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Homework Help: Sums of subspaces?

  1. Feb 17, 2009 #1
    1. The problem statement, all variables and given/known data
    Let U and W be subspaces of V. What are U+U, U+V? Is U+W=W+U?

    2. Relevant equations

    3. The attempt at a solution
    It is easy to show that U+U and U+V are spaces too under closed addition and scalar multiplication, but I'm not sure where they lie. For example, is U+U a subspace of V or V+V. The space U+V confuses me altogether; does it lie outside of V? And for the last one, does the question mean the direct sum of U + W even if the plus doesn't have a little circle around it? If it is a direct sum, wouldn't the statement be true because U and W together form the basis of V? I'm just having a really difficult time picturing all of this.
  2. jcsd
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