Optimization Theory. A Concise Introduction. Jiongmin Yong


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1 October 11, :5 wsbook9x6 Book Title Optimization Theory Lecture Notes page 1 1 Optimization Theory A Concise Introduction Jiongmin Yong
2 Optimization Theory Lecture Notes page
3 Optimization Theory Lecture Notes page iii Contents 1. Mathematical Preparations 5 1 Basics in R n Some Results of Linear Algebra Functions: Limits and Continuity Differentiation Optimization Problems and Existence of Optimal Solutions 47 1 Optimization Problems Some Examples of Optimization Problems Existence of Optimal Solutions A Constructive Method Necessary and Sufficient Conditions of Optimal Solutions 71 1 Unconstrained Problems Problems with Equality Constraints Problems with Equality and Inequality Constraints Problems without Regularity of the Constraints Problems with Convexity and QuasiConvexity Conditions Convex Sets and Convex Functions Optimization Problems under Convexity Conditions Lagrange Duality QuasiConvexity and Related Optimization Problems Linear Programming Standard and Canonical Forms iii
4 Optimization Theory Lecture Notes page 4 4 Optimization Theory A Concise Introduction Geometric Considerations and the Fundamental Theorem of Linear Programming The Simplex Method General consideration Phase II Phase I Endless cycling Sensitivity Analysis Duality Theory The Dual Simplex and the Primal Dual Methods
5 Optimization Theory Lecture Notes page 5 Chapter 1 Mathematical Preparations In this chapter, we will briefly present some basic results of advanced calculus (analysis) and linear algebra, which will be used in the later chapters. We will mainly concentrate on the definitions and statements of relevant results, with short proofs. Lengthy proofs will be omitted, and they can be found in standard textbooks of analysis and linear algebra. Also, we do not try to be exhaustive. Two types of readers can skip this chapter: Those who have solid knowledge of analysis and linear algebra, and those who are only interested in computational aspects of optimization theory. 1 Basics in R n Let R be the sets of all real numbers. For any n 1, we define { x 1 R n =. } xi R, i = 1,,, n, x n and identify R 1 = R. Elements in R n are called vectors and briefly denoted by x, y, z, etc., i.e., x = x 1. x n, y = y 1. y n, z = z 1. z n. For notational convenience and presentation consistency, we will let all vectors in R n all be column vectors, and denote (x 1,, x n ) = x 1. x n, 5
6 Optimization Theory Lecture Notes page 6 6 Optimization Theory A Concise Introduction call it a row vector, with the superscript being called the transpose. Now, let us introduce some binary relations between two vectors. For any x (x 1,, x n ), y (y 1,, y n ) R n, we write x = y, if x i = y i, i = 1,,, n, x y, if x i y i, i = 1,,, n, x > y, if x i > y i, i = 1,,, n. Note that when n = 1, for any x, y R, one must have either x y, or y x. (1.1) We refer to the relation an order in R. Further, we can refine the above as follows: For any x, y R, one and only one of the following must be true: x = y, x > y, y > x. (1.) However, if n, there are x, y R n for which none of (1.) holds. For example, for x = (1, 0) and y = (0, 1), none of relations in (1.1) holds. Hence, for the case n, we usually refer to the relation as a partial order, since it only gives relations to some pairs of vectors. Next, we define R n + = {x R n x 0}, R n ++ = {x R n x > 0}, which are called nonnegative and strictly positive orthants of R n, respectively. It is clear that x = y, x y and y x. Here, means if and only if. Also, we define x y, if y x, x < y, if y > x. Let us look at some basics on the sets in R n. If A is a set in R n consisting of vectors x R n satisfying certain property, denoted by (P), then we write For example, A = { x R n x satisfies (P) }. A = {x R x 0}. (1.3) If A is a set in R n, and a vector x is an element of A, we denote x A, and say x belongs to A, or x is in A, otherwise, we denote x / A, and say x is not in A. For example, if A is defined by (1.3), then ( ) ( ) 1 1 A, / A. 1 1
