# Math Camp Lecture 4: Linear Algebra. Xiao Yu Wang. Aug 2010 MIT. Xiao Yu Wang (MIT) Math Camp /10 1 / 88

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1 Math Camp 2010 Lecture 4: Linear Algebra Xiao Yu Wang MIT Aug 2010 Xiao Yu Wang (MIT) Math Camp /10 1 / 88

2 Linear Algebra Game Plan Vector Spaces Linear Transformations and Matrices Determinant Diagonalization Inner Product Spaces Definiteness Xiao Yu Wang (MIT) Math Camp /10 2 / 88

3 Linear Algebra Vector Spaces and Subspaces Definition (Real Vector Space) A triple (V, +, ) consisting of a set V, addition +: V V V (x, y) x + y and multiplication : R V V (λ, x) λ x is called a Real Vector Space if the following 8 conditions hold: Xiao Yu Wang (MIT) Math Camp /10 3 / 88

4 Linear Algebra Vector Spaces and Subspaces 1 (x + y) + z = x + (y + z) for all x, y, z V (assoc of add) x + y = y + x for all x, y V (commut of add) There is an element 0 V (the zero vector) s.t. x + 0 = x x V For each element x V, there is an element y V such that x + y = 0 (additive inverse) 1 x = x for every x V (identity) λ (µ x) = (λµ) x for all λ, µ R, x V (assoc mult) λ (x + y) = λ x + λ y for all λ R, x, y V (distr) (λ + µ) x = λ x + µ x for all λ, µ R, x V (distr) Xiao Yu Wang (MIT) Math Camp /10 4 / 88

5 Linear Algebra Vector Spaces and Subspaces Definition (Subspace) Let V be a real vector space. A subset U V is called a subspace of V if U is nonempty and for any x, y U and λ R, x + y U and λ x U That is: A subspace is a subset that is closed under addition and multiplication. The smallest subspace of any vector space is the Null Space {0}. Any intersection of subspaces of a vector space V is a subspace of V. Xiao Yu Wang (MIT) Math Camp /10 5 / 88

6 Linear Algebra Vector Spaces and Subspaces Example Finite Euclidean Space: R n = R R R = {(x 1,..., x n ) : x i R, i} Note that by x = (x... x ) we denote a vector in n 1,, n R. One of the conventions is to call that vector a row vector and employ the notation x T for a column vector. Xiao Yu Wang (MIT) Math Camp /10 6 / 88

7 Linear Algebra Vector Spaces and Subspaces Definition Let x = (x 1,..., x n ) and y = (y 1,..., y n ) be two vectors. Define the sum by: Definition x + y = (x 1,..., x n ) + (y 1,..., y n ) = (x 1 + y 1,..., x n + y n ) Let x = (x 1,..., x n ) be a vector and λ a real number. Define multiplication by: λ x = λ(x 1,..., x n ) = (λx 1,..., λx n ) One can verify that all 8 conditions from the definition of the vector space hold true for Rn so it is a vector space. Xiao Yu Wang (MIT) Math Camp /10 7 / 88

8 Linear Algebra Vector Spaces and Subspaces Example A more complicated vector space is the set of all functions from a given set E to R. E can be anything. Define f + g such that (f + g)(x) = f (x) + g(x) and λf such that (λf )(x) = λf (x). Subspaces are given by any property that is conserved by addition and multiplication; for example, the set of continuous functions from E into R denoted by C (E ), or the set of n-times continuously differentiable functions C n (E ) are vector spaces. Be careful and distinguish between 0 V and 0 R; we use the same notation for convenience. Xiao Yu Wang (MIT) Math Camp /10 8 / 88

9 Linear Algebra Vector Spaces and Subspaces Example The set of all polynomials with coeffi cients from F is denoted by P(F). Example The set of all polynomials up to degree n with coeffi cients from F is denoted by P n (F). Example The set of all matrices of dimension m n with coeffi cients from F is denoted by M m n (F). Xiao Yu Wang (MIT) Math Camp /10 9 / 88

10 Linear Algebra Linear Independence Definition (Linear Combination) Let V be a real vector space. A vector v V is called a linear combination of vectors if there exist a finite number of vectors v 1,..., v n V and scalars a 1,..., a n R such that v = a 1 v 1 + a 2 v a n v n. If the scalars are nonnegative and sum to 1 then we may also use the term convex combination. Definition (Span) Let S be a nonempty subset of a vector space V. The span of S, denoted span(s), is the set consisting of all linear combinations of the vectors in S. For convenience, we define span( ) = {0}. Theorem The span of any subset S of a vector space V is a subspace of V. Moreover, any subspace of V that contains S must also contain the span of S. Xiao Yu Wang (MIT) Math Camp /10 10 / 88

