Math Linear Algebra II. 1. Inner Products and Norms


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1 Math Linear Algebra II Notes 1. Inner Products and Norms One knows from a basic introduction to vectors in R n Math 254 at OSU) that the length of a vector x = x 1 x 2... x n ) T R n, denoted x, or x, is given by the formula x = x x x 2 n. We will call this this function : R n [0, ) the standard) norm on R n. Recall the familiar dot product on R n : Let x = x 1 x 2... x n ) T, y = y 1 y 2... y n ) T R n. Then n x y = x i y i. We will call the dot product on R n the standard) inner product on R n, and we will use the fancier notation x, y) for x y. Formally, we can regard the inner product as a function, ) : R n R n R. An easy and important consequence is that x, x) = x 2, equivalently, x = x, x). We also have another expression for this inner product: For all x, y R n, x, y) = y T x same as x T y).
2 Now let s explore the standard) norm and the standard) inner product on C n : Recall that for z C, z = a + bi, and z 2 = a 2 + b 2 = z z really just the R 2 standard) norm!) Now let z = z 1 z 2... z n ) T C n with z j = a j + b j i for j = 1, 2,..., n. It is natural to define the standard) norm on C n by n n z = z j 2 = a 2 j + b2 j ) for z Cn. j=1 j=1 Furthermore, with w = w 1 w 2... w n ) T C n, we define the standard) inner product on C n by z, w) = n z i w i. Again, an easy and important consequence is that z, z) = z 2, equivalently, z = z, z). Let A M mn have entries in C. Let A = Ā)T where Ā determined by applying the conjugate operation entrywise. This combination conjugate transpose) is the hermitian adjoint or just the adjoint of A. For w, w = w) T = w 1 w 2... w n ) and thus z, w) = w z.
3 Now let s get more abstract. We will let F denote either R or C. Let V be an arbitrary vector space over F. An inner product on V is a function, ) : V V F, that takes in two vectors u, v V and outputs a scalar u, v) F, satisfying these four fundamental properties: 1) Positivity aka nonnegativity): For all v V, v, v) R and v, v) 0. 2) Definiteness aka nondegeneracy): Let v V. Then v, v) = 0 iff v = 0. 3) Linearity: For all c F, for all u, v, w V, u + cv, w) = u, w) + cv, w). 4) Conjugate symmetry: For all u, v V, u, v) = v, u) just symmetry if F = R.) If V is a vector space over F and, ) is an inner product on V then we call V and inner product space over F. Examples: In all of these, if F = R then there is no need for the conjugation.) 1) Let V = F n. We have already covered standard aka Euclidean) inner product on V given by For all u, v V, u, v) = v u. ELFY) Show that this function does indeed satisfy the four fundamental properties of an inner product... 2) Let V = P n, polynomials of degree at most n over F. Let the interval I = [a, b]. Then for fx), gx) P n define fx), gx)) I = b a fx)gx)dx. ELFY) Show that this function does indeed satisfy the four fundamental properties of an inner product...
4 3) Let V = M m,n over F). For A, B M mn define A, B) = traceb A). ELFY) Show that this function does indeed satisfy the four fundamental properties of an inner product... Note: traceb A) = n B A) ii = n b 1i a 1i + b 2i a 2i + + b mi a mi ). Properties of inner products: From here until otherwise indicated page 12), let V be an inner product space over F). ELFY) Show that For all c F, for all u, v, w V, u, v + cw) = u, v) + cu, w). Hint: Use conjugate symmetry, linearity, and then conjugate symmetry again! Proposition: Let x V. Then, x = 0 iff for all y V, x, y) = 0. Proof : The forward direction is trivial: For all y V, 0, y) = 0y, y) = 0y, y) = 0... also y, 0) = y, 0y) = 0y, y) = 0). Now assume x V satisfies for all y V, x, y) = 0. Then in particular, x, x) = 0. Using the definite property of an inner product we get x = 0 ELFY) Use the proposition above to prove that for all x, y V, x = y iff for all z V, x, z) = y, z).