7 Optimization Theory Lecture Notes page 7 Mathematical Preparations 7 If A and B are two sets in R n such that any elements in A is in B, we call A a subset of B, denoted by A B. If A B and there is at least one element which is in B but not in A, then A is called a proper subset of B, denoted by A B. Two sets are equal if A B and B A. The set that contains no element is called the empty set, denoted by. Let A λ be a family of sets in R n with λ Λ, where Λ is an index set which could be a finite set or an infinite set. We define { A λ x R } n x A λ, for some λ Λ, λ Λ { A λ x R } n x A λ, for all λ Λ, λ Λ and they are called the union and intersection of A λ (λ Λ), respectively. Also, for any A, B R n, we define A \ B {x A x / B}, which is called A minus B. As a special case, we denote B c = R n \ B, which is called the complementary of B. It is clear that We have the following result. A B B c A c. Proposition 1.1. (de Morgan s Law) Let {A λ, λ Λ} be a family of sets in R n. Then ( ) c ( ) c A λ = A c λ, A λ = A c λ. (1.4) λ Λ λ Λ Proof. Look at the following: ( x λ Λ A λ)c x / λ Λ λ Λ x A c λ, λ Λ x λ Λ A c λ, λ Λ A λ x / A λ, λ Λ proving the first relation in (1.4). Similarly, we can prove the second one, which is left to the readers. We now introduce two operations: Addition: For any x = (x 1,, x n ), y = (y 1,, y n ) R n, define x + y = (x 1 + y 1,, x n + y n ). (1.5)
8 Optimization Theory Lecture Notes page 8 8 Optimization Theory A Concise Introduction Scalar multiplication: For any x = (x 1,, x n ) R n and λ R, define λx = (λx 1,, λx n ). (1.6) The above are called linear operations (or algebraic operations ) on R n. Inspired by the above, we introduce some abstract notions which will give us a better view on more general problems. Definition 1.. A nonempty set X is called a linear space (over R) if on X, two operations are defined: one is called addition: (x, y) x + y X, x, y X, and the other is called scalar multiplication: (λ, x) λx X, λ R, x X. These operations satisfy the following eight axioms: (i) For any x, y X, x + y = y + x; (ii) For any x, y, z X, (x + y) + z = x + (y + z); (iii) There exists a unique element called zero in X, denoted by 0, such that x + 0 = 0 + x = x, for all x X; (iv) For any x X, there exists a unique y X such that x + y = y + x = 0. Such an y is denoted by x; (v) For any x, y X and λ R, λ(x + y) = λx + λy; (vi) For any λ, µ R and x X, (λ + µ)x = λx + µx; (vii) For any x X, 1x = x; (viii) For any λ, µ R and x X, (λµ)x = λ(µx). A linear space is also called a vector space. space is called a vector. Any element in a linear As an important example, we have that R n is a linear space under the addition and scalar multiplication defined by (1.5) and (1.6) 1. If X = {0}, we call it the zero linear space whose structure is trivial. If a linear space X {0}, we call it a nonzero linear space. To study the 1 There are many other linear spaces, besides R n, for examples, the set of all (n m) matrices, the set of all polynomials, the set of all continuous functions, to mention a few.