11 Linear Algebra Linear Independence Definition (Generates (or spans)) A subset S of a vector space V generates (or spans) V if span(s) = V. Example The vectors (1, 1, 0), (1, 0, 1), and (0, 1, 1) span R3 since an arbitrary vector (a 1, a 2, a 3 3) R is a linear combination of the three given vectors. In fact, the scalars r, s, t for which r(1, 1, 0) + s(1, 0, 1) + t(0, 1, 1) = (a 1, a 2, a 3 ) 1 are r = 2 (a 1 + a 2 a 3 ), s = 1 2 (a 1 a 2 + a 3 ), t = 1 2 ( a 1 + a 2 + a 3 ) Note that (1, 0, 0), (0, 1, 0), and (0, 0, 1) also span R 3. Xiao Yu Wang (MIT) Math Camp /10 11 / 88

12 Linear Algebra Linear Independence Definition (Linearly Dependent) A subset S of a vector space V is called linearly dependent if there exists a finite number of distinct vectors u 1, u 2,..., u n in S and scalars a 1, a 2,..., a n, not all zero, such that a 1 u 1 + a 2 u a n u n = 0 In this case, we also say that the vectors of S are linearly dependent. Example Let S = {(1, 3, 4, 2), (2, 2, 4, 0), (1, 3, 2, 4), ( 1, 0, 1, 0)} R 4. S is linearly dependent if we can find scalars a 1, a 2, a 3, and a 4, not all zero, such that a 1 (1, 3, 4, 2) + a 2 (2, 2, 4, 0) + a 3 (1, 3, 2, 4) + a 4 ( 1, 0, 1, 0) = 0 One such solution is (a 1, a 2, a 3, a 4 ) = (4, 3, 2, 0). So these vectors are linearly dependent (alt, S is linearly dependent). Xiao Yu Wang (MIT) Math Camp /10 12 / 88

13 Linear Algebra Linear Independence Definition (Linearly Independent) A subset S of a vector space that is not linearly dependent is called linearly independent. As before, we say the vectors of S are linearly independent. Properties 1 The empty set is linearly independent 2 A set consisting of a single nonzero vector is linearly independent 3 A set is linearly independent iff the only representations of 0 as linear combinations of its vectors are trivial representations (i.e., coeffi cients all zero) Xiao Yu Wang (MIT) Math Camp /10 13 / 88

14 Linear Algebra Basis and Dimension Definition (Basis) A basis β for a vector space V is a linearly independent subset of V that generates V. Examples is a basis for {0} The standard basis of Rn is {e i : e i = (0,..., 1,..., 0), i = 1,..., n} The standard basis of P n (F) is the set {1, x, x 2,..., x n } A basis of P(F) is the set {1, x, x 2,...} Xiao Yu Wang (MIT) Math Camp /10 14 / 88

15 Linear Algebra Basis and Dimension Theorem Let V be a vector space and β = {u 1,..., u n } be a subset of V. Then β is a basis for V iff each v V can be uniquely expressed as a linear combination of vectors of β. Proof Let β be a basis for V. If v V, then v span(β). Thus v is a linear combination of the vectors of β. Suppose that n v = a i u i = b i u i i=1 i=1 are two such representations of v. Subtracting these two equations yields n 0 = (a i b i )u i i=1 Since β is linearly independent, a i b i = 0 i, so v is uniquely expressible as a linear combination of the vectors of β. Exercise. Xiao Yu Wang (MIT) Math Camp /10 15 / 88 n

16 Linear Algebra Basis and Dimension Theorem (Replacement Theorem) Let V be a vector space that is generated by a set G (span(g ) = V ) containing exactly n vectors, and let L be a linearly independent subset of V containing exactly m vectors. Then m n and there exists a subset H of G containing exactly n m vectors such that L H generates V. Corollary Let V be a vector space having a finite basis. Then every basis for V contains the same number of vectors. (If you added a vector, it would break linear independence; if you took a vector away, it would break span.) Xiao Yu Wang (MIT) Math Camp /10 16 / 88

