# Linear Algebra review Powers of a diagonalizable matrix Spectral decomposition

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1 Linear Algebra review Powers of a diagonalizable matrix Spectral decomposition Prof. Tesler Math 283 Fall 2016 Also see the separate version of this with Matlab and R commands. Prof. Tesler Diagonalizing a matrix Math 283 / Fall / 32

2 Matrices A matrix is a square or rectangular table of numbers. An m n matrix has m rows and n columns. This is read m by n. This matrix is 2 3: A = [ 1 2 ] The entry in row i, column j, is denoted A i, j or A ij. A 1,1 = 1 A 1,2 = 2 A 1,3 = 3 A 2,1 = 4 A 2,2 = 5 A 2,3 = 6 Prof. Tesler Diagonalizing a matrix Math 283 / Fall / 32

3 Matrix multiplication A B = C [ ] ] [ = } {{ } } {{ } 2 3 } {{ } Let A be p q and B be q r. The product AB = C is a certain p r matrix of dot products: C i, j = q A i,k B k, j = dot product (i th row of A) (j th column of B) k=1 The number of columns in A must equal the number of rows in B (namely q) in order to be able to compute the dot products. Prof. Tesler Diagonalizing a matrix Math 283 / Fall / 32

4 Matrix multiplication [ ] = [ 2 ] C 1,1 = 1(5) + 2(0) + 3( 1) = = 2 Prof. Tesler Diagonalizing a matrix Math 283 / Fall / 32

5 Matrix multiplication [ ] = [ 2 18 ] C 1,2 = 1( 2) + 2(1) + 3(6) = = 18 Prof. Tesler Diagonalizing a matrix Math 283 / Fall / 32

6 Matrix multiplication [ ] = [ ] C 1,3 = 1(3) + 2(1) + 3(4) = = 17 Prof. Tesler Diagonalizing a matrix Math 283 / Fall / 32

7 Matrix multiplication [ ] = [ ] C 1,4 = 1(2) + 2( 1) + 3(3) = = 9 Prof. Tesler Diagonalizing a matrix Math 283 / Fall / 32

8 Matrix multiplication [ ] = [ ] C 2,1 = 4(5) + 5(0) + 6( 1) = = 14 Prof. Tesler Diagonalizing a matrix Math 283 / Fall / 32

9 Matrix multiplication [ ] = [ ] C 2,2 = 4( 2) + 5(1) + 6(6) = = 33 Prof. Tesler Diagonalizing a matrix Math 283 / Fall / 32

10 Matrix multiplication [ ] = [ ] C 2,3 = 4(3) + 5(1) + 6(4) = = 41 Prof. Tesler Diagonalizing a matrix Math 283 / Fall / 32

11 Matrix multiplication [ ] = [ ] C 2,4 = 4(2) + 5( 1) + 6(3) = = 21 Prof. Tesler Diagonalizing a matrix Math 283 / Fall / 32

12 Transpose of a matrix Given matrix A of dimensions p q, the transpose A is q p, obtained by interchanging rows and columns: (A ) ij = A ji. [ ] = Transpose of a product reverses the order and transposes the factors: (AB) = B A [ ] = = [ ] Prof. Tesler Diagonalizing a matrix Math 283 / Fall / 32

13 Multiplying several matrices Multiplication is not commutative: usually, AB BA. AB and BA are both defined only if A and B are both n n. Even then, they are usually not equal. Multiplication is associative: (AB)C = A(BC) Suppose A is p 1 p 2 B is p 2 p 3 C is p 3 p 4 D is p 4 p 5 Then ABCD is p 1 p 5. By associativity, it may be computed in many ways, such as A(B(CD)), (AB)(CD),... or directly by: (ABCD) i, j = p 2 p 3 p 4 k 2 =1 k 3 =1 k 4 =1 A i, k2 B k2, k 3 C k3, k 4 D k4, j This generalizes to any number of matrices. Powers A 2 = AA, A 3 = AAA,... are defined for square matrices. Prof. Tesler Diagonalizing a matrix Math 283 / Fall / 32

14 Inverse matrix The n n identity matrix I is I = I i, j = { 1 if i = j (main diagonal); 0 if i j (elsewhere). The inverse of an n n matrix A is an n n matrix A 1 such that A A 1 = I and A 1 A = I. It may or may not exist. For 2 2 matrices [ ] a b A = c d A 1 = 1 ad bc [ d ] b c a unless det(a) = ad bc = 0, in which case A 1 is undefined. For n n matrices, use the row reduction algorithm (a.k.a. Gaussian elimination) in Linear Algebra. If A, B are invertible and the same size: (AB) 1 = B 1 A 1 The order is reversed and the factors are inverted. Prof. Tesler Diagonalizing a matrix Math 283 / Fall / 32

15 Span, basis, and linear (in)dependence The span of vectors v 1,..., v k is the set of all linear combinations α 1 v α k v k α 1,..., α k R Example 1 In 3D, 1 0 span 0, 0 = 0 1 x 0 : x, z R z = xz plane Here, the span of these two vectors is a 2-dimensional space. Every vector in it is generated by a unique linear combination. Prof. Tesler Diagonalizing a matrix Math 283 / Fall / 32