9 Optimization Theory Lecture Notes page 9 Mathematical Preparations 9 structure of a nonzero linear space X (which could be R n ), we introduce the following important notions. Definition 1.3. Let X be a nonzero linear space over R. (i) A set {ξ 1, ξ,, ξ k } X of finitely many vectors is said to be linearly dependent if there are constants λ 1,, λ k R, at least one of them is nonzero, such that k λ 1 ξ 1 + λ ξ + + λ k ξ k λ i ξ i = 0. (1.7) Otherwise, these vectors are said to be linearly independent. The left hand side of (1.7) is called a linear combination of ξ 1,, ξ k. (ii) A set {ξ 1,, ξ k } X is called a basis of X if they are linearly independent and for any x X, there are constants λ 1,, λ k R such that k x = λ i ξ i. (1.8) i=1 i=1 (iii) A subset X 0 X is called a (linear) subspace of X if λx + µy X 0, x, y X 0, λ, µ R. For any linear space X, there are two trivial subspaces: X and {0}. Other subspaces are called proper subspaces. The following result is fundamentally important. Proposition 1.4. Let X be a nonzero linear space over R. (i) Let ξ 1,, ξ k X. Then the span of {ξ 1,, ξ k } defined by { k } span {ξ 1,, ξ k } = λ i ξ i λ i R, 1 i k, is a linear subspace of X. i=1 (ii) If X 0 {0} is a subspace of X, then there is a set {ξ 1,, ξ k } X 0 of linearly independent vectors such that X 0 = span {ξ 1,, ξ k }. (1.9) In this case, {ξ 1,, ξ k } must be a basis of X 0. Further, if {η 1,, η l } is another basis of X 0, then it must be true that l = k. (iii) If {ξ 1,, ξ k } is a basis of X, then any vector x X admits a unique representation of form (1.8).
10 Optimization Theory Lecture Notes page Optimization Theory A Concise Introduction Because of (ii) in Proposition 1.4, we call k the dimension of X, denoted by dim X = k. This is the maximum possible number of linearly independent vectors in the space X. If we let e i R n be the vector with the ith component being 1 and all other components being zero, then {e 1,, e n } is a basis of R n, which is called the canonical basis of R n. From this, we see that dim R n = n. Note that to find a basis for a nonzero linear space X, one can start from any nonzero vector, call it ξ 1. If any other vector in X is a multiple of ξ 1, we are done and {ξ 1 } is a basis of X. Otherwise, one can find a vector ξ X which is not a multiple of ξ 1. Then ξ 1 and ξ have to be linearly independent. If any vector in X is a linear combination of ξ 1 and ξ, we are done and {ξ 1, ξ } is a basis of X. Otherwise, we can continue the above procedure, and eventually, we will find a basis of X. If the above procedure stops at some finite step, i.e., dim X = k for some k, we say that X is a finitedimensional space. Otherwise, X is called an infinitedimensional space. Note that for a given linear space, the basis is not unique. From the above discussion, we see that thanks to the linear operations of R n and the notion of basis, the structure of any subspace X 0 (including R n itself) of R n becomes very simple: As long as a basis {ξ 1,, ξ k } of X 0 is found, X 0 can be represented by (1.9). We refer to the above described as the linear structure (or algebraic structure) of R n. Next, on R n, we introduce two more notions: Dot product: For any x = (x 1,, x n ), y = (y 1,, y n ) R n, let n x y x 1 y 1 + x y + + x n y n x i y i. (1.10) Euclidean norm: For any x = (x 1,, x n ) R n, define ( n x = x i ) 1. (1.11) More generally, we introduce the following abstract notions. i=1 Definition 1.5. Let X be a linear space over R. (i) A binary operation, denoted by (x, y) x y from X X to R, is called an inner product if the following are satisfied: For example, the set of all continuous functions defined on, say, [0, 1], is an infinitedimensional linear space. i=1
11 Optimization Theory Lecture Notes page 11 Mathematical Preparations 11 (a) positivity: For any x X, x x 0, and x x = 0 if and only if x = 0; (b) symmetry: For any x, y X, x y = y x; (c) linearity: For any x, y, z X, and λ, µ R, (λx + µy) z = λ(x z) + µ(y z). When a linear space X admits an inner product, it is called an inner product space. (ii) A map x x from X to R is called a norm if the following are satisfied: (a) positivity: For any x X, x 0 and x = 0 if and only if x = 0; (b) positive homogeneity: For any λ R and x X, λx = λ x ; (c) triangle inequality: For and x, y X and x + y x + y. When a linear space X admits a norm, it is called a normed linear space. The following proposition reveals some relations between inner product and norm. Proposition 1.6. Let X be an inner product space. (i) (Cauchy Schwarz inequality) For any x, y X, it holds x y (x x)(y y). (1.1) (ii) The map x x defined by the following: x = x x, x X is a norm on X, under which X is a normed linear space. In this case is called the norm induced by the inner product. Note that a norm is not necessarily induced by an inner product. Proof. (i) First of all, if y = 0, (1.1) is trivially true. Now, let y 0. For any λ R, we have 0 (x + λy) (x + λy) = (x x) + (x y)λ + (y y)λ f(λ). In particular, Hence, (1.1) follows. ( 0 f x y ) (x y) = (x x) y y (y y).