17 Linear Algebra Basis and Dimension Definition (Dimension) A vector space is called finite-dimensional if it has a basis consisting of a finite number of vectors. The unique number of vectors in each basis for V is called the dimension of V and is denoted by dim(v ). A vector space that is not finite-dimensional is called infinite-dimensional. Examples The vector space {0} has dimension 0 n The vector space R has dimension n. (Recall the standard basis (1, 0.., 0), (0, 1, 0,..., 0),...) C 0 ([0, 1]) has infinite dimension. (Cts functions on [0, 1].) Intuition: Weierstrass. (Allow polynomial to have infinite terms (countable).) Xiao Yu Wang (MIT) Math Camp /10 17 / 88

18 Linear Algebra Basis and Dimension Corollary Let V be a vector space with dimension n. a) Any finite generating set for V contains at least n vectors, and a generating set for V that contains exactly n vectors is a basis for V. b) Any linearly independent subset of V that contains exactly n vectors is a basis for V. c) Every linearly independent subset of V can be extended to a basis for V Proof of (c) Let β be a basis for V. If L is a linearly independent subset of V containing m vectors, then the replacement theorem asserts that there is a subset H of β containing exactly n m vectors such that L H generates V. Now L H contains at most n vectors; therefore (a) implies that L H contains exactly n vectors and that L H is a basis for V. Xiao Yu Wang (MIT) Math Camp /10 18 / 88

19 Linear Algebra Basis and Dimension Example Do the polynomials x 3 2x 2 + 1, 4x 2 x + 3, and 3x 2 generate P 3 (R)? (Hint: think of standard basis.) Example Is {(1, 4, 6), (1, 5, 8), (2, 1, 1), (0, 1, 0)} a linearly independent subset of R 3? (Hint: think of standard basis.) Xiao Yu Wang (MIT) Math Camp /10 19 / 88

20 Linear Transformations Operations on Matrices Rank of a Matrix Xiao Yu Wang (MIT) Math Camp /10 20 / 88

21 Linear Transformations Definition (Linear Transformation) Let V and W be real vector spaces. We call a function T : V W a linear transformation from V to W if, for all x, y V and c R, we have a) T (x + y) = T (x) + T (y) and b) T (cx) = ct (x) Xiao Yu Wang (MIT) Math Camp /10 21 / 88

22 A Matrix Linear Transformations and Matrices Linear Transformations An m n matrix A with entries from R is a rectangular array of the form a a a 1n a 21 a 22 a 2n A =... a m1 a m2 a mn where each entry a ij (1 i m, 1 j n) R. We call the entries a ii the diagonal entries of the matrix. An m 1 matrix is an element of Rm and is called a column vector. A 1 n matrix is an element of Rn and is called a row vector. Xiao Yu Wang (MIT) Math Camp /10 22 / 88

23 Linear Transformations Matrix representation of a linear transformation The matrix A is associated with the linear mapping from x y R m that satisfies n y i = a ij x j j=1 We write y = Ax. R n to Xiao Yu Wang (MIT) Math Camp /10 23 / 88

24 Example Linear Transformations and Matrices Linear Transformations Let T : R2 R 3 be the linear transformation defined by T (a 1, a 2 ) = (a 1 + 3a 2, 0, 2a 1 4a 2 ). Then the matrix associated with this mapping is 1 3 M T = since M T a = (a 1 + 3a 2, 0, 2a 1 4a 2 ). Xiao Yu Wang (MIT) Math Camp /10 24 / 88

25 Operations on Matrices The set of all m n matrices with real entries is a vector space, which we denote by M m n (R), with the following operations of matrix addition and scalar multiplication: For A, B M m n (R) and c R, for 1 i m, 1 j n. (A + B) ij = A ij + B ij (ca) ij = ca ij Xiao Yu Wang (MIT) Math Camp /10 25 / 88

26 Operations on Matrices Properties of Matrices A + B = B + A (A + B) + C = A + (B + C ) (λ + µ)a = λa + µa λ(a + B) = λa + λb λ(µa) = (λµ)a The null matrix is the matrix that has all elements equal to 0 and is denoted by 0. Xiao Yu Wang (MIT) Math Camp /10 26 / 88

27 Operations on Matrices Gaussian Elimination Definition ((Row) Echelon Form) A matrix is in (row) echelon form if a) All rows that contain only zeros are grouped at the bottom of the matrix b) For each row that does not contain only zeros, the first nonzero entry of the row (the pivot) appears strictly to the right of the pivot of each row that appears above it. Definition A matrix is in reduced row echelon form if in addition, every leading coeffi cient is 1 and is the only nonzero entry in its column. Xiao Yu Wang (MIT) Math Camp /10 27 / 88

