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# Lecture 6 Eigenvalue and Vector Space

Lecture 6 Eigenvalue and Vector Space. Lat Time - Matrix Computation Packages (Matlab and …) - Evaluation of Determinants - Geometric Interpretations - Properties of Determinants. Elementary Linear Algebra R. Larsen et al. (5 Edition) TKUEE 翁慶昌 -NTUEE SCC_10_2007.

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## Lecture 6 Eigenvalue and Vector Space

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1. Lecture 6 Eigenvalue and Vector Space Lat Time - Matrix Computation Packages (Matlab and …) - Evaluation of Determinants - Geometric Interpretations - Properties of Determinants Elementary Linear Algebra R. Larsen et al. (5 Edition) TKUEE翁慶昌-NTUEE SCC_10_2007

2. Lecture 6: Eigenvalue and Vectors Today • Properties of Determinants • Introduction to Eigenvalues • Applications of Determinants • Vectors in Rn • Vector Spaces Reading Assignment: Secs 3.4 – 3.5, 4.1-4.2 of Textbook Next Time • Subspaces of Vector Spaces • Spanning Sets and Linear Independence • Basis and Dimension • Rank of a Matrix and Systems of Linear Equations Reading Assignment: Secs 4.3- 4.6 Homework #3 due

3. Lecture 6: Elementary Matrices & Determinants Today • Properties of Determinants (Cont.) • Introduction to Eigenvalues • Applications of Determinants • Vectors in Rn • Vector Spaces

4. What Did You Actually Learn about Determinant?

5. 3.3 Properties of Determinants • Notes: • Thm 3.5: (Determinant of a matrix product) det (AB) = det (A) det (B) (1) det (EA) = det (E) det (A) (2) (3)

6. Ex 1: (The determinant of a matrix product) Find |A|, |B|, and |AB| Sol:

7. Check: |AB| = |A| |B|

8. Ex 2: • Thm 3.6: (Determinant of a scalar multiple of a matrix) If A is an n × n matrix and c is a scalar, then det (cA) = cn det (A) Find |A|. Sol:

9. Thm 3.7: (Determinant of an invertible matrix) • Ex 3: (Classifying square matrices as singular or nonsingular) A square matrix A is invertible (nonsingular) if and only if det (A)  0 Sol: A has no inverse (it is singular). B has inverse (it is nonsingular).

10. Ex 4: • Thm 3.8: (Determinant of an inverse matrix) • Thm 3.9: (Determinant of a transpose) (a) (b) Sol:

11. If A is an n × n matrix, then the following statements are equivalent. • Equivalent conditions for a nonsingular matrix: (1) A is invertible. (2) Ax = b has a unique solution for every n × 1 matrix b. (3) Ax = 0 has only the trivial solution. (4) A is row-equivalent to In (5) A can be written as the product of elementary matrices. (6) det (A)  0

12. Sol: (a) This system does not have a unique solution. (b) This system has a unique solution.

13. Eigenvalue and eigenvector: A：an nn matrix ：a scalar x： a n1nonzero column matrix Eigenvalue Eigenvector (The fundamental equation for the eigenvalue problem) 3.4 Introduction to Eigenvalues • Eigenvalue problem: If A is an nn matrix, do there exist n1 nonzero matrices x such that Ax is a scalar multiple of x？

14. Eigenvalue Eigenvalue Eigenvector Eigenvector • Ex 1: (Verifying eigenvalues and eigenvectors)

15. Note: (homogeneous system) If has nonzero solutions iff . • Characteristic equation of AMnn: • Question: Given an nn matrix A, how can you find the eigenvalues and corresponding eigenvectors?

16. Sol: Characteristic equation: Eigenvalue: • Ex 2: (Finding eigenvalues and eigenvectors)

17. Ex 3: (Finding eigenvalues and eigenvectors) Sol: Characteristic equation:

18. Chapter 4 Vector Spaces 4.1 Vectors in Rn 4.2 Vector Spaces 4.3 Subspaces of Vector Spaces 4.4 Spanning Sets and Linear Independence 4.5 Basis and Dimension 4.6 Rank of a Matrix and Systems of Linear Equations 4.7 Coordinates and Change of Basis

19. n-space: Rn the set of all ordered n-tuple 4.1 Vectors in Rn • An ordered n-tuple: a sequence of n real number

20. n = 1 R1 = 1-space = set of all real number n = 2 R2 = 2-space = set of all ordered pair of real numbers n = 3 R3 = 3-space = set of all ordered triple of real numbers n = 4 R4 = 4-space = set of all ordered quadruple of real numbers • Ex:

21. a point a vector • Notes: (1) An n-tuple can be viewed as a point in Rn with the xi’s as its coordinates. (2) An n-tuple can be viewed as a vector in Rnwith the xi’s as its components. • Ex:

22. (two vectors in Rn) • Equal: • if and only if • Vector addition (the sum of u and v): • Scalar multiplication (the scalar multiple of u by c): • Notes: The sum of two vectors and the scalar multiple of a vector in Rn are called the standard operations in Rn.