12 Optimization Theory Lecture Notes page 1 1 Optimization Theory A Concise Introduction The proof of (ii) is left to the readers. From the above, we see that the Euclidean norm defined by (1.11) is a norm on R n, which is induced by the dot product defined by (1.10), and R n is a inner product space under the dot product and a normed linear space under. In what follows, we call R n a Euclidean space if the linear operations are defined by (1.5) (1.6), and the inner product and the norm are defined by (1.10) (1.11). Having the norm in a linear space X, we can define the distance between two vectors x, y X by x y. We call x y the Euclidean distance between x, y R n, which is a natural generalization of the usual distances on the line, on the plane, and in the 3dimensional space. Clearly, x is nothing but the distance of the vector x to the zero vector 0 = (0,, 0), which can also be called the length of the vector x. Note that when n = 1, x is nothing but the absolute value of x R: { x, x 0, x = x, x < 0. A vector x in a normed linear space X is called a unit vector if x = 1. Clearly, a vector x in a normed linear space is nonzero if and only if x > 0. From the Cauchy Schwarz inequality, we can define the angle θ [0, π] between two nonzero vectors x and y in an inner product space X through the following: cos θ = x y x y. Then two vectors x, y X are said to be orthogonal, denoted by x y, if x y = 0 (which is equivalent to cos θ = 0 with θ being the angle between these two vectors). For any finitedimensional inner product space X, if {ξ 1,, ξ k } is a basis of X such that { 1, i = j, ξ i ξ j = 0, i j, which means that {ξ 1,, ξ k } is a set of mutually orthogonal unit vectors, we call {ξ 1,, ξ k } an orthonormal basis of X. It is clear that {e 1,, e n } is an orthonormal basis of R n under the dot product. Note that for a given inner product space, the orthonormal basis is not unique. The following result provides some facts about the orthogonality. Proposition 1.7. Let X be an inner product space.
13 Optimization Theory Lecture Notes page 13 Mathematical Preparations 13 (i) Suppose G is a nonempty subset of X. Then the annihilator G of G defined by G {y X y x = 0, x G} is a subspace of X, and { } (G ) = span (G) = λ i ξ i ξ i G, λ i R, i 1 G. (1.13) and i 1 (ii) If G 1 G X, then (iii) If G is a subspace of X, then G 1 G. (1.14) (G ) = G, (1.15) X = G + G {x + y x G, y G }, G G = {0}. (1.16) The proof is left to the readers. Note that by definition, one has Thus, we could simply write x y, x G, y G. When the two properties in (1.16) hold, we write G G. (1.17) X = G G, and call it an orthogonal decomposition of X. Hence, in the case G is a subspace of X, we call G the orthogonal complementary of G. To be specific, in the rest of this subsection, we will only consider Euclidean space R n. Now, we introduce the following definition. Proposition 1.8. (i) The open ball B r (x) and the closed ball Br (x) centered at x with radius r are defined by the following: { B r (x) = y R } { n y x < r, Br (x) = y R } n y x r ; (ii) A set G R n is called an open set if for any x G, there exists a δ > 0 such that B δ (x) G. (ii) A set G R n is called a closed set if G c R n \ G is open.