28 Example Linear Transformations and Matrices Operations on Matrices This matrix is in reduced row echelon form: Xiao Yu Wang (MIT) Math Camp /10 28 / 88

29 Operations on Matrices Every matrix can be transformed to a matrix in row echelon form using (a finite number of) elementary row operations, which are defined as: a) Add a multiple of one row to another b) Interchange two rows c) Multiply a row by a nonzero constant This is Gaussian elimination. Xiao Yu Wang (MIT) Math Camp /10 29 / 88

30 Operations on Matrices The matrix product of A and B is defined when A is m n and B is n p. The product is then defined by: n (AB) ik = a ij b jk j=1 and AB is an m p matrix. This definition is motivated by the fact that if A and B represent linear mappings then AB represents the composition of the mappings A B. Xiao Yu Wang (MIT) Math Camp /10 30 / 88

31 Example Linear Transformations and Matrices Operations on Matrices [ ] 1 [ ] = 15 2 = = 17 3 Xiao Yu Wang (MIT) Math Camp /10 31 / 88

32 Operations on Matrices Properties (AB)C =A(BC) (assoc) A(B+C)=AB+AC (distr) (A+B)C=AC+BC (distr) Xiao Yu Wang (MIT) Math Camp /10 32 / 88

33 Operations on Matrices Properties Note that if AB exists, BA need not be defined and even if it exists, it is not usually true that AB = BA. However, commutativity holds for symmetric matrices. Definition (Symmetric matrix) An n m matrix A is symmetric iff a ij = a ji for i {1,..., n} and j {1,..., m}. Xiao Yu Wang (MIT) Math Camp /10 33 / 88

34 Operations on Matrices Several other types of matrices are important: Definition (Diagonal matrix) A diagonal matrix is a square matrix in which all elements off the main diagonal are zero. That is, d ij = 0 for all i = j, i, j {1,..., n} For example: the identity matrix is diagonal Xiao Yu Wang (MIT) Math Camp /10 34 / 88

35 Operations on Matrices Definition (Triangular matrix) An m n matrix is triangular if either all the elements above the main diagonal and the main diagonal are nonzero, and all other elements are zero, or all the elements below the main diagonal and the main diagonal are zero. The first matrix is called "upper triangular", and the second, "lo wer triangular" For example: is upper triangular Xiao Yu Wang (MIT) Math Camp /10 35 / 88

36 Operations on Matrices Definition (Permutation matrix) A permutation matrix is a square n n binary matrix that has exactly one entry 1 in each row and each column and 0 s elsewhere. Each such matrix represents a specific permutation of n elements and, when used to multiply another matrix, can produce that permutation in the rows or columns of the other matrix = For ex: Definition (Doubly stochastic matrix) A doubly stochastic matrix is a square matrix of nonnegative real numbers, each of whose rows and columns sums to 1. Xiao Yu Wang (MIT) Math Camp /10 36 / 88

37 Operations on Matrices The transpose of an m n matrix A is denoted by A T or A. If A has elements a ij, then A has elements a ji. Note that if the matrix is symmetric, then it equals its transpose. Properties (A ) = A (A + B) = A + B (AB) = B A Xiao Yu Wang (MIT) Math Camp /10 37 / 88

38 Operations on Matrices We call A non-singular (invertible) if there exists a matrix A 1 such that AA = A A = I where I = is the identity matrix. Note that these are square matrices. Nonsquare matrices are not invertible, though they may have "left" or "right" inverses (not important). Noninvertible square matrices are called singular. Xiao Yu Wang (MIT) Math Camp /10 38 / 88

39 Operations on Matrices Properties (AB) 1 = B 1 A 1 (A ) 1 = (A 1 ) The mapping that corresponds to the inverse of a matrix A corresponds to the inverse of the mapping associated with A. Xiao Yu Wang (MIT) Math Camp /10 39 / 88

40 Operations on Matrices Algorithm for Computing Matrix Inverse Augment matrix A with identity matrix Apply elementary row operations to this augmented matrix to transform A to I The matrix in the place of the original identity matrix is now A 1 Xiao Yu Wang (MIT) Math Camp /10 40 / 88

41 Operations on Matrices Example A = Xiao Yu Wang (MIT) Math Camp /10 41 / 88

42 Operations on Matrices Perform elementary operations: multiply the second row by 1 and add it to the first row Xiao Yu Wang (MIT) Math Camp /10 42 / 88