5 Proposition: Let T, S LV ). Suppose that for all x, y V, T x, y) = Sx, y). Then T = S. Proof : Let T, S LV ). *) Assume for any x, y V that T x, y) = Sx, y). Assume T S. Then there exists x V such that T x Sx. However, T x, y) = Sx, y) for all y V by assumption *). Using the exercise left for you on the previous page, T x = Sx, which is a contradiction. So T = S An inner product on V induces a norm on V corresponding to the inner product, which is a function : V R given by Basic norm property: v = v, v) for v V. cv = cv, cv) = c cv, v) = c 2 v, v) = c v. Also, we call two vectors u, v V orthogonal if u, v) = 0 by conjugate symmetry, v, u) = 0 would also hold). ELFY) if u, v are orthogonal, then u + v 2 = u 2 + v 2 Pythagorean Theorem).
6 Orthogonal decomposition: Let u, v V. Assume v 0. Then v, v) 0. Let c F. Clearly, u = cv + u cv). Let s find c such that v, u cv) are orthogonal: u cv, v) = 0 u, v) cv, v) = 0 u, v) = cv, v) c = u, v) v, v) = u, v) v 2. We will call proj v u) = u,v) v 2 v the orthogonal projection of u onto v. We will call u = proj v u) + u proj v u)) the orthogonal decomposition of u along v Consider that ) u, v) ) proj v u) = v = v 2 u, v) v. Notice that since u proj v u), v) = 0 it follows that u proj v u), proj v u)) = u proj v u), u, v) u, v) v) = v 2 v u proj vu), v) = 0. 2 Thus if u = proj v u) + u proj v u)) is the orthogonal decomposition of u along v, then the vector terms satisfy the Pythagorean Theorem: ) u 2 = proj v u) 2 + u proj v u) 2. Then ) and ) implies u proj v u) 2 = u 2 u, v) 2 v 2.
7 You could of course calculate this directly: u proj v u) 2 = u proj v u), u proj v u)) = u, u proj v u)) proj v u), u proj v u)) = u, u) u, proj v u)) proj v u), u) + proj v u), proj v u)) = u 2 + u, v) 2 v 2 u, proj v u)) u, proj v u)) = u 2 + = u 2 + u, v) 2 v 2 u, u, v) 2 v 2 u, v) v u, v) u, v) v) u, v 2 v v) 2 u, v) u, v) u, v) 2 v 2 = u 2 + u, v) 2 v 2 u, v) 2 v 2 u, v) 2 v 2 = u 2 u, v) 2 v 2. In the last step we used the fact that conjugation does nothing to a real number: u, v) 2 v 2 = u, v) 2 v 2.
8 The following propositions is of great importance. Proposition CauchySchwarz Inequality): If u, v V then u, v) u v. Proof : Let u, v V. If v = 0 then the inequality holds as both sides are clearly 0. So we assume that v 0. Consider the orthogonal decomposition u = proj v u) + w where w = u proj v u)). 0 w 2 = u 2 u, v) 2 v 2 u, v) 2 v 2 u, v) v u 2 u u, v) u v
9 Proposition Triangle Inequality): If u, v V then u + v u + v. Proof : Let u, v V. Consider that for all z = a + bi C, Rea + bi) = a a = a 2 a 2 + b 2 = a + bi. Then u + v 2 = u + v, u + v) = u, u) + v, v) + u, v) + v, u) = u 2 + v 2 + u, v) + u, v) = u 2 + v 2 + 2Re[u, v)] u 2 + v u, v) u 2 + v u v = u + v ) 2. Hence, u + v u + v
10 These are the socalled polarization identities: Proposition: For all x, y V, 1) x, y) = 1 4 x + y 2 x y 2) if F = R. 2) x, y) = 1 4 x + y 2 x y 2 + i x + iy 2 i x iy 2) if F = C. Proof : Let x, y V. Assume F = R. Then conjugation does nothing and x, y) = y, x). Consider that 1 x + y 2 x y 2) = 1 x + y, x + y) x y, x y)) = 4 4 = 1 x, x) + 2x, y) + y, y) [x, x) 2x, y) + y, y)]) = x, y). 4 Assume F = C. Then z = 1 4 x + y 2 x y 2 + i x + iy 2 i x iy 2) = = 1 x + y, x + y) x y, x y) + ix + iy, x + iy) ix iy, x iy)) 4 Let s split z into two parts: and c 1 = 1 x + y, x + y) x y, x y)), 4 c 2 = 1 ix + iy, x + iy) ix iy, x iy)). 4 Clearly z = c 1 + c 2.