14 Optimization Theory Lecture Notes page Optimization Theory A Concise Introduction (iii) The interior G of a set G R n is the largest open set that is contained in G; the closure Ḡ of a set G Rn is the smallest closed set that contains G; the boundary G of G is defined as follows: G = Ḡ G c. (iv) G is called a bounded set if there exists a constant K such that G B K (0); G is called a compact set if it is bounded and closed 3. (v) G is said to be connected if there are no nonempty subsets G 1, G G such that G = G 1 G, Ḡ 1 G =, G 1 Ḡ =. (vi) G is called a domain if it is open and connected. Note that the dot product leads to the norm, the distance, and the notion of open balls, then leads to the open sets in R n (and hence closed sets, bounded sets, compact sets, etc.). Anything related to open sets, distance, and so on, is referred to as a topological property. Therefore, the dot product determines the topological structure of R n. Now, we consider sequences in R n. The most important notion for sequences is the following: Proposition 1.9. A sequence {x k } k 1 R n is convergent to x R n, denoted by x k x or lim x k = x, if for any ε > 0, there exists a K 1 k (depending on ε > 0) such that x k x < ε, k K. In this case, x is called the limit of the sequence {x k } k 1. The following proposition collects relevant results concerning the sequences in R n, whose proof is left to the readers. Proposition Let {x k } k 1 be a sequence in R n. Then the following statements are equivalent: (i) {x k } k 1 is convergent; (ii) {x k } k 1 is a Cauchy sequence, i.e., for any ε > 0, there exists a K 1 (depending on ε > 0) such that x k x l < ε, k, l K. (iii) Every subsequence of {x k } k 1 is convergent. (iv) For each i = 1,,, n, the sequence {x i k } k 1 is convergent, where x k = (x 1 k,, xn k ). 3 This definition only works for finitedimensional normed linear space.
15 Optimization Theory Lecture Notes page 15 Mathematical Preparations 15 For a sequence {x k } k 1, a subsequence, denoted by {x kj } j 1, is a sequence whose terms are selected from {x k } k 1. For example, one can choose k j = j, then {x kj } = {x j } j 1 = {x, x 4, }. In general, for any strictly increasing map σ : {1,, } {1,, }, {x σ(j) } j 1 gives a subsequence of {x k } k 1. Note that by the above result, if in a sequence {x k } k 1, one can find two subsequences which converge to two different limits, then the original sequence is not convergent. An easy example is x k = ( 1) k, k 1. Also, one sees that if a sequence {x k } k 1 is convergent, the limit must be unique. The following result is useful when one wants to prove a set to be closed. Proposition Let G R n. Then G is closed if and only if for any convergent sequence {x k } G, the limit x G. Now, let us say a little bit more on the case n = 1. Recall that on R, we have an order determined by (or ). This order enables us to define the following notion which will play a fundamental role in the optimization theory. Definition 1.1. Let G R. (i) A real number a inf G R is called the infimum (also called the greatest lower bound) of G if { x a, x G, ε > 0, x G, such that x < a + ε. In the case that inf G G, we call inf G the minimum of G, denote it by min G. (ii) A real number b sup G R is called the supremum (also called the least upper bound) of G if { x b, x G, ε > 0, x G, such that x > b ε. In the case that sup G G, we call sup G the maximum of G, denote it by max G. We have the following basic result concerning the supremum and the infimum. Proposition Let G R be bounded. Then inf G and sup G uniquely exist. Moreover, there are sequences {x k } k 1, {y k } k 1 G such that lim x k = inf G, lim y k = sup G. k k
