Linear Algebra with Computer Science Application February 5, 208 Review. Review: linear combinations Given vectors v, v 2,..., v p in R n and scalars c, c 2,..., c p, the vector w defined by w = c v + + c p v p is called a linear combination. Here, vectors v,..., v p and weights c,..., c p are known..2 Review: linear independence An indexed set of vectors {v,..., v p } in R n is said to be linearly independent if the vector equation x v + x 2 v 2 + + x p v p = 0 has only the trivial solution. Here, weights x,..., x p are unknown. Express this using the definition of linear combination. Set with one vector? Set with two vectors?.3 Review: generalization to more vectors An indexed set S = {v,..., v p } of two or more vectors is linearly dependent if and only if at least one of the vectors in S is a linear combination of the others. In fact, if S is linearly dependent and v = 0, then some v j (with j > ) is a linear combination of the preceding vectors, v,..., v j..4 Review: maximum number of independent vectors in R n If a set contains more vectors than there are entries in each vector i.e., the dimension of the vectors n then the set is linearly dependent. That is, any set {v,..., v p } in R n is linearly dependent if p > n..5 Review: matrix vector product If A is an m n matrix, with columns a,..., a n, and if x is in R n, then the product of A and x, denoted by Ax, is the linear combination of the columns of A using the corresponding entries in x as weights; that is, x Ax = [ ] x 2 a a 2... a n. = x a + x 2 a 2 + + x n a n. x n Row-vector rule. If the product Ax is defined, then the ith entry in Ax is the sum of the products of corresponding entries from row i of A and from the vector x. /2
2 Linear transformations 2. Matrices as operators The difference between a matrix equation Ax = b and the associated vector equation x a + + x n a n = b is merely a matter of notation. A matrix expression Ax can arise in a way that is not directly connected to solution of linear equations. When we think of the matrix A as an object that acts on a vector x by multiplication to produce a new vector called Ax. For instance the equations below say that multiplication by A transforms x into b and transforms u into the zero vector. [ ] [ ] 4 3 3 5 2 0 5 = 8 and [ 4 3 ] 3 2 0 5 4 3 [ ] 0 =. 0 From this new point of view, solving the equation Ax = b amounts to finding all vectors x in R 4 that are transformed into the vector b in R 2 under that action of multiplication by A. The correspondence from x to Ax is a function from one set of vectors to another. This concept generalizes the common notion of a function as a rule that transforms one real number to another. 2.2 Transformations A transformation (or function or mapping) T from R n to R m is a rule that assigns to each vector x in R n a vector T(x) in R m. R n is called the domain of T. R m is called the codomain of T. The notation T : R n R m indicates that the domain of T is R n and the codomain is R m. For x in R n, the vector T(x) in R m is called the image of x (under the action of T). The set of all images T(x) is called the range of T. 2/2
2.3 Matrix transformations For each x in R n, T(x) is computed as Ax, where A is an m n matrix. For simplicity, we sometimes denote such a matrix transformation by x Ax. The domain of T is R n when A has n columns and the codomain of T is R m when each column of A has m entries. The range of T is the set of all linear combinations of the columns of A, because each image T(x) is of the form Ax. 2.4 Example 3 Let A = 3 5, u = 7 [ 3 2, b = 2, c = ] 5 3 2 and define a transformation T : R 2 R 3 by 5 3 [ ] T(x) = Ax = 3 5 x. x 7 2. Find T(u), the image of u under the transformation T. 2. Find an x in R 2 whose image under T is b. 3. Is there more than one x whose image under T is b? 4. Determine whether c is in the range of the transformation T. Solution. 3 [ 5. Compute T(u) = Au = 3 5 2 =. ] 7 9 2. Solve T(x) = b for x. That is, solve Ax = b, the augmented matrix is Hence x = 3 3 3 5 2 7 5 3 3 0 4 7 0 4 2 3 3 0.5 0 0 0 [ ].5. The image of x under T is the given vector b..5 0.5 0.5 0 0 0 2. By the solution to part (2), it is clear that there is a unique solution. So there is exactly one x whose image is b. 3. The vector c is in the range of T if c is the image of some x in R 2, that is, if c = T(x) for some x. This is just another way of asking if the system Ax = c is consistent. To find the answer, row reduce the augmented matrix: 3 3 3 5 2 7 5 3 3 0 4 7 0 4 8 3 3 0 2 0 4 7 3 3 0 2 0 0 35 The third equation, 0 = 35, shows that the system is inconsistent. So c is not in the range of T. 3/2