43 Operations on Matrices Thus A 1 = Xiao Yu Wang (MIT) Math Camp /10 43 / 88

44 Operations on Matrices [ a Trick for inverting 2 2 matrices c 1 Calculate (ad bc) = D [ 2 1 d Then the inverse is: D c ] b a ] b. d Xiao Yu Wang (MIT) Math Camp /10 44 / 88

45 Rank of a Matrix Definition (Column/Row Rank) The column rank of an m n matrix A is the largest number of linearly independent column vectors A contains. The row rank is the largest number of linearly independent row vectors A contains. It can be shown that the row rank and column rank are equal. The row and column rank of a matrix is often referred to simply as the rank of the matrix. We denote the rank of A by rk(a). Definition (Column Space) The column space of A is the set of vectors generated by the columns of A, denoted by CS(A) = {y : y = Ax, x R n }. The dimension of CS(A) = rk(a). Xiao Yu Wang (MIT) Math Camp /10 45 / 88

46 Rank of a Matrix Definition (Null Space) The null space of A, N(A) = {x Theorem n R : Ax = 0}. If A is an m n matrix, then rk(a)+ dimn(a) = n. Theorem Let A be an n n matrix. Then the following are equivalent a) A is nonsingular (invertible) b) N(A) = {0} c) rk(a) = n Xiao Yu Wang (MIT) Math Camp /10 46 / 88

47 Rank of a Matrix Determining Rank and Bases for Row, Column, and Null Spaces Let A M m n( R) and U be a matrix in row echelon form obtained from A by row operations. Then RS(A) = RS(U) and N(A) = N(U) The nonzero rows of U form a basis for RS(U) The columns of U that contain the pivots form a basis for the CS(U) Whenever certain columns of U form a basis for CS(U), the corresponding column vectors of A form a basis for the CS(A) Xiao Yu Wang (MIT) Math Camp /10 47 / 88

48 Rank of a Matrix Example Find a basis for the row space, column space, and null space, and rk(a), where A = Xiao Yu Wang (MIT) Math Camp /10 48 / 88

49 Rank of a Matrix Row echelon form gives U = Xiao Yu Wang (MIT) Math Camp /10 49 / 88

50 Rank of a Matrix A basis for RS(A) is {(2, 1, 3), (0, 4, 5)} A basis for the CS(A) is 2 2, 3. And clearly rk(a) = Xiao Yu Wang (MIT) Math Camp /10 50 / 88

51 Rank of a Matrix To find a basis for the N(A) = N(U) we can examine the vectors, x, such 5 that Ux = 0. This means that x2 = 4 s and x 1 = 1 2 x s where x 3 = s. Thus x 1 x = 1 5 x 2 = 2 4 s 3 2 s s 8 = s 5 4 x 3 s 1 7 So, is a basis for N( A). 1 Xiao Yu Wang (MIT) Math Camp /10 51 / 88

52 Determinant Definition (Determinant) Let A M n n (R). If n = 1, so that A = (A 11), we define det(a) = A 11. For n 2, we define det(a) recursively as n det( A) = ( 1) i+j A ij det(ã ij ) j=1 for any i {1,..., n} where Ã ij denotes the (n 1) (n 1) matrix obtained from A by deleting row i and column j. The scalar det(a) is called the determinant of A and is also denoted by A. The scalar c ij = ( 1) i+j det(ã ij ) is called the cofactor of the entry of A in row i, column j. Xiao Yu Wang (MIT) Math Camp /10 52 / 88

53 Determinant Example Find the determinant of matrix A: A = Xiao Yu Wang (MIT) Math Camp /10 53 / 88

54 Determinant Example [ ] [ ] 5 2 [ ] Then: det(a) = 1 det 3 det 3 det = (26) 3( 32) = 40. Xiao Yu Wang (MIT) Math Camp /10 54 / 88

55 Determinant Theorem For any A, B M n n (R), det(ab) = det(a) det(b). Corollary A matrix A M n n (R) is invertible iff det(a) = 0. Furthermore, if A is invertible, then det( A 1 ) = 1 det(a). Theorem For any A M n n (R), det(a ) = det(a). Xiao Yu Wang (MIT) Math Camp /10 55 / 88