11 c 1 = 1 x, x) + x, y) + y, x) + y, y) [x, x) x, y) y, x) + y, y)]) 4 c 1 = 1 4 2x, y) + 2y, x)) = 1 x, y) + y, x)). 2 c 2 = i x, x) + x, iy) + iy, x) + iy, iy) [x, x) x, iy) iy, x) + iy, iy)]) 4 c 2 = i 4 2x, iy) + 2iy, x)) = i 2 [ i)x, y) + iy, x)] = 1 x, y) y, x)). 2 Finally, z = c 1 + c 2 = x, y) Proposition Parallelogram Identity): For all x, y V, x + y 2 + x y 2 = 2 x 2 + y 2). I leave the proof of this one to you...it s pretty easy.
12 Now let V be a vector space over F, but we no longer assume that there is an inner product on V. The function n : V R is a norm on V if it satisfies the following properties: 1) Homogeneity: For all v V and for all c F, ncv) = c nv). 2) Triangle Inequality: For all u, v V, nu + v) nu) + nv). 3) Positivity aka nonnegativity): For all v V, nv) 0. 4) Definiteness aka nondegeneracy): Let v V. Then nv) = 0 iff v = 0. Of course, the norm : V R induced from an inner product space V satisfies these properties: We have already verified that 1) and 2) hold. Properties 3) and 4) are direct consequences of the positivity and definiteness of the inner product. So all inner product spaces are normed spaces, via the induced norm v = v, v). However, there are normed spaces where the norm does not correspond to an inner product. When V be a normed vector space, with norm n, we will still use the notation nv) = v.
13 Let V = R n or V = C n. p : V R given by It turns out that for any p, 1 p <, following function x p = n x i p ) 1 p, is a norm called the pnorm. Of course the 2norm aka Euclidean norm) is the norm induced by the standard inner product on V. We can also define a norm called the norm: x = max{ x i : i = 1, 2,..., n}. For p and it is easy to check ELFY) that they satisfy defining properties 1), 3), and 4) of a norm. The triangle inequality is a different story. Of course we already have proven the triangle inequality for 2. The triangle inequality for p isn t too hard to check ELFY) when p = 1 and p =. For arbitrary values of p it is fairly involved to prove the triangle inequality, but it does hold, and is called Minkowski s Inequality. We won t do that here... It turns out that p for p 2 cannot be induced by an inner product they don t satisfy the parallelogram inequality)! The proof is beyond the scope of this course because of the reverse implication), but the following proposition characterizes norms induced from inner products: Proposition: Let V be a normed space with a norm. The norm is induced from an inner product, ) on V iff satisfies the parallelogram inequality, that is, for all x, y V, x + y 2 + x y 2 = 2 x 2 + y 2). Suggested Homework: 1.1, 1.2, 1.3, 1.5, 1.7, and 1.9.