23. Difference: • Zero vector: • Notes: (1) The zero vector 0 in Rn is called the additive identity in Rn. (2) The vector –v is called the additive inverse of v. • Negative:

24. (1) u+v is a vector inRn (2) u+v = v+u (3) (u+v)+w = u+(v+w) (4) u+0 = u (5) u+(–u) = 0 (6) cu is a vector inRn (7) c(u+v) = cu+cv (8) (c+d)u = cu+du (9) c(du) = (cd)u (10) 1(u) = u • Thm 4.2: (Properties of vector addition and scalar multiplication) Let u, v, and w be vectors inRn , and let c and d be scalars.

25. Ex 5: (Vector operations in R4) Let u=(2, – 1, 5, 0), v=(4, 3, 1, – 1), and w=(– 6, 2, 0, 3) be vectors in R4. Solve x for x in each of the following. (a) x = 2u– (v + 3w) (b) 3(x+w) = 2u –v+x Sol: (a)

26. (b)

27. (1) The additive identity is unique. That is, if u+v=v, then u = 0 (2) The additive inverse of v is unique. That is, if v+u=0, then u = –v (3) 0v=0 (4) c0=0 (5) If cv=0, then c=0 or v=0 (6) –(–v) = v • Thm 4.3: (Properties of additive identity and additive inverse) Let v be a vector inRn and c be a scalar. Then the following is true.

28. Ex 6: Given x = (– 1, – 2, – 2), u = (0,1,4), v = (– 1,1,2), and w = (3,1,2) in R3, find a, b, and c such that x = au+bv+cw. Sol: • Linear combination: The vector x is called a linear combination of , if it can be expressed in the form

29. Notes: A vector in can be viewed as: a 1×n row matrix (row vector): or a n×1 column matrix (column vector): (The matrix operations of addition and scalar multiplication give the same results as the corresponding vector operations)

30. Vector addition Scalar multiplication

31. Keywords in Section 4.1: • ordered n-tuple：有序的n項 • n-space：n維空間 • equal：相等 • vector addition：向量加法 • scalar multiplication：純量乘法 • negative：負向量 • difference：向量差 • zero vector：零向量 • additive identity：加法單位元素 • additive inverse：加法反元素

32. Addition: (1) u+vis in V (2) u+v=v+u (3) u+(v+w)=(u+v)+w (4) V has a zero vector 0 such that for every u in V, u+0=u (5) For every u in V, there is a vector in V denoted by –u such that u+(–u)=0 4.2 Vector Spaces • Vector spaces: Let V be a set on which two operations (vector addition and scalar multiplication) are defined. If the following axioms are satisfied for every u, v, and w in V and every scalar (real number) c and d, then V is called a vector space.

33. Scalar multiplication: (6) is in V. (7) (8) (9) (10)

34. is called a vector space zero vector space (2) • Notes: (1) A vector space consists of four entities: a set of vectors, a set of scalars, and two operations V：nonempty set c：scalar vector addition scalar multiplication

35. (2) Matrix space:(the set of all m×n matrices with real values) Ex:：(m = n = 2) vector addition scalar multiplication • Examples of vector spaces: (1) n-tuple space:Rn vector addition scalar multiplication

36. (4) Function space: (the set of all real-valued continuous functions defined on the entire real line.) (3) n-th degree polynomial space: (the set of all real polynomials of degree n or less)

37. Thm 4.4: (Properties of scalar multiplication) Let vbe any element of a vector space V, and let c be any scalar. Then the following properties are true.

38. Ex 6:The set of all integer is not a vector space. (it is not closed under scalar multiplication) Pf: scalar noninteger integer • Ex 7:The set of all second-degree polynomials is not a vector space. Pf: Let and (it is not closed under vector addition) • Notes: To show that a set is not a vector space, you need only find one axiom that is not satisfied.

39. the set (together with the two given operations) is not a vector space • Ex 8: V=R2=the set of all ordered pairs of real numbers vector addition: scalar multiplication: Verify V is not a vector space. Sol:

40. Keywords in Section 4.2: • vector space：向量空間 • n-space：n維空間 • matrix space：矩陣空間 • polynomial space：多項式空間 • function space：函數空間

41. Trivial subspace: Every vector space V has at least two subspaces. (1)Zero vector space {0} is a subspace of V. (2) V is a subspace of V. 4.3 Subspaces of Vector Spaces • Subspace: : a vector space : a nonempty subset ：a vector space (under the operations of addition and scalar multiplication defined in V) Wis a subspace ofV

42. Thm 4.5: (Test for a subspace) If W is a nonempty subset of a vector space V, then W is a subspace of V if and only if the following conditions hold. (1) If u and v are in W, then u+v is in W. (2) If uis in W and c is any scalar, then cuis in W.

43. Ex: Subspace of R3 • Ex: Subspace of R2

44. Ex 2: (A subspace of M2×2) Let W be the set of all 2×2 symmetric matrices. Show that W is a subspace of the vector space M2×2, with the standard operations of matrix addition and scalar multiplication. Sol:

45. Ex 3: (The set of singular matrices is not a subspace of M2×2) Let W be the set of singular matrices of order 2. Show that W is not a subspace of M2×2 with the standard operations. Sol:

46. (not closed under scalar multiplication) • Ex 4: (The set of first-quadrant vectors is not a subspace of R2) Show that , with the standard operations, is not a subspace of R2. Sol:

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