2.5 Example 0 0 If A = 0 0, what is the codomain and range of the transformation? 0 0 0 Solution. The transformation x Ax does not modify x and x 2 components, it zeros x 3. Therefore, it projects points in R 3 onto the x x 2 plane because x 0 0 x x x 2 0 0 x 2 = x 2. x 3 0 0 0 x 3 0 2.6 Example [ ] 3 Let A =. The transformation T : R 0 2 R 2 defined by T(x) = Ax is called a shear transformation. Solution. To show that T shears its input, We need to show that. T maps horizontal segments onto line segments. 2. check that the corners of the square map onto the vertices of the parallelogram. For item, T maps horizontal strip [ x x 2 ] to [ x + 3x 2 x 2 ]. Therefore points with the same x 2 stay on the same For item 2, we have [ ] [ ] [ ] 3 0 0 =, 0 0 0 [ ] [ ] [ ] 3 0 6 =, 0 2 2 [ ] [ ] [ ] 3 2 2 =, 0 0 0 [ ] [ ] [ ] 3 2 8 =. 0 2 2 2.7 Linear Transformations Definition A transformation (or mapping) T is linear if:. T(u + v) = T(u) + T(v) for all u, v in the domain of T. 2. T(cu) = ct(u) for all scalars c and all u in the domain of T. 4/2
. From (2), we have T(0) = 0. 2. Items () and (2) are true if and only if T(cu + dv) = ct(u) + dt(v) for all vectors u, v in the domain of T and all scalars c, d. Linear transformations preserve the operations of vector addition and scalar multiplication. 2.8 Example Given a scalar r, define T : R 2 R 2 by T(x) = rx. Let r = 3, and show that T is a linear transformation. Solution. Let u, v be in R 2 and let c, d be scalars. Then T(cu + dv) = 3(cu + dv) = 3cu + 3dv = c(3u) + c(3v) = ct(u) + dt(v). Thus T is a linear transformation by definition. T is called a contraction when 0 r < and a dilation when r >. 2.9 Example Define a linear transformation T : R 2 R 2 by Find the images under T of u = [ ] 4, v = T(x) = [ 0 0 [ ] 2, and u + v. 3 ] [ ] x x 2 Solution. We have T(u) = [ 0 0 ] [ 4 ] = [ ], T(v) = 4 [ 0 0 ] [ 2 3 ] = [ ] 3. 2 and, because T is linear, T(u + v) = [ ] + 4 [ ] 3 = 2 [ ] 4, 6 T rotates its input vector counter clockwise about the origin through 90. In fact, T transforms the entire parallelogram determined by u and v into the one determined by T(u) and T(v). 5/2
2.0 Example A company manufactures two products, B and C with some associated cost. We construct a unit cost matrix, U = [ b c ], whose columns describe the costs per dollar of output for the products: U =.45.40 Material.25.35 Labor.5.5 Overhead Let x = (x, x 2 ) be a production vector, corresponding to x dollars of product B and x 2 dollars of product C, and define T : R 2 R 3 by Total material cost T(x) = Ux = Total labor cost Total overhead cost. The mapping T transforms a list of production quantities (measured in dollars) into a list of total costs. The linearity of this mapping is reflected in two ways:. If production is increased by a factor, say 4, from x to 4x, then the costs will increase by the same factor, from T(x) to 4T(x). 2. If x and y are production vectors, then the total cost vector associated with the combined production x + y is precisely the sum of the cost vectors T(x) and T(y). 2. The Matrix of a linear transformation For transformation, we would like to have a formula Today we will show that each linear transformation T : R n R m is a matrix transformation Revisit the existence and uniqueness question in the context of linear/matrix transformations 2.2 The Matrix of a linear transformation Example: The columns of the identity matrix in two dimensions, I 2 = [ ] 0 e 2 =. Suppose T is a linear transformation from R to R 3 such that 5 3 T(e ) = 7 and T(e 2 ) = 8. 2 0 [ ] 0 are e 0 = [ ] and 0 With no additional information, find a formula for the image of an arbitrary x in R 2. Solution. Write x = [ x x 2 Since T is a linear transformation, ] [ ] [ ] [ ] x 0 = I 2 = x x + x 2 0 2 = x e + x 2 e 2. T(x) = x T(e ) + x 2 T(e 2 ) 5 3 = x 7 + x 2 8. 2 0 6/2
This shows that the knowledge of T(e ) and T(e 2 ) is sufficient to determine T(x) for any x. Moreover, since we express T(x) as a linear combination of vectors, we can put these vectors into the columns of a matrix A 2.3 Linear transformation matrix T(x) = T(x e + x 2 e 2 ) = x T(e ) + x 2 T(e 2 ) = [ T(e ) T(e 2 ) ] [ ] 5 3 [ ] x = 7 8 x x 2 x 2 0 2 = Ax Theorem Theorem Let T : R n R m be a linear transformation. Then there exists a unique matrix A such that T(x) = Ax for all x R n. In fact, A is the m n matrix whose jth column is the vector T(e j ), where e j is the jth column of the identity matrix in R n : A = [ T(e ) T(e 2 )... T(e n ) ]. This matrix A is called the standard matrix for the linear transformation T. 2.4 Example Find the standard matrix A for the dilation transformation T(x) = 3x, for x in R 2. Solution. Therefore, Write T(e ) = 3e = [ ] 3 0 A = and T(e 2 ) = 3e 2 = [ ] 3 0. 0 3 [ ] 0. 3 2.5 Example Let T : R 2 R 2 be the transformation that rotates each point in R 2 about the origin through an angle, with counter-clockwise rotation for a positive angle. We could show geometrically that such a transformation is linear. Find the standard matrix A of this transformation. Solution. Vector e = [ ] rotates to 0 [ ] cos φ sin φ [ ] 0, and e 2 = rotates to [ ] cos φ sin φ A =. sin φ cos φ [ ] sin φ. Therefore cos φ 7/2