14 2. Orthogonal and Orthonormal Bases Recall two vectors u, v in an inner product space V are orthogonal if u, v) = 0. We will use the notation u v. A vector v V is orthogonal to a subspace U of V if for all u U, u v. We use the notation v U. Two subspaces U 1, U 2 are orthogonal subspaces if for all u 1 U 1, u 1 is orthogonal to the subspace U 2. Furthermore, we can define for any subspace U of V, the orthogonal complement to U in V ): U = {v V v U}. Also u is called a unit vector if u, u) = 1 and thus u = 1). The vectors v 1, v 2,..., v k are called an orthogonal system if they are pairwise orthogonal, and an orthonormal system if they are an orthogonal system of unit vectors. Proposition: Any orthogonal system of nonzero vectors is linearly independent. Proof : Let v 1, v 2,..., v k be an orthogonal system of nonzero vectors. Suppose c i v i = 0 for c i F, i = 1, 2,..., k. Then for any j = 1, 2,..., k we get ) c i v i, v j = c i v i, v j ) = c j v j 2 = 0. For all j = 1, 2,..., k, v j 2 0 as v j 0. Therefore, c 1 = c 2 =... = c k = 0, which completes the proof that the orthogonal system is linearly independent
15 Proposition General Pythagorean Theorem): Let v 1, v 2,..., v k be an orthogonal system. Then for all c 1, c 2,..., c k F, 2 c i v i = c i 2 v i 2. Proof : Let v 1, v 2,..., v k be an orthogonal. Let c i F for i = 1, 2,..., k. 2 c i v i = c i v i, ) c j v j = j=1 c i c j v i, v j ) = j=1 c i c i v i, v i ) = c i 2 v i 2 A basis B = {v 1, v 2,..., v n } of V is an orthogonal basis orthonormal basis) if B is an orthogonal system orthonormal system). Due to a previous proposition, if n = dim V, then any orthogonal orthonormal) system of n nonzero vectors forms a orthogonal orthonormal) basis. Let B = {v 1, v 2,..., v n } be an orthogonal basis of V. Suppose x V. Usually finding [x] B = c 1 c 2 c n ) T involves solving a linear system, but it is easier when B is an orthogonal basis. For any j = 1, 2,..., n, n ) x, v j ) = c i v i, v j = n c i v i, v j ) = c j v j, v j ) = c j v j 2 c j = x, v j) v j 2. This should look familiar! Recall that proj v u) = u,v) v 2 v.) Furthermore, if B is an orthonormal basis, this result reduces to c j = x, v j ).
16 When B = {v 1, v 2,..., v n } is an orthonormal basis, then for any v V, the formula v = n v, v i )v i, is called an orthogonal Fourier decomposition. Suggested Homework: 2.2, 2.3, 2.5, 2.6, 2.8, and 2.9.
17 3. Orthogonalization and GramSchmidt Let W be a subspace of V. For each v V, w W is called an orthogonal projection of v onto W if v w) W. Proposition: Let W be a subspace of V and let w be an orthogonal projection of v onto W. Then 1) For all x W, v w v x w minimizes distance from v to W ). 2) If x W and v w = v x then x = w orthogonal projections are unique). Proof : Let W be a subspace of V and let w W be an orthogonal projection of v onto W. Let x W. Set y = w x W. Then v x = v w) + y. Since v w) y as y W, by the Pythagorean Theorem we get v x 2 = v w 2 + y 2 v x v w, and v x = v w iff y = 0. Hence v x = v w iff x = w Now we can say the orthogonal projection of v onto W.) Let W be a subspace of V with an orthogonal basis B = {v 1, v 2,..., v k }. Let v V and P W v be the orthogonal projection of v onto W. Let w = v, v i ) v i 2 v i W. Then v w = v v, v i ) v i 2 v i.
18 Let x W. Then there exists scalars c 1, c 2,..., c k such that x = k c iv i. Then v w, x) = = v v, v, v i ) v i 2 v i, ) c j v j j=1 ) c i v i v, v i ) v i 2 v i, ) c j v j j=1 = = = = c i v, v i ) c i v, v i ) c i v, v i ) c i v, v i ) j=1 v, v i ) v i 2 c j v i, v j ) v, v i ) v i 2 c i v i, v i ) v, v i ) v i 2 c i v i 2 c i v, v i ) = 0. This shows that w W satisfies v w) W, and so, w is the orthogonal projection of v onto W. Hence we get the formula: v, v i ) P W v = v i v i. 2 Claim: Let W be a subspace of V. P W LV ). Let u, v V and c F. Let w W. Then P W u + cp W v W and [u + cv] [P W u + cp W v], w) = [u P W u] + c[v P W v], w) = u P W u, w) c v P W v, w) = 0. Thus P W u + cv) = P W u + cp W v showing that P W is a linear operator on V.
