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Linear transformation arbitrary question

  1. Mar 1, 2012 #1
    1. The problem statement, all variables and given/known data

    Suppose A is an mxn matrix and b is a vector in R^m. Define a function T:R^n --> R^m by T(x) = Ax + b. Prove that if T is a linear transformation then b=0.

    2. Relevant equations

    For the second part of the question, a transformation is linear if:

    1) T(u+v) = T(u) + T(v) for all u,v in domain of T

    2) T(cu) = cT(u) for all u & c (scalars)

    3. The attempt at a solution

    For the first part of the question, I am thrown off by the "+b" as I never saw anything other than T(x) = Ax.

    However, I am leaning towards using the identity matrix I to prove T(x) = Ax, but I don't feel this will be complete...

    I really need a hand getting started and I think I'll be able to pick it up from there.

    Thank you!
  2. jcsd
  3. Mar 1, 2012 #2
    Calculate T(u+v) and T(u)+T(v) with your definition of T. Are those two equal?
  4. Mar 1, 2012 #3
    Thank you!

    So I gave numerical values to the matrix A, & vectors b, u &v (making b a zero vector) and was able to conclude that T(u+v) = T(u) + T(v).

    Since I made b a zero vector, this helps me with the second part of the problem. However, how can I connect this to the first part of the question?

    Does proving this define the function? What does it mean to define the function?
  5. Mar 1, 2012 #4


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    Put u and v equal to the zero vector. So if T is linear, then T(0)+T(0) should equal T(0). If T(u)=Au+b, what does that tell you about b?
  6. Mar 1, 2012 #5
    Okay, I made u & v zero vectors so that T(u+v) = T(u) + T(v). (T is linear)

    Now, one rule I have says that to define the function (or for T to be linear), T(0) = 0 (has to be so)

    So this means that if T(u) = A(u) + b, then b has to be a zero vector as well right?

    Does proving that a transformation T is linear help me in defining a function? What does it mean to define a function? (this may be a very basic question, but I can't grasp it!)
  7. Mar 1, 2012 #6


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    Sure. If T is a linear function then T(0) has to equal 0. So b has to be 0. I'm a little vague on what the rest of your question is.
  8. Mar 1, 2012 #7
    You have helped a lot, thank you!

    The rest of my question is really about the first part of the statement:

    "Suppose A is an mxn matrix and b is a vector in R^m. Define a function T:R^n --> R^m) by T(x) = Ax + b."

    Perhaps I'm over thinking it, but what does it mean to define a function in this way? How does this relate to proving that b=0 if the transformation is linear?
  9. Mar 1, 2012 #8


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    Yeah, maybe overthinking it. A is a matrix with a bunch of numbers in it. No matter what those numbers are A operating on the zero vector is the zero vector. Any number times 0 is 0. So T(0)=b.
  10. Mar 1, 2012 #9
    hehe Okay, I understand, but...

    Just one last question:

    Does what we did here (proving that b = 0 makes the transformation linear) ALSO define the function as asked in the first part of the problem statement?
  11. Mar 1, 2012 #10
    I really don't understand. Nobody is asking you to define a function. There are no two parts in the problem statement.
    The only thing they say is that you have a function f(x)=Ax+b for a matrix A and a column vector b.
  12. Mar 1, 2012 #11


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    No, T(x)=Ax+b defines a function. For it to be linear means b is zero. That doesn't define A at all. Why do you think it should?
  13. Mar 1, 2012 #12
    I'm not sure... I think I'm really just having trouble figuring out with this question is asking.

    From what I understand, together, we did the proof that was required (b=0). Is that all that is being asked for?
  14. Mar 1, 2012 #13
    Definitely over thinking this, think I just had a light bulb.

    Thank you guys for your time and patience!
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