2.6 Geometric linear transformation in R 2 The figure below is referred to as the unit square in R 2 Understanding the effect of a linear transformation on the unit square give us enough information about its general behavior (by definition of linearity). The following tables illustrate other common geometric linear transformations of the plane, how they affect the unit square, and their corresponding matrices. 2.7 Reflection 8/2
2.8 Contractions & expansions 2.9 Shear 2.20 Projection 9/2
2.2 Onto map Definition 2 A mapping T : R n R m is said to be onto R m if each b in R m is the image of at least one x in R n. Equivalently, T is onto R m when the range of T is all of the codomain R m. That is, T maps R n onto R m if, for each b in the codomain R m, there exists at least one solution of T(x) = b. The question Does T map R n onto R m? is an existence question. The mapping T is not onto when there is some b in R m for which the equation T(x) = b has no solution. 2.22 One-to-one map Definition 3 A mapping T : R n R m is said to be one-to-one if each b in R m is the image of at most one x in R n. Equivalently, T is one-to-one if, for each b in R m, the equation T(x) = b has either a unique solution or none at all. Whether T is one-to-one or not is a uniqueness question. The mapping T is not one-to-one when some b in R m is the image of more than one vector in R n. If there is no such b, then T is one-to-one. Which of the geometric transformations we saw are onto? Which are one-to-one? 2.23 Example Let T be the linear transformation whose standard matrix is 4 8 A = 0 2 3. 0 0 0 5 Does T map R 4 onto R 3? Is T a one-to-one mapping? Solution. Since A happens to be in echelon form, we can see at once that A has a pivot position in each row. For each b in R 3, the equation Ax = b is consistent. In other words, the linear transformation T maps R 4 (its domain) onto R 3. However, since the equation Ax = b has a free variable (because there are four variables and only three basic variables), each b is the image of more than one x. That is, T is not one-to-one. 0/2
2.24 Solutions of a one-to-one map Theorem 2 Let T : R n R m be a linear transformation. Then T is one-to-one if and only if the equation T(x) = 0 has only the trivial solution. 2.25 Matrix of one-to-one and onto maps Theorem 3 Let T : R n R m be a linear transformation and let A be the standard matrix for T. Then. T maps R n onto R m if and only if the columns of A span R m. 2. T is one-to-one if and only if the columns of A are linearly independent. Both statements follow from definition 2.26 Example Let T(x, x 2 ) = (3x + x 2, 5x + 7x 2, x + 3x 2 ). Show that T is a one-to-one linear transformation. Does T map R 2 onto R 3? Solution. When x and T(x) are written as column vectors, you can determine the standard matrix of T by inspection, visualizing the row-vector computation of each entry in Ax: 3x + x 2?? [ ] 3 [ ] T(x) = 5x + 7x 2 =?? x = 5 7 x x x + 3x 2?? 2 x 3 2 So T is indeed a linear transformation, with its standard matrix A shown above. The columns of A are linearly independent because they are not multiples of each other. Therefore, T is one-to-one. To decide if T is onto R 3, examine the span of the columns of A. - Since A is 3 2, the columns of A span R 3 if and only if A has 3 pivot positions. (We had a theorem for this, to span R 3, A should have a pivot in each row). This is impossible, since A has only 2 columns. So the columns of A do not span R 3, and the associated linear transformation is not onto R 3. 2.27 Example Find the map T : R 2 R 2 that reflects points through the horizontal axis x and then the line x 2 = x. Find the map that rotates points by 90 (counter clock-wise). 2.28 Example True/false for linear transformation T. Draw the echelon form of the corresponding matrices? T : R 3 R 4 is one-to-one T : R 4 R 3 is onto Complete the statement /2
T : R n R m is one-to-one if and only if A has pivot columns. Why? T : R n R m is onto if and only if A has pivot columns. Why? 2/2