19 The GramSchmidt algorithm: Input: A linearly independent set of vectors {x 1, x 2,..., x k } spanning a subspace W of V. Output: An orthogonal system {v 1, v 2,..., v k } such that the span is W. Here are the steps: 1) Set v 1 = cx 1 for any nonzero c F usually 1). Set W 1 = span{x 1 } = span{v 1 }. ) 2) Set v 2 = cx 2 P W1 x 2 ) = c x 2 x 2,v 1 ) v 1 v 2 1 for any nonzero c F. Set W 2 = span{v 1, v 2 }. Note that W 2 = span{x 1, x 2 }. ) 3) Set v 3 = cx 3 P W2 x 3 ) = c x 3 x 3,v 1 ) v 1 v 2 1 x 3,v 2 ) v 2 v 2 2 for any nonzero c F. Set W 3 = span{v 1, v 2, v 3 }. Note that W 3 = span{x 1, x 2, x 3 }. 4) Continue in this manner until v k is determined it s not necessary to determine W k ). Suppose V = R 3. Consider the basis B = 1 2 3, 1 1, Let s apply the GramSchmidt procedure to turn this into an orthogonal basis: 1) Set v 1 = and W 1 = span
20 2) Consider that P W = = , = = So for convenience set v 2 = 5 4 1, and set W 2 = span 1 2, = 3) Consider that , 1 = P W = 2 1, = = So for convenience set v 3 =
21 In conclusion B = 1 2 3, 5 4, 1 1, is an orthogonal basis 1 1 such that the first i vectors of B and B have the same spans for i = 1, 2, 3. Recall if W is a subspace of V, then W = {v V v W }. Claim: Let W be a subspace of V. W orthogonal complement) is also a subspace of V. This is pretty easy to justify: First notice W is not empty as 0 W. Let u, v W and c F. Let w W. Then u + cv, w) = u, w) + cv, w) = 0. Thus u + cv W proving that W is a subspace of V. More generally, if U, W are subspaces of V then U + W = {u + w u U, w W } is called the subspace sum. ELFY) U + W is a subspace of V for any subspaces U, W. A subspace sum U + W is called a direct sum, denoted U W, if U W = {0}. Proposition: Let U, W be subspaces of V. V = U W iff each vector in v V has a unique representation v = u + w for u U, w W. Proof : Let U, W be subspaces of V. Assume V = U W. Let v V. Then there exists u U and w W such that v = u+w. Suppose v = u 1 +w 1 where u 1 U and w 1 W. Then v = u + w = u 1 + w 1 implies u u 1 = w w 1. Then u + u 1 = w w 1 U V = {0}. Hence u = u 1 and w = w 1, so we have uniqueness.
22 Now assume for all v V there exists unique vectors u U, w W such that v = u + w. We are assuming each vector in V has a unique decomposition into a vector from U plus a vector from W. Clearly V = U + W. The question is whether it s a direct sum. To yield a contradiction, suppose U W {0}. Let y U W such that y 0. Let v V. Since V = U + W there exists u U, w W such that v = u + w. Set u 1 = u + y U. Since y 0, u 1 u. Set w 1 = w y U. Since y 0, w 1 w. Then u 1 + w 1 = u + w = v, which contradicts the uniqueness of the decomposition. This completes the proof Let v V and W be a subspace at V. Then let v 1 = P W v W and v 2 = v P W v W. We have already shown that P W v W is the unique vector for which v P W v W. Therefore we have a unique decomposition of v into a vector in W plus a vector in W. Hence V = W W. ELFY) For any subspace W of V, we get W ) = W. Suggested Homework: 3.2, 3.3, 3.4, 3.5, 3.7, and 3.8.
23 4. Least Squares Solution Let A M mn and b F m. How do we determine an x F n such that Ax is as close as possible to b? If we can find x such that Ax b = 0 then Ax = b is consistent and we say x is an exact solution. In the case where we cannot find x such that Ax b = 0 then we call an x which minimizes Ax b a least squares solution to Ax = b which is inconsistent). Why least squares? Well, minimizing Ax b is equivalent to minimizing m m Ax b 2 = Ax) i b i 2 n 2 = A ij x j b i. j=1 Let W = range A = {Ax x F n }. W is a subspace of F m. In particular, the minimum value of Ax b is the distance from b to W, which is satisfied by an x which solves the equation *) Ax = P W b. Furthermore, if v 1, v 2,..., v k is an orthogonal basis for W = range A then we get an explicit formula for P W b: P W b = b, v i ) v i 2 v i. So, if we have a basis for W which we can create by taking columns of A that are pivot columns) then with GramSchmidt we can create an orthogonal basis and compute P W b and then solve *). That is actually a lot of work...maybe there is an easier method? Here is a nice approach: Let A = [a 1 a 2 a n ]. Ax = P W b iff b Ax) W iff b Ax) a i for i = 1, 2,..., n. Hence Ax = P W b iff b Ax), a i ) = a i b Ax) = 0 for i = 1, 2,..., n. Here is the clever part: A b Ax)) i = a i b Ax) = 0 for i = 1, 2,..., n. So Ax = P W b iff A b Ax) = 0 iff A Ax = A b.