16 Optimization Theory Lecture Notes page Optimization Theory A Concise Introduction Further, if G is closed (and hence it is compact), then sup G = max G G, inf G = min G G. In the above case, we call {x k } k 1 and {y k } k 1 a minimizing sequence and a maximizing sequence, respectively. The proof of the existence of the supremum and infimum is much more involved, for which we prefer not to get into details. However, the proof of the existence of maximizing and minimizing sequences easily follows the definition of the supremum and infimum, together with the following proposition. Proposition Let {x k } k 1 R be a sequence. (i) If x k is bounded from below and nonincreasing, i.e. for some M > 0, then x k converges to inf k 1 x k. x k M, x k x k+1, k 1, (ii) If x k is bounded from above and nondecreasing, i.e. for some M > 0, then x k converges to sup x k. k 1 x k M, x k x k+1, k 1, The following result characterizes compact sets. Proposition (Bolzano Weierstrass) Let G R n. Then G is compact, i.e., G is bounded and closed, if and only if any sequence {x k } G admits a subsequence which is convergent to some x G. Now, let {x k } k 1 be a bounded sequence in R. For any l 1, the tail sequence {x k } k l of {x k } k 1 is also bounded. Thus, by Proposition 1.13, y l inf k l x k, z l sup x k k l exist. Moreover, it is ready to see that {y l } l 1 is bounded and nondecreasing, and {z l } l 1 is bounded and nonincreasing. Therefore, by Proposition 1.14, the following limits exist: lim x k lim y l lim inf x k, k l l k l lim x k lim z l lim sup x k. k l l k l
17 Optimization Theory Lecture Notes page 17 Mathematical Preparations 17 They are called the limit inferior (liminf, for short) and limit superior (limsup, for short) of sequence {x k } k 1, respectively. We have the following simple and interesting result. Proposition For any bounded sequence {x k } k 1 R, lim x k and k lim x k exist. Moreover, if x is any limit point of {x k } k 1 (i.e., there exists k a subsequence {x kj } j 1 of {x k } k 1 such that x kj x), then Further, lim x k x lim x k. k k lim x k exists lim x k = lim x k. k k k From the above, we see that for any bounded sequence {x k } k 1, if K is the set of all limit points of the sequence, then lim x k = inf K, k lim x k = max K. k Exercises 1. Let A and B be given by the following. Calculate A B, A B, A \ B and B \ A: (i) A = [0, ] and B = [0, 1); (ii) A = {1,, 3}, B = {a, b, c}; (iii) A = {x R x 0}, B = {x R x > 0}; (iv) A = {a + b a, b Q}, B = {a + b 3 a, b Q}, where Q is the set of all rational numbers.. Prove the following: ( 3. Let λ Λ ) c A λ = A c λ. A = {a + b { a + b a, b Q}, B = c + d a, b, c, d Q, c + d 0 }. Prove or disprove A = B. λ Λ
18 Optimization Theory Lecture Notes page Optimization Theory A Concise Introduction 4. Let ξ 1,, ξ k R n. Show that { k } span {ξ 1,, ξ k } α i ξ i αi R, 1 i k is a linear subspace of R n. 5. Let ξ 1 = (0, 1, 1) and ξ = (1, 0, 0). Find span {ξ 1, ξ }. i=1 6. Determine linear independence of each given set of vectors: (i) ξ 1 = (1, 1, 0), ξ = (1, 0, 1), ξ 3 = (0, 1, 1) ; (ii) η 1 = (1, 1, 1), η = (, 0, ), η 3 = (0, 1, 0). 7. Define ( n x p = i=1 x i p) 1 p, 1 p <, x = max 1 i n x i, Show that p, 1 p, are norms on R n. x R n. 8. Let p be defined as above. Show that for any p 1, p [1, ], there is a constant K(p 1, p ) such that x p1 K(p 1, p ) x p, x R n. What can you say about this result? 9. Prove Proposition For G R n defined below, find Ḡ and G : (i) n = 1, G = {1,, 3, 4}; (ii) n =, G = {(x, y) x + y = 1}; (iii) n = 3, G = {(x, y, z) R 3 x + y + z < 1}. 11. Define sequence {x k } k 1 as below. Calculation lim x k and lim x k. k k Identify if the sequence is convergent, given your reason(s): (i) x k = ( 1)k k, k 1; (ii) x k = + ( 1) k, k 1; (iii) x k = sin kπ, k 1.
