Linear Transformations Prepared by Vince Zaccone For Campus Learning Assistance Services at UCSB

Linear Transformations A transformation (or function or mapping) T from ℝn to ℝm is a rule that assigns to each vector x in ℝn a vector T (x) in ℝm . The set ℝn is called the domain of T, and ℝm is called the codomain of T. The notation T: ℝn → ℝm says the domain of T is ℝn and codomain is ℝm . For x in ℝn , the vector T (x) in ℝm is called the image of x. The set of all images T (x) is called the range of T. Prepared by Vince Zaccone For Campus Learning Assistance Services at UCSB

Linear Transformations For each x in ℝn , T(x) is computed as Ax, where A is an mxn matrix. For simplicity, we denote such a matrix transformation by x↦Ax. The domain of T is ℝn when A has n columns and the codomain of T is ℝm when each column of A has m entries. So an mxn matrix transforms vectors from ℝn into vectors from ℝm. Here are a few examples of transformation matrices: Prepared by Vince Zaccone For Campus Learning Assistance Services at UCSB

Linear Transformations This matrix is 2x2, so it transforms vectors from ℝ2 into (other) vectors from ℝ2. To see what this matrix does, we can where it takes a few specific vectors. Prepared by Vince Zaccone For Campus Learning Assistance Services at UCSB

Linear Transformations This matrix is 2x2, so it transforms vectors from ℝ2 into (other) vectors from ℝ2. To see what this matrix does, we can where it takes a few specific vectors. Prepared by Vince Zaccone For Campus Learning Assistance Services at UCSB

Linear Transformations This matrix is 2x2, so it transforms vectors from ℝ2 into (other) vectors from ℝ2. To see what this matrix does, we can where it takes a few specific vectors. Domain (ℝ2) Range (ℝ2) This is a shear transformation. Prepared by Vince Zaccone For Campus Learning Assistance Services at UCSB

Linear Transformations This matrix is 2x2, so it transforms vectors from ℝ2 into (other) vectors from ℝ2. To see what this matrix does, we can where it takes a few specific vectors. Prepared by Vince Zaccone For Campus Learning Assistance Services at UCSB

Linear Transformations This matrix is 2x2, so it transforms vectors from ℝ2 into (other) vectors from ℝ2. To see what this matrix does, we can where it takes a few specific vectors. Prepared by Vince Zaccone For Campus Learning Assistance Services at UCSB

Linear Transformations This matrix is 2x2, so it transforms vectors from ℝ2 into (other) vectors from ℝ2. To see what this matrix does, we can where it takes a few specific vectors. Domain (ℝ2) Range (ℝ2) This matrix is a combination of a rotation through 45° and a stretch by a factor of √2. We will have more to say about this type of matrix when we cover Chapter 5. Check out your textbook for more discussion of 2x2 transformation matrices. Prepared by Vince Zaccone For Campus Learning Assistance Services at UCSB

Linear Transformations This matrix is 3x2, so it transforms vectors from ℝ2 into vectors from ℝ3. This transformation takes a vector from ℝ2 and maps it to a vector in ℝ3. There is more we can say though. The range of this transformation is not the entire 3-dimensional ℝ3 space. The images must be in a subset of ℝ3 that has dimension (at most) 2 – a plane. Prepared by Vince Zaccone For Campus Learning Assistance Services at UCSB

Linear Transformations This matrix is 3x2, so it transforms vectors from ℝ2 into vectors from ℝ3. This transformation takes a vector from ℝ2 and maps it to a vector in ℝ3. There is more we can say though. The range of this transformation is not the entire 3-dimensional ℝ3 space. The images must be in a subset of ℝ3 that has dimension (at most) 2 – a plane. Domain (ℝ2) Range (plane in ℝ3) The images all lie on a plane. The Range can’t have a larger dimension than the Domain. We will see this as a more general rule later, but for now we need to know the concepts of ONE-TO-ONE and ONTO. This transformation is not ONTO because it does not span all of ℝ3. Prepared by Vince Zaccone For Campus Learning Assistance Services at UCSB

Linear Transformations DEFINITIONS: A mapping T:ℝn↦ℝm is said to be ONTO (or surjective) if each b in ℝm is the image of at least one x in ℝn. Domain ℝn Range is All of ℝm T Domain ℝn Range is a subspace of ℝm T T is onto T is not onto A mapping T:ℝn↦ℝm is said to be ONE-TO-ONE (or injective) if each b in ℝm is the image of at most one x in ℝn. T T is one-to-one T T is not one-to-one Prepared by Vince Zaccone For Campus Learning Assistance Services at UCSB

Linear Transformations A couple of quick tests to see if a transformation is one-to-one or onto: More Columns than Rows – it can’t be One-to-One More Rows than Columns – it can’t be Onto More precisely: A transformation is onto iff the columns of A span ℝm. A transformation is one-to-one iff the columns are linearly independent. Prepared by Vince Zaccone For Campus Learning Assistance Services at UCSB

Linear Transformations This matrix is 3x5, so it transforms vectors from ℝ5 into (other) vectors from ℝ3. To see what this matrix does, we can where it takes a few specific vectors. Prepared by Vince Zaccone For Campus Learning Assistance Services at UCSB

Linear Transformations This matrix is 3x5, so it transforms vectors from ℝ5 into (other) vectors from ℝ3. To see what this matrix does, we can where it takes a few specific vectors. This one is a bit harder to visualize, but we are starting with vectors from ℝ5, and mapping them to vectors in ℝ3. The transformation is definitely not ONE-TO-ONE because the dimension of the range (at most 3) is certainly lower than the domain (5). The transformation will be ONTO as long as the set of column vectors in the matrix spans all of ℝ3. This can be checked in the usual way by row reducing the matrix and seeing that there are 3 pivot positions in the RREF form (see below). RREF for this matrix is (check this yourself!) Prepared by Vince Zaccone For Campus Learning Assistance Services at UCSB

Linear Transformations So far we have seen a few linear transformations, but what makes them LINEAR? Prepared by Vince Zaccone For Campus Learning Assistance Services at UCSB

Linear Transformations So far we have seen a few linear transformations, but what makes them LINEAR? To be linear, a transformation must have the following properties: For any vectors u and v in the domain of T For all scalars c and every vector u in the domain of T The basic idea is that for vector addition and scalar multiplication, the results are the same if you perform the operation before or after you apply the transformation. An important special case of the scalar multiplication rule is that This gives an easy way to test a transformation for linearity. Prepared by Vince Zaccone For Campus Learning Assistance Services at UCSB

Linear Transformations Are the following transformations linear? 𝑎) 𝑇 𝑥,𝑦 =(2𝑥,𝑥+𝑦) 𝑏) 𝑇 𝑥,𝑦 =(𝑥−3𝑦,𝑥𝑦) 𝑐) 𝑇 𝑥,𝑦,𝑧 =(𝑥,𝑦,0) 𝑑) 𝑇 𝑥,𝑦,𝑧 =(2𝑥,2𝑦,2) e) 𝑇 𝑥,𝑦,𝑧,𝑤 =(2𝑥+𝑦,2𝑦+𝑧,2𝑧+𝑤) For the ones that are linear, find the matrix representation (in the standard basis). Find the dimensions of the Domain and Co-Domain, and determine whether the transformation is one-to-one or onto. Prepared by Vince Zaccone For Campus Learning Assistance Services at UCSB