56 Kronecker Product Definition (Kronecker product) If A is an n m matrix and B is a p q matrix, the Kronecker product of A and B is denoted by A B, where this equals: a 11 B a 1m B a n1 B a nm B Xiao Yu Wang (MIT) Math Camp /10 56 / 88

57 Diagonalization In what follows, A M n n (R). Definition (Eigenvalue/Eigenvector) If Ax = λx and x R n \{0}, we say that λ is an eigenvalue of A and x is an eigenvector associated with λ. Theorem The eigenvalues are the roots of the characteristic polynomial of A, defined as P A (λ) = det(a λi ). There are at most n eigenvalues. Theorem If x 1,..., x k are eigenvectors corresponding to λ 1,..., λ k distinct eigenvalues of A, then {x 1,..., x k } is linearly independent. Xiao Yu Wang (MIT) Math Camp /10 57 / 88

58 Diagonalization Definition (Similar) A is said to be similar to B if there exists an invertible matrix such that Definition (Diagonalizable) A = PBP 1 A is diagonalizable if A is similar to a diagonal matrix, i.e. there exists an invertible matrix P and a diagonal matrix Λ such that A = PΛP 1 Xiao Yu Wang (MIT) Math Camp /10 58 / 88

59 Diagonalization Theorem A is diagonalizable iff A has n linearly independent eigenvectors. Theorem If A has n real, distinct eigenvalues, then A is diagonalizable. Xiao Yu Wang (MIT) Math Camp /10 59 / 88

60 Diagonalization Definition (Splits) A polynomial f (t) splits over R if there are scalars c, a 1,..., a n (not necessarily distinct) in R such that f (t) = c(t a 1 )(t a 2 ) (t a n ) Theorem The characteristic polynomial of any diagonalizable matrix splits. Xiao Yu Wang (MIT) Math Camp /10 60 / 88

61 Diagonalization Definition (Multiplicity) Let λ be an eigenvalue of a matrix with characteristic polynomial f (t). The multiplicity of λ is the largest possible integer k for which (t λ) k is a factor of f (t). Theorem A is diagonalizable iff i) The characteristic polynomial of A splits ii) For each eigenvalue λ of A, the multiplicity of λ equals n rk(a λi ). Xiao Yu Wang (MIT) Math Camp /10 61 / 88

62 Diagonalization Example A = The characteristic polynomial of A is det(a λi ) = (1 λ) 2 (2 λ), which splits and has eigenvalues λ 1 = 1 and λ 2 = 2 with multiplicities 2 and 1, respectively. Xiao Yu Wang (MIT) Math Camp /10 62 / 88

63 Diagonalization Example Note that rk(a λ 1 I ) = 3 rk = = 2 so, from the last theorem, A is diagonalizable. We can also find the basis composed of eigenvectors for R 3. To do so, we find the null space of (A λ i I ) for i = 1, 2. Xiao Yu Wang (MIT) Math Camp /10 63 / 88

64 Diagonalization Example For λ 1 = 1, the corresponding nullspace is x x 1 3 x2 R : x 2 = 0 x x 3 Xiao Yu Wang (MIT) Math Camp /10 64 / 88

65 Diagonalization Example This is the solution space for the system x 2 + x 3 = 0 and has a basis of 1 0 γ 1 = 0, Xiao Yu Wang (MIT) Math Camp /10 65 / 88

66 Diagonalization Example For λ 2 = 2, the corresponding nullspace is x x 1 x 2 R 3 1 : x 2 = 0 x x 3 Xiao Yu Wang (MIT) Math Camp /10 66 / 88

67 Diagonalization Example This is the solution space for the system x 1 = x 3 and has a basis of 1 γ 2 = 0 1 Xiao Yu Wang (MIT) Math Camp /10 67 / 88

68 Diagonalization Example Let Then γ is an ordered basis for R γ = 0, 1, consisting of eigenvectors of A. Xiao Yu Wang (MIT) Math Camp /10 68 / 88

69 Diagonalization Example One can verify that A = PΛP 1 where P = 0 1 0, P 1 = 0 1 0, andλ = Note that the columns of P are the eigenvectors corresponding to the eigenvalues, which are on the diagonal of Λ. Xiao Yu Wang (MIT) Math Camp /10 69 / 88

70 Diagonalization Trace Definition (Trace) The trace of A, denoted tr(a), is the sum of the diagonal entries of A. Properties: 1 tr(a) + tr(b) = tr(a + B), for square matrices. 2 tr(ca) = ctr(a), c R, for square matrices. 3 If A is an m n matrix and B is an n m matrix, then tr(ab) = tr(ba). Xiao Yu Wang (MIT) Math Camp /10 70 / 88

