1. Lecture 7 Vector Spaces Lat Time Properties of Determinants Introduction to Eigenvalues Applications of Determinants Vectors in R n Elementary Linear Algebra R. Larsen et al. (6th Edition) TKUEE 翁慶昌 -NTUEE SCC_11_2008

2. 7 - 2 Lecture 7: Eigenvalue and Vectors Today Vectors in R n (Cont.) Vector Spaces Subspaces of Vector Spaces Spanning Sets and Linear Independence Basis and Dimension Reading Assignment: Secs 4.1-4.5 of Textbook Homework #4 due

3. 7 - 3 Lecture 7: Eigenvalue and Vectors Next Time Midterm Exam (15:30 – 17:10, 11/28) Scope: Chapters 1-4.6 Homework-like problems: 80% Last Year: http://recipe.ee.ntu.edu.tw/Frame//LA/ 作業及解答.html http://recipe.ee.ntu.edu.tw/Frame///LA/homework_solution/midterm%20exam_Fall_2007_%20soluti on_v2.pdf In-class Q&A type and advanced problems: 20% Rank of a Matrix and Systems of Linear Equations

4. 7 - 4 - Lecture 7: Vector Spaces Today Vectors in R n (Cont.) Vector Spaces Subspaces of Vector Spaces Spanning Sets and Linear Independence Basis and Dimension

9. 7 - 9 4.1 Vectors in R n a sequence of n real number An ordered n-tuple: the set of all ordered n-tuple  n-space: R n

10. 7 - 10 n = 4 = set of all ordered quadruple of real numbers R 4 = 4-space R 1 = 1-space = set of all real number n = 1 n = 2 R 2 = 2-space = set of all ordered pair of real numbers n = 3 R 3 = 3-space = set of all ordered triple of real numbers Ex:

11. 7 - 11 Notes:  Ex: a pointa vector (1) An n-tuple can be viewed as a point in R n with the x i ’s as its coordinates. (2) An n-tuple can be viewed as a vector in R n with the x i ’s as its components.

12. 7 - 12 (two vectors in R n )  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 R n are called the standard operations in R n.

13. 7 - 13  Negative:  Difference:  Zero vector:  Notes: (1) The zero vector 0 in R n is called the additive identity in R n. (2) The vector –v is called the additive inverse of v.

14. 7 - 14 Thm 4.2: (Properties of vector addition and scalar multiplication) Let u, v, and w be vectors in R n, and let c and d be scalars. (1) u+v is a vector in R n (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 in R n (7) c(u+v) = cu+cv (8) (c+d)u = cu+du (9) c(du) = (cd)u (10) 1(u) = u

15. 7 - 15 Ex 5: (Vector operations in R 4 ) Sol: (a) Let u=(2, – 1, 5, 0), v=(4, 3, 1, – 1), and w=(– 6, 2, 0, 3) be vectors in R 4. Solve x for x in each of the following. (a) x = 2u – (v + 3w) (b) 3(x+w) = 2u – v+x

16. 7 - 16 (b)

17. 7 - 17 Thm 4.3: (Properties of additive identity and additive inverse) Let v be a vector in R n and c be a scalar. Then the following is true. (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

18. 7 - 18 Linear combination:  Ex 6: Given x = (– 1, – 2, – 2), u = (0,1,4), v = (– 1,1,2), and w = (3,1,2) in R 3, find a, b, and c such that x = au+bv+cw. Sol: The vector x is called a linear combination of, if it can be expressed in the form

19. 7 - 19  Notes: A vector in can be viewed as: ( The matrix operations of addition and scalar multiplication give the same results as the corresponding vector operations) or a n×1 column matrix (column vector): a 1×n row matrix (row vector):

20. 7 - 20 Vector additionScalar multiplication

21. 7 - 21 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

22. 7 - 22 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. Addition: (1) u+v is 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

23. 7 - 23 Scalar multiplication: (6) is in V. (7) (8) (9) (10)

24. 7 - 24 Notes: (1) A vector space consists of four entities: (2) zero vector space V ： nonempty set c ： scalar vector addition scalar multiplication is called a vector space a set of vectors, a set of scalars, and two operations

25. 7 - 25 Examples of vector spaces: (1) n-tuple space: R n (2) Matrix space: (the set of all m×n matrices with real values) Ex: ： (m = n = 2) vector addition scalar multiplication vector addition scalar multiplication

26. 7 - 26 (3) n-th degree polynomial space: (the set of all real polynomials of degree n or less) (4) Function space : ( the set of all real-valued continuous functions defined on the entire real line.)