16 Span, basis, and linear (in)dependence Example 2 In 3D, span 0, 1, 0 = 0 0 1/2 x y : x, y, z R = R3. z Note that x y = (x y) 0 + y 1 2z 0 z 0 0 1/2 Here, the span of these three vectors is a 3-dimensional space. Every vector in R 3 is generated by a unique linear combination. Prof. Tesler Diagonalizing a matrix Math 283 / Fall / 32

17 Span, basis, and linear (in)dependence Example 3 In 3D, span 0, 0, 0 = x 0 : x, z R z = xz plane This is a plane (2D), even though it s a span of three vectors. Note that v 2 = v 1 + v 3, or v 1 v 2 + v 3 = 0. There are multiple ways to generate each vector in the span: for all x, z, t, x = x 0 + z 0 + t ( v 1 v 2 + v 3 ) = (x + t) 0 t 0 + (z + t) 0 } {{ } z = 0 Prof. Tesler Diagonalizing a matrix Math 283 / Fall / 32

18 Span, basis, and linear (in)dependence Given vectors v 1,..., v k, if there is a linear combination α 1 v α k v k = 0 with at least one α i 0, the vectors are linearly dependent (Ex. 3). Otherwise they are linearly independent (Ex. 1 2). Linearly independent vectors form a basis of the space S they span. Any vector in S is a unique linear combination of basis vectors (vs. it s not unique if v 1,..., v k are linearly dependent). One basis of R n is a unit vector on each axis: but there are other possibilities, e.g., Example 2: [ [ ] [ ] [ ] ,, ], [ ], [ 0 0 1/2 ] Prof. Tesler Diagonalizing a matrix Math 283 / Fall / 32

19 Eigenvectors Eigenvalues and eigenvectors Let A be a square matrix (k k) and v 0 be a column vector (k 1). If A v = λ v for a scalar λ, then v is an eigenvector of A with eigenvalue λ. Example [8 [ ] ] [ ] 1 3 = [ ] (8)(1) + ( 1)(3) (6)(1) + (3)(3) is an eigenvector with eigenvalue 5. = [ ] 5 15 = 5 [ ] 1 3 But this is just a verification. How do we find eigenvalues and eigenvectors? Prof. Tesler Diagonalizing a matrix Math 283 / Fall / 32

20 Finding eigenvalues and eigenvectors We will work with the example P = [ ] Form the identity matrix of the same dimensions: [ ] 1 0 I = 0 1 The formula for the determinant depends on the dimensions of the matrix. For a 2 2 matrix, [ ] a b det = ad bc c d Prof. Tesler Diagonalizing a matrix Math 283 / Fall / 32

21 Finding eigenvalues and eigenvectors Compute the determinant of P λi: [ ] 8 λ 1 det(p λi) = det 6 3 λ = (8 λ)(3 λ) ( 1)(6) = 24 11λ + λ = λ 2 11λ + 30 This is the characteristic polynomial of P. It has degree k in λ. The characteristic equation is det(p λi) = 0. Solve it for λ. For k = 2, use the quadratic formula: λ = 11 ± ( 11) 2 4(1)(30) 2 The eigenvalues are λ = 5 and λ = 6. = 5, 6 Prof. Tesler Diagonalizing a matrix Math 283 / Fall / 32

22 Finding the (right) eigenvector for λ = 5 Let v = [ ] a. We will solve for a, b. b The equation P v = λ v is equivalent to (P λi) v = 0. [ ] 0 0 = (P 5I) v = [ ] [ ] 3 1 a 6 2 b = [ ] 3a b 6a 2b so 3a b = 0 and 6a 2b = 0 (which are equivalent). Solving gives b = 3a. Thus, [ ] a v = = b Any nonzero scalar multiple of eigenvalue 5. [ ] a 3a [ ] 1 3 = a [ ] 1 3 is an eigenvector of P with Prof. Tesler Diagonalizing a matrix Math 283 / Fall / 32

23 Finding the (right) eigenvector for λ = 6 Let v = [ ] a. We will solve for a, b. b The equation P v = λ v is equivalent to (P λi) v = 0. [ ] 0 0 = (P 6I) v = [ ] [ ] 2 1 a 6 3 b = [ ] 2a b 6a 3b so 2a b = 0 and 6a 3b = 0 (which are equivalent). Solving gives b = 2a. Thus, [ ] a v = = b Any nonzero scalar multiple of eigenvalue 6. [ ] a 2a [ ] 1 2 = a [ ] 1 2 is an eigenvector of P with Prof. Tesler Diagonalizing a matrix Math 283 / Fall / 32

24 Verify the eigenvectors [ ] [ ] [ ] [ ] = = [ ] 8(1) 1(3) 6(1) + 3(3) [ ] 8(2) 1(4) 6(2) + 3(4) = = [ ] 5 15 [ ] [ ] 1 = 5 3 [ ] 2 = 6 4 Prof. Tesler Diagonalizing a matrix Math 283 / Fall / 32