24 So we get the following result: So the easiest way to find a least squares solution to Ax = b is to solve the socalled normal equation A Ax = A b. Also for a least squares solution x, P W b = Ax In particular, this means that a least squares solution is unique iff A A M nn is invertible. In this case P W b = AA A) 1 A b, which would hold for all b F m, so then the matrix of P W is AA A) 1 A. Proposition: Let A M mn. Ker A = KerA A). In other words, A A is invertible iff rank A = n. Proof : Let A M mn. Suppose x Ker A. Then Ax = 0 and thus A A)x = A Ax) = A 0 = 0. Hence x KerA A) and Ker A KerA A). Suppose x KerA A). Then A A)x = A Ax = 0. Thus Ax 2 = Ax, Ax) = Ax) Ax = x A )Ax = x A Ax) = x 0 = 0. This implies that Ax = 0. So x Ker A and KerA A) Ker A. This completes the proof Line of best fit: A classic application is finding a line y = mx+b to best fit some given data {x 1, y 1 ), x 2, y 2 ),..., x k, y k )}. The best solution minimizes the following sum of square differences: mx i + b y i 2.
25 Equivalently, we are looking for a least squares solution to x 1 1 x x k 1 m b ) = y 1 y 2... y k. As long as not all xcoordinates are the same, a least squares solution is unique. EXAMPLE: Suppose we have the following data points in the xyplane: {1, 2), 3, 5), 4, 5), 6, 8), 6, 9), 7, 10)}. Find an equation for the line of best fit. So we are looking for the least squares solution to m b ) = First we create the normal equation: ) m b ) = ) ) m b ) = ) m b ) = ) ) =
26 Hence the line of best fit is... y = x General approach to curvefitting or surfacefitting: 1) Find equations your data should fit if there was an exact solution. The equations must be linear with respect to the parameters.) 2) Write these equations as a linear system in a matrix form inconsistent unless there is an exact solution). 3) Find the least squares solution leftmultiply both sides by the conjugate transpose, aka adjoint, of the coefficient matrix and solve). Suggested Homework: 4.2, 4.3, and 4.4.
27 5. Adjoint of a Linear Transformation Let V = F n. Let A M mn with entries in F R or C). Recall the adjoint of A is A = A T. Proposition: Let A M mn. For all x, y V, Ax, y) = x, A y). Proof : Let A M mn and x, y V. Then Ax, y) = y, Ax) = Ax) y = x A )y = x A y) = A y, x) = x, A y) This proposition gives rise to the notion of an adjoint operator. Now let V, W be finite dimensional) inner product spaces and T LV, W ). S LW, V ) is called an adjoint operator if T x, y) = x, Sy) for all x V and y W. We will use the notation T for S. Does it exist? Yes. Let T LV, W ). By having B, C be orthonormal bases for V, W respectively, define T LW, V ) by having [T ] BC = [T ] CB ). Let s justify that it is indeed adjoint: Let x V and y W. Let B = {v 1, v 2,..., v n } and C = {w 1, w 2,..., w m } be orthonormal bases for V, W respectively. Then T v i = c 1i w 1 + c 2i w c mi w m and y = d 1 w 1 + d 2 w d m w m, where c ij, d k are scalars for i = 1, 2,..., m, j = 1, 2,..., n, and k = 1, 2,..., m. Then T v i, y) = m k=1 c kid k = [T ] CB [v i ] B, [y] C ) = [v i ] B, [T ] CB ) [y] C ) for i = 1, 2,..., n. By linearity, we get T x, y) = [T ] CB [x] B, [y] C ) = [x] B, [T ] CB ) [y] C ). Also T w i = a 1i v 1 + a 2i v a ni v n and x = b 1 v 1 + b 2 v b n v n, where a ij, b k are scalars for i = 1, 2,..., n, j = 1, 2,..., m, and k = 1, 2,..., n. Then x, T w i ) = n k=1 a kib k = [x] B, [T ] BC [w i ] C ) = [x] B, [T ] CB ) [w i ] C ) for i = 1, 2,..., m. By linearity, we get x, T y) = [x] B, [T ] CB ) [y] C ) = T x, y). So T is indeed an adjoint operator to T. Is it unique? Suppose S is another adjoint operator. Then for all x V and y W, T x, y) = x, T y) = x, Sy) implies that for all y W, T = Sy. Thus S = T. So the adjoint operator is unique.
