senior Member
    Joined
    Jan 2006
    From
    Brussels, Belgium
    Posts
    405
    Thanks
    3
    That's right, H has to be non-empty (easy to check: 0 has to be an element) and you can combine the other two properties into one: it has to be closed under lineair combinations, i.e. if v and w (vectors) are solutions, then kv+lw (k,l scalars) has to be a solution too.

    1) 0 is a solution, trivial. If (a,b,c,d) and (e,f,g,h) are solutions, is k(a,b,c,d)+l(e,f,h,h) a solution too? Check it

    2) Try it.
    Follow Math Help Forum on Facebook and Google+

  3. #3
    Member
    Joined
    May 2006
    Posts
    148
    Thanks
    1
    Hmm. Okay so there are 3 things you have to check for each. So I believe its something along check if the zero vector is an element of H, and it H is "closed" under adddition and multiplication.

    I dont really get what the a + b + c + d = 0 is for... it seems obvious that H is a subspace for a...what is the basis for it? How do I show it?

    #2: again, like #1, it seems obvious H is a subspace...same questions. A detailed answer will help me do some of the other homework problems.

    Thanks
    Follow Math Help Forum on Facebook and Google+

  4. #4
    Member
    Joined
    May 2006
    Posts
    148
    Thanks
    1
    Tried doing some other of the others, though they're all like this and ask the same question. I thought it would be sufficient to prove if the 0 zero is an element of H only, but not according to my prof...

    Anyone familiar with subspace?
    Follow Math Help Forum on Facebook and Google+

  5. #5
    TD!
    TD! is offline
    Senior Member
    Joined
    Jan 2006
    From
    Brussels, Belgium
    Posts
    405
    Thanks
    3
    Taking the two vectors I suggested, we want to know whether the following is in H too:

    k\left( {a,b,c,d} \right) + l\left( {e,f,g,h} \right) = \left( {ka + le,kb + lf,kc + \lg ,kd + lh} \right)

    This is only true if ka+le + kb+lf + kc+lg + kd+lh = 0. But we can rewrite:

    ka + le + kb + lf + kc + \lg  + kd + lh = k\underbrace {\left( {a + b + c + d} \right)}_{0} + l\underbrace {\left( {e + f + g + h} \right)}_{0} = 0

    Which is exactly what we wanted, so H is indeed a subspace.
    Follow Math Help Forum on Facebook and Google+

Similar Math Help Forum Discussions

  1. Subspace spanned by subspace
    Posted in the Advanced Algebra Forum
    Replies: 3
    Last Post: February 9th 2010, 08:47 PM
  2. Determining a δ
    Posted in the Calculus Forum
    Replies: 1
    Last Post: September 4th 2009, 11:11 AM
  3. determining whether a subset is subspace
    Posted in the Advanced Algebra Forum
    Replies: 2
    Last Post: July 3rd 2008, 09:43 AM
  4. Determining if subspace of P4
    Posted in the Advanced Algebra Forum
    Replies: 1
    Last Post: April 22nd 2008, 08:05 PM
  5. nid Help on determining A, B, C, and D T_T
    Posted in the Algebra Forum
    Replies: 1
    Last Post: September 29th 2006, 08:22 AM

Search Tags


/mathhelpforum @mathhelpforum