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Set of non-invertible matrices is unbounded

  1. May 3, 2009 #1
    Question:
    How do I prove the set of non-invertible matrices is unbounded?

    Attempt:
    Let A be an element of set of non-invertible matrices.
    det(A)=0
    det(A)=0 is just the line y=0 if you have det(A) as the y-axis and the set of non-invertible matrices on the x-axis. y=0 is unbounded, so the set of non invertible matrices is unbounded?
     
  2. jcsd
  3. May 3, 2009 #2

    Hurkyl

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    For this to make any sense at all, "the set of non-invertible matrices on the x-axis" would have to be a subset of "the set of real numbers"....


    Let's start with an easier question: can you find a non-invertible matrix whose norm is bigger than 10?

    (p.s. what norm are you using?)
     
  4. May 3, 2009 #3
    I guess the problem is that I don't know which norm to use, so I used det as the norm. How do you define a norm for matrices other than the det function?
     
  5. May 3, 2009 #4

    Hurkyl

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    There are infinitely many different norms you can define for matrices, several of which are in common use. This is a question I cannot answer for you -- you will have to check your homework problem / textbook / class notes to find out what norm you're supposed to be using.

    (Incidentally, det isn't a norm. And even if it was, then the set of all non-invertible matrices would be bounded with respect to it)
     
  6. May 3, 2009 #5

    HallsofIvy

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    Since every non-invertible matrix has determinant 0, it would be very bounded!
     
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