27. 7 - 27  Thm 4.4: (Properties of scalar multiplication) Let v be any element of a vector space V, and let c be any scalar. Then the following properties are true.

28. 7 - 28 Notes: To show that a set is not a vector space, you need only find one axiom that is not satisfied.  Ex 7: The set of all second-degree polynomials is not a vector space. Pf: Let and (it is not closed under vector addition) ( it is not closed under scalar multiplication ) scala r Pf:  Ex 6: The set of all integer is not a vector space. integer noninteger

29. 7 - 29 Ex 8: V=R 2 =the set of all ordered pairs of real numbers vector addition : scalar multiplication: the set (together with the two given operations) is not a vector space Verify V is not a vector space. Sol:

30. 7 - 30 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

31. 7 - 31 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) W is a subspace of V  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.

32. 7 - 32 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 u is in W and c is any scalar, then cu is in W.

33. 7 - 33 Ex: Subspace of R 3 Ex: Subspace of R 2

34. 7 - 34  Ex 2: (A subspace of M 2×2 ) Let W be the set of all 2×2 symmetric matrices. Show that W is a subspace of the vector space M 2×2, with the standard operations of matrix addition and scalar multiplication. Sol:

35. 7 - 35 Ex 3: (The set of singular matrices is not a subspace of M 2×2 ) Let W be the set of singular matrices of order 2. Show that W is not a subspace of M 2×2 with the standard operations. Sol:

36. 7 - 36  Ex 4: (The set of first-quadrant vectors is not a subspace of R 2 ) Show that, with the standard operations, is not a subspace of R 2. Sol: (not closed under scalar multiplication)

37. 7 - 37  Ex 6: (Determining subspaces of R 2 ) Which of the following two subsets is a subspace of R 2 ? (a) The set of points on the line given by x+2y=0. (b) The set of points on the line given by x+2y=1. Sol: (closed under addition) (closed under scalar multiplication)

38. 7 - 38 (b) (Note: the zero vector is not on the line)

39. 7 - 39  Ex 8: (Determining subspaces of R 3 ) Sol:

40. 7 - 40  Thm 4.6: (The intersection of two subspaces is a subspace)

41. 7 - 41 Keywords in Section 4.3: subspace ：子空間 trivial subspace ：顯然子空間

42. 7 - 42 4.4 Spanning Sets and Linear Independence  Linear combination:

43. 7 - 43  Ex 2: (Finding a linear combination) Sol:

44. 7 - 44 (this system has infinitely many solutions)

45. 7 - 45

46. 7 - 46 If S={v 1, v 2,…, v k } is a set of vectors in a vector space V, then the span of S is the set of all linear combinations of the vectors in S,  the span of a set: span (S)  a spanning set of a vector space: If every vector in a given vector space can be written as a linear combination of vectors in a given set S, then S is called a spanning set of the vector space.

47. 7 - 47  Notes:

48. 7 - 48  Ex 5: (A spanning set for R 3 ) Sol:

49. 7 - 49

50. 7 - 50  Thm 4.7: (Span(S) is a subspace of V) If S={v 1, v 2,…, v k } is a set of vectors in a vector space V, then (a)span (S) is a subspace of V. (b)span (S) is the smallest subspace of V that contains S. (Every other subspace of V that contains S must contain span (S).)

51. 7 - 51  Linear Independent (L.I.) and Linear Dependent (L.D.): : a set of vectors in a vector space V