28 Proposition: Let T LV, W ) where V, W are inner product spaces. Then 1) Ker T = range T ). 2) Ker T = range T ). 3) range T = Ker T ). 4) range T = Ker T ). Proof : First recall that for any subspace U, U ) = U. Thus 1) and 3) are equivalent as well as 2) and 4). It turns out for any linear transformation T, T ) = T ELFY  Use conjugate symmetry!). Thus Thus 1) and 2) are equivalent as well as 3) and 4). So we can just prove 1). It s pretty straightforward: y range T ) iff T x, y) = x, T y) = 0 for all x V. Also, x, T y) = 0 for all x V iff T y = 0, which means y Ker T This completes the proof Suggested Homework: 5.2. Hint: If P W is the orthogonal projection onto a subspace W of an inner product space V then I V P W is the orthogonal projection onto W.)
29 6. Isometries and Unitary Operators Let V and W be inner product spaces over F. T LV, W ) is an isometry if for all x V, T x = x. Proposition: T LV, W ) is an isometry iff for all x, y V, T x, T y) = x, y). Proof : Let s start with the reverse direction. Suppose for all x, y V, T x, T y) = x, y). Then for all x V, T x = T x, T x) = x, x) = x. So T is an isometry. Now assume T is an isometry. We split this direction into two cases, F = R and F = C: Assume F = R. Let x, y V. Then by the polarization identity, T x, T y) = 1 4 T x + T y 2 T x T y 2) = 1 4 T x + y) 2 T x y) 2) = = 1 4 x + y 2 x y 2) = x, y). Assume F = C. Let x, y V. Then by the polarization identity, T x, T y) = 1 4 T x + T y 2 T x T y 2 + i T x + it y 2 i T x it y 2) = = 1 4 T x + y) 2 T x y) 2 + i T x + iy) 2 i T x iy) 2) = = 1 4 x + y 2 x y 2 + i x + iy 2 i x iy 2) = x, y) So an isometry preserves the inner product.
30 Proposition: T LV, W ) is an isometry iff T T = I V. Proof : Let T LV, W ) be an isometry. Let y V. Then for all x V, x, T T y) = T x, T y) = x, y). Hence, T T y = y for all y V. Thus T T = I V. Now assume T T = I V. Let x, y V. Then T x, T y) = x, T T y) = x, I V y) = x, y). Thus T is an isometry The proposition above guarantees an isometry is leftinvertible, but not necessarily invertible. An isometry T LV, W ) is a unitary operator if it is invertible. A matrix A M mn is an isometry if A A = I n. Also, a square matrix U M nn is unitary if U U = I n. ELFY) The columns of a matrix A M nn form an orthonormal basis iff A is unitary. If F = R then a unitary matrix is also called orthogonal. Basic properties of unitary matrices: Let U be a unitary matrix. 1) U 1 = U is also unitary. 2) If v 1, v 2,..., v n is an orthonormal basis of F n then Uv 1, Uv 2,..., Uv n is an orthonormal basis of F n. Suppose V, W have the same dimension. Let v 1, v 2,..., v n be an orthonormal basis of V and w 1, w 2,..., w n be an orthonormal basis of W. Define T LV, W ) by T v i = w i for i = 1, 2,...n. Then T is a unitary operator: Let x V. There exists c 1, c 2,..., c n F such that x = c 1 v 1 + c 2 v c n v n and x 2 = c c c n 2. Then n ) 2 T x 2 = T n 2 n 2 c i v i = c i T v i = c i w i = n c i 2 = x 2.