25 Normalization: Which scalar multiple should we use? In some applications, any nonzero multiple is fine. In others, a particular scaling is required. Markov chains / Stochastic matrices Scale the vector [ ] so that the entries sum up to 1. 1 For v = a, the sum is a (1 + 3) = 4a = 1, so a = 1/4: 3 [ ] 1/4 v = 3/4 Principal component analysis Scale it to be a unit vector, so that the sum of the squares of its entries equals 1: 1 = a 2 ( ) = 10a 2 ±1 so a = = ± v = ± [ ] 1/ 10 3/ 10 (two possibilities) Prof. Tesler Diagonalizing a matrix Math 283 / Fall / 32

26 Finding the left eigenvector for λ = 5 Let v = [ a b ]. We will solve for a, b. The equation vp = λ v is equivalent to v(p λi) = 0. [ ] [ ] [ ] = v(p 5I) = a b 6 2 = [ 3a + 6b a 2b ] so 3a + 6b = 0 and a 2b = 0 (which are equivalent). Solving gives b = a/2. Thus, v = [ a b ] = [ a a/2 ] = a [ 1 1/2 ] Any nonzero scalar multiple of [ 1 with eigenvalue 5. 1/2 ] is a left eigenvector of P Prof. Tesler Diagonalizing a matrix Math 283 / Fall / 32

27 Finding the left eigenvector for λ = 6 Let v = [ a b ]. We will solve for a, b. The equation vp = λ v is equivalent to v(p λi) = 0. [ ] [ ] [ ] = v(p 6I) = a b 6 3 = [ 2a + 6b a 3b ] so 2a + 6b = 0 and a 3b = 0 (which are equivalent). Solving gives b = a/3. Thus, v = [ a b ] = [ a a/3 ] = a [ 1 1/3 ] Any nonzero scalar multiple of [ 1 with eigenvalue 6. 1/3 ] is a left eigenvector of P Prof. Tesler Diagonalizing a matrix Math 283 / Fall / 32

28 Verify the left eigenvectors [ ] [ ] = [ 2(8) + 1(6) 2( 1) + 1(3) ] = [ 10 5 ] = 5 [ 2 1 ] [ ] [ ] = [ 1.5(8).5(6) 1.5( 1).5(3) ] = [ 9 3 ] = 6 [ ] Prof. Tesler Diagonalizing a matrix Math 283 / Fall / 32

29 Diagonalizing a matrix This procedure assumes there are k linearly independent eigenvectors, where P is k k. If the characteristic polynomial has k distinct roots, then there are k such eigenvectors. But if roots are repeated, there may or may not be a full set of eigenvectors. We ll explore this complication later. Prof. Tesler Diagonalizing a matrix Math 283 / Fall / 32

30 Diagonalizing a matrix Put the right eigenvectors r 1, r 2,... into the columns of a matrix V. Form diagonal matrix D with eigenvalues λ 1, λ 2,... in the same order: V = [ [ ] [ ] [ ] ] 1 2 λ r 1 r 2 = D = = λ [ ] [ ] Compute V 1 l = = l 2 3/2 1/2 Its rows are the left eigenvectors l 1, l 2,... of P, in the same order as the eigenvalues in D, scaled so that l i r i = 1. This gives the diagonalization P = VDV 1 : P = V D V 1 [ ] [ ] [ ] [ ] = /2 1/2 Prof. Tesler Diagonalizing a matrix Math 283 / Fall / 32

31 Matrix powers using the spectral decomposition An expansion of P n is P n = (VDV 1 )(VDV 1 ) (VDV 1 ) = VD n V 1 : [ ] [ ] [ ] P n = VD n V 1 5 n 0 = V 0 6 n V 1 5 n 0 = V V V n V 1 V [ ] 5 n 0 V 1 = 0 0 [ ][ ][ ] [ n (1)(5 = n )( 2) (1)(5 n ] )(1) (3)(5 n )( 2) (3)(5 n )(1) ] [ ] [ 2 ] 1 = n λ1 r 1 l 1 = 5 n 2 1 = 5 n [ V [ ] n V 1 = [ ] [ ] [ ] [ ( n = n )(1.5) 2(6 n ] )(.5) (6 n )(1.5) 4(6 n )(.5) ] [ ] [1.5 ].5 = n λ2 r 2 l 2 = 6 n 3 1 = 6 n [ Prof. Tesler Diagonalizing a matrix Math 283 / Fall / 32

32 Matrix powers using the spectral decomposition Continue computing P n : P n = VD n V 1 = V [ ] = 5 n [ ] 5 n n + 6 n [ V 1 = V ] [ ] 5 n 0 V 1 + V 0 0 [ ] n V 1 General formula (with k = 2 and two distinct eigenvalues): P n = VD n V 1 = λ 1 n r 1 l 1 + λ 2 n r 2 l 2 General formula: If P is k k and is diagonalizable, this becomes: P n = VD n V 1 = λ 1 n r 1 l 1 + λ 2 n r 2 l λ k n r k l k What if the matrix is not diagonalizable? We will see a generalization called the Jordan Canonical Form. Prof. Tesler Diagonalizing a matrix Math 283 / Fall / 32

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