71 Diagonalization This yields the following useful theorem: Theorem Let A be an n n matrix that is similar to an upper triangular matrix and has the distinct eigenvalues λ 1,..., λ k with corresponding multiplicities m 1,..., m k. Then a) det(a) = (λ 1 ) m 1(λ 2 ) m 2 ( λ m k ) k k b) tr(a) = i=1 m i λ i Xiao Yu Wang (MIT) Math Camp /10 71 / 88

72 Inner Product Spaces Inner Products and Norms Orthonormal Basis and Projections Symmetric Matrices Xiao Yu Wang (MIT) Math Camp /10 72 / 88

73 Inner Product Spaces Inner Products and Norms Definition (Inner Product) Let V be a real vector space. An inner product on V is a function that assigns, to every ordered pair of vectors x, y V, a real scalar, denoted x, y, such that for all x, y, z V and all real scalars c, the following hold a) x + z, y = x, y + z, y b) cx, y = c x, y c) x, y = y, x d) x, x > 0 if x = 0 Xiao Yu Wang (MIT) Math Camp /10 73 / 88

74 Inner Product Spaces Inner Products and Norms Example When V = R n, the standard inner product is given by: x, y = x y = x i y i i=1 In terms of matrix notations: x and y are n-dimensional vectors, so they can be written as n 1 matrices and then the inner product is equivalent to the matrix multiplication: x, y = x y n Xiao Yu Wang (MIT) Math Camp /10 74 / 88

75 Inner Product Spaces Inner Products and Norms Example Definition (Norm) Let V be a vector space endowed with an inner product. For x V we define the norm of x by: x = x, x Example When V = R n, then x = n i=1 xi 2 is the Euclidean definition of length. Xiao Yu Wang (MIT) Math Camp /10 75 / 88

76 Inner Product Spaces Inner Products and Norms Let V be a vector space endowed with an inner product. Then for all x, y V and c R, the following are true: cx = c x x = 0 iff x = 0. In any case, x 0. (Cauchy-Shwarz Inequality) x, y x y (Triangle Inequality) x + y x + y Xiao Yu Wang (MIT) Math Camp /10 76 / 88

77 Inner Product Spaces Orthonormal Basis and Projections Definition (Orthogonal/Orthonormal) Let V be a vector space endowed with an inner product. Vectors x, y V are orthogonal if x, y = 0. A susbset S of V is orthogonal if any two distinct vectors in S are orthogonal. A vector x V is a unit vector if x = 1. Finally, a subset S of V is orthonormal if S is orthogonal and consists entirely of unit vectors. We say that an n n matrix A is orthonormal iff its column vectors form an orthonormal basis of R n. It should be clear that A is orthonormal iff A A = I (which implies that A = A 1 for orthonormal matrices; A A is a diagonal matrix for A orthogonal). Example: standard basis of R n. Xiao Yu Wang (MIT) Math Camp /10 77 / 88

78 Inner Product Spaces Orthonormal Basis and Projections Theorem Any subspace S of Rn has an orthonormal basis. Xiao Yu Wang (MIT) Math Camp /10 78 / 88

79 Inner Product Spaces Orthonormal Basis and Projections Definition (Orthogonal Complement) Let S be a nonempty subset of an inner product space V. We define S to be the set of all vectors in V that are orthogonal to every vector in S; that is, S = {x V : x, y = 0 for all y S}. The set S is called the orthogonal complement of S. It should be clear that S is a subspace of V for any subset S of V. Examples {0} = V and V = {0} for any inner product space V If V = R 3 and S = {e 3 }, then S equals the xy plane Xiao Yu Wang (MIT) Math Camp /10 79 / 88

80 Inner Product Spaces Orthonormal Basis and Projections The vector u in the corollary is called the orthogonal projection of y on W. Using all of this, it is straightforward to find the ONB of W, but we never do this in econ so I won t go through the "formula", but it s basically normalizing stuff. Xiao Yu Wang (MIT) Math Camp /10 80 / 88 Theorem Let W be a finite-dimensional subspace of an inner product space V and let y V. Then there exists unique vectors u W and z W such that y = u + z. Furthermore, if {v 1,..., v k } is an orthonormal basis for W, then Corollary k u = y, v i v i i=1 In the notation of the preceding theorem, the vector u is the unique vector in W that is "closest" to y; that is, for any x W, y x y u, and this inequality is an equality iff x = u.

