# Is there a Linear Transformation

• Chillguy
In summary, the conversation discusses the existence of a linear transformation from R^3 to R^2 with specific coordinates. The requirements for a linear transformation are discussed, including the use of linearly independent vectors to span R^3 and the resulting transformation of a linear combination of these vectors. It is noted that the transformation is not unique and can go to any place in R^2.

## Homework Statement

From Hoffman and Kunze:

Is there a linear transformation T from $$R^3$$ to $$R^2$$ such that T(1,-1,1)=(1,0) and T(1,1,1)=(0,1)?

## Homework Equations

$$T(c\alpha+\beta)=cT(\alpha)+T(\beta)$$

## The Attempt at a Solution

I don't really understand how to prove that there is a linear transformation with these coordinates. I think I begin by defining another arbitrary, linearly independent, transformation such as T(1,0,0)=(1,1). Then I don't really know where to go from here.

Chillguy said:
I think I begin by defining another arbitrary, linearly independent, transformation such as T(1,0,0)=(1,1).

Think about how this helps you along with the requirement that T is a linear transformation. For example, what does your requirement that having the transformation of another linearly independent vector in R3 give you?

Orodruin said:
Think about how this helps you along with the requirement that T is a linear transformation. For example, what does your requirement that having the transformation of another linearly independent vector in R3 give you?
It would allow me to span R3 with these three vectors

Chillguy said:
It would allow me to span R3 with these three vectors

And thus the transformation of any vector in R3 can be written as ...

... can be written as $$T(c\alpha_i+\beta_i)?$$
where alpha and beta are vectors in R3

Well, you need to define what your ##\alpha## and ##\beta## etc are. You have three vectors with which you can express any vector in R3 as a linear combination. What will be the resulting transformation of such a linear combination? Will it fulfil the requirements you have?

The resulting combination will be a vector in R^2, I think it could be any vector in R^2?

No, not if you already have fixed it for three vectors. Use the linear property of T! Let us say you have the vectors ##v_1, v_2, v_3## and have fixed ##T(v_i) = u_i## for ##i = 1,2,3##. Now you take a linear combination of those ##v = \sum_i c_i v_i##. What is ##T(v)##?

$$T(v)=T(c*v_1+c*v_2+c*v_3)$$
Which I can then apply the definition of a linear transformation as defined before?

Yes. So does this linear transformation fulfil the requirements?

Yes! it does! Thank you so much. It makes much more sense now. I just have one other question, why do the vectors need to span all of R^3 in the first place? Is it so the transformation can go to any place in R^2?

Unless you specify the transformation for any arbitrary vector in R3 you have not really found a linear transformation from R3 to R2 but only from a two-dimensional subspace. A more complete answer would be: Yes, it exists, but is not unique. (You could have selected any vector in R2 as the image of your third linearly independent vector in R3).