31 Proposition: Let U M nn be a unitary matrix. 1) det U = 1. For an orthogonal matrix U, det U = ±1.) 2) If λ is an eigenvalue of U then λ = 1. Proof : Let U M nn be a unitary matrix. Since det U = det U, it follows that 1 = det I n = detu U) = det U ) det U) = det Udet U = det U 2. Thus det U = 1, which proves 1). Let λ be an eigenvalue of U. Then there is a nonzero x F n such that Ux = λx. Then x = Ux = λx = λ x implies λ = 1, which proves 2) Two operators S, T LV ) or matrices S, T M nn ) are called unitarily equivalent if there exists and unitary operator U LV ) or matrix U M nn ) such that S = UT U. In particular, since U 1 = U, the operators/matrices would also be similar. However, not all similar operators/matrices are unitarily equivalent. Proposition: A M nn is unitarily equivalent to a diagonal matrix D iff A has an orthogonal basis of eigenvectors. Proof : Let A M nn. Assume A is unitarily equivalent to a diagonal matrix D. Then there is a unitary matrix U such that A = UDU. Since U U = I n, it is easy to check that the vectors Ue i for i = 1, 2,..., n are eigenvectors of A ELFY). Finally, since the vectors e 1, e 2,..., e n form an orthogonal orthonormal) basis, so do the eigenvectors Ue 1, Ue 2,..., Ue n since U is unitary. Now assume A has an orthogonal basis of eigenvectors v 1, v 2,..., v n. Let u i = 1 v i v i for i = 1, 2,..., n. Then B = {u 1, u 2,..., u n } is an orthonormal basis of eigenvectors of A. Let D = [A] BB and U be the matrix whose columns are u 1, u 2,..., u n. Clearly D is diagonal and U is unitary columns are an orthonormal basis). In particular, U = [I n ] SB, where S is the standard basis. Hence U = U 1 = [I n ] BS and A = [A] SS = [I n ] SB [A] BB [I n ] BS = UDU. So A is unitarily equivalent to D Suggested Homework: 6.1, 6.4, 6.6, and 6.8.
32 7. Rigid Motions in R n A rigid motion in an inner product space V is a transformation f : V V not necessarily linear) that preserves distance, that is, for all x, y V, fx) fy) = x y. Clearly every unitary operator T LV ) is a rigid motion. A nice example of a rigid motion that is not a linear operator is translation: Let y V be nonzero. Let f y : V V be given by f y x) = x + y. This operator is not linear since f0) 0. Proposition: Let f be a rigid motion in a real inner product space V. Let T : V V be given by T x) = fx) f0). Then for all x, y V, 1) T x) = x. 2) T x) T y) = x y. 3) T x), T y)) = x, y). Proof : Let f be a rigid motion in a real inner product space V. Let T : V V be given by T x) = fx) f0). Let x, y V. T x) = fx) f0) = x 0 = x. T x) T y) = fx) f0)) fy) f0)) = fx) fy) = x y. Since V is a real inner product space, it follows that x y 2 = x 2 2x, y) + y 2 and T x) T y) 2 = T x) 2 2T x), T y)) + T y) 2. By using 1) and 2) in the equations above, T x), T y)) = x, y)
33 The following proposition states that every rigid motion in a real inner product space is a composition of a translation and an orthogonal transformation. Proposition: Let f be a rigid motion in a real inner product space V. Let T : V V be given by T x) = fx) f0). T is an orthogonal transformation unitary operator in a real inner product space). Proof : Let x, y V and c R. Consider that T x + cy) T x) + ct y)) 2 = T x + cy) T x)) ct y)) 2 = = T x + cy) T x) 2 2cT x + cy) T x), T y)) + c 2 T y) 2 = = x + cy x 2 2cT x + cy), T y)) + 2cT x), T y)) + c 2 y 2 = = c 2 y 2 2cx + cy, y) + 2cx, y) + c 2 y 2 = = 2c 2 y 2 2cx, y) 2ccy, y) + 2cx, y) = 2c 2 y 2 2c 2 y, y) = 0. So T x + cy) T x) + ct y)) = 0, which means T x + cy) = T x) + ct y). Therefore T is linear, and thus by the previous proposition, T is an isometry. However, since T is an isometry from V to itself, it follows that T is invertible and thus unitary, which in a real inner product space is called an orthogonal transformation Note: We will not cover section 8 of this chapter.
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