81 Inner Product Spaces Orthonormal Basis and Projections Proof We have y = u + z, where z W. Let x W. Then u x is orthogonal to z, so we have y x 2 = u + z x 2 = (u x) + z 2 u x 2 = + z 2 z 2 = y u 2 where the third equality can be shown to be true by applying the definition of a norm and using the linearity of the inner product. Now suppose that y x = y u. Then the inequality above becomes an equality, and therefore u x = 0, and hence u = x. The proof of the converse is obvious. Xiao Yu Wang (MIT) Math Camp /10 81 / 88

82 Inner Product Spaces Orthonormal Basis and Projections Application: Ordinary Least Squares Regression (OLS) Consider a vector of explained data y R n and vectors x1,..., xm R n of explanatory variables. Here, n is the number of observations; usually n >> m. In OLS we look for a linear combination of explanatory data that is closest to the explained data. It is given by a projection of y on the subspace of Rn spanned by the explanatory variables. The coeffi cients on the explanatory variables in the projected vector are called the regression parameters. Xiao Yu Wang (MIT) Math Camp /10 82 / 88

83 Inner Product Spaces Orthonormal Basis and Projections Diagonalization and Symmetric Matrices Theorem Let A be an n n symmetric matrix. Then A is diagonalizable on an orthonormal basis of eigenvectors and all of the eigenvalues are real. Hence, A can be expressed as A = QΛQ where Q is an orthonormal matrix and Λ is a diagonal matrix. Xiao Yu Wang (MIT) Math Camp /10 83 / 88

84 Inner Product Spaces Orthonormal Basis and Projections Definition (Idempotent Matrix) A n n matrix A is said to be idempotent if MM = M. Theorem A symmetric idempotent matrix has eigenvalues equal to either 0 or 1. Proof Let λ be an eigenvalue of a matrix A. Then Ax = λx. Therefore, 2 λx = Ax = AAx = Aλx = λax = λ x. Since x is nonzero and λ is real, it follows that λ can only be 0 or 1. Xiao Yu Wang (MIT) Math Camp /10 84 / 88

85 Definiteness Let A be an n n matrix and x n R. Then x Ax = a ij x i x j i,j is a quadratic form. Note that x Ax is a scalar. Therefore it follows that x 1 Ax = x A x such that we can write x Ax = 2 x (A + A )x where A+A 2 is symmetric. Thus, assume, without loss of generality, that A is symmetric. Then A is positive (negative) definite if x Ax > (<)0 x = 0. positive (negative) semidefinite if x Ax ( )0 x Xiao Yu Wang (MIT) Math Camp /10 85 / 88

86 Definiteness Theorem For any matrix A, A A is positive semidefinite. Proof Let y = Ax. Then x A Ax = y y = n i=1 y 2 i 0. Theorem A positive definite matrix is nonsingular. Proof Assume that A is positive definite and singular. Then there exists an x = 0 such that Ax = 0. But then x Ax = 0 which contadicts that A is positive definite. Xiao Yu Wang (MIT) Math Camp /10 86 / 88

87 Definiteness Theorem If A is m n matrix with m n and rk(a) = n, then A A is positive definite. Proof Let y = Ax and observe that Ax is zero only if x = 0 because A is of full column rank n. Then for x = 0, y = 0, so x A Ax = y y > 0. Theorem Any symmetric matrix with strictly positive eigenvalues is positive definite. Any symmetric matrix with nonnegative eigenvalues is positive semidefinite. In particular, any symmetric idempotent matrix is positive semidefinite. Xiao Yu Wang (MIT) Math Camp /10 87 / 88

88 Definiteness A Determinantal Test of Definiteness For any T S matrix M, we denote by t M the t S submatrix of M where only the first t T rows are retained. Similarly, we let M s be the T s submatrix of M where only the first s S columns are retained. Finally, we let t M s be the t s submatrix of M where only the first t T rows and s S columns are retained. Also, if M is an N N matrix, then for any permutation π of the indices {1,..., N} we denote by M π the matrix in which rows and columns are correspondingly permuted. Theorem Let M be an N N symmetric matrix. i) Then M is negative definite iff ( 1) r r M r > 0 for every r = 1,..., N. ii) Then M is negative semidefinite iff ( 1) r r M π r 0 for every r = 1,..., N and for every permutation π of the indices. Xiao Yu Wang (MIT) Math Camp /10 88 / 88

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