19 Optimization Theory Lecture Notes page 19 Mathematical Preparations Let G be given by the following. Find supremum, infimum, maximum and minimum. (i) G = {x [0, 1] x is irrational}; (ii) G = { 1 k k N}; (iii) G = {1 + ( 1)k k k N}; (iv) G = {x [0, π] sin x > 1 }. Some Results of Linear Algebra In this section, we briefly present some results from linear algebra. First of all, we recall that an (m n) matrix A is defined to be the following arrays: a 11 a 1 a 1n a 1 a a n A =.... (.1) a m1 a m a mn We also call the above A as a matrix of order (m n). Sometimes, we use notation A = (a ij ) when the order is clear. The set of all (m n) matrices is denoted by R m n. Two matrices A = (a ij ) and B = (b ij ) are equal if and only if they have the same order and a ij = b ij for all i and j within their ranges. When m = n, A is called a square matrix. There are two special matrices: the identity matrix I and the zero matrix 0 which are defined by the following: I =......, 0 = Sometimes, we use I n to represent the (n n) identity matrix, indicating the size of the matrix. Also, zero matrix does not have to be square. On R m n, we define addition and scalar multiplication as follows: If A = (a ij ), B = (b ij ) R m n, and λ R, then A + B = (a ij + b ij ), λa = (λa ij ). (.) One can show that R m n is a linear space under the above defined addition and scalar multiplication (see Definition 1.). Obviously, R m n is quite
20 Optimization Theory Lecture Notes page 0 0 Optimization Theory A Concise Introduction different from R n. But, as far as linear spaces are concerned, they have the same nature. If for any 1 i m, 1 j n, we let E(i, j) R m n be the matrix that the entry at (i, j) position is 1 and all other entries are zero, then {E(i, j) 1 i m, 1 j n} form a basis of R m n. Hence, Actually, we have A (a ij ) = m i=1 j=1 dim R m n = mn. n a ij E(i, j), A (a ij ) R m n. In what follows, E(i, j) might be defined in different R m n and whose order (m n) can be identified from the context. Next, for A = (a ij ) R m n given by (.1), we define its transpose A R n m by the following: a 11 a 1 a m1 A a 1 a a m =.... (.3) a 1n a 1n a mn Thus, A and A have different orders unless m = n. When A R n n, and A = A, it is called a symmetric matrix. The set of all symmetric matrices of order (n n) is denoted by S n. Also, for any square matrix A = (a ij ) R n n, we define n tr (A) = a ii, (.4) which is called the trace of A. Any A = (a ij ) R m n has n columns which are called the column vectors of A (all of them are in R m ). Likewise, A has m rows which are called the row vectors of A (whose transpose are the column vectors of A and belong to R n ). On the other hand, any vector in R n can be regarded as an (n 1) matrix, and any m vectors in R n can form an (n m) matrix. Such an observation is very useful. Next, for A R m n and B R n l given by a 11 a 1 a 1n b 11 b 1 b 1l a 1 a a n A =..., B = b 1 b b l..., a m1 a m a mn b n1 b n b nl i=1
21 Optimization Theory Lecture Notes page 1 Mathematical Preparations 1 we define the product AB = (c ij ) R m l of A and B with the entries c ij given by the following: n c ij = a ik b kj, 1 i m, 1 j l. k=1 In particular, regarding any x, y R n as x, y R n 1, we see that n x y = x k y k = x y. k=1 Hereafter, in R n, we will identify x y with x y. Note that in order the product AB makes sense, the number of columns of A should be equal to the number of rows of B. Thus, when AB is defined, BA is not necessarily define. Further, if both AB and BA are all defined, they may have different orders, and even if AB and BA have the same order, we may still not necessarily have AB = BA. Also, it is interesting that as long as AB is defined, B A will also be defined and (AB) = B A. Next, in the case A R n n, sometimes there exists a B R n n such that AB = BA = I. When this happens, we say that A is invertible (or nonsingular), and in this case, one can show that the matrix B satisfying the above must be unique. Hence, we denote such a B by A 1 which is called the inverse of A. We have the following result concerning the inverse of matrices. Proposition.1. (i) Let A R n n be invertible. Then so is A and (A ) 1 = (A 1 ). (ii) Let A, B R n n be invertible. Then so are AB and BA, and (AB) 1 = B 1 A 1. Next, we note that for any A, B R m n, the product AB R m m and A B R n n are always defined. We further have the following proposition. Proposition.. (i) Let A R m n and B R n m. Then tr [AB] = tr [BA]. (.5) (ii) The map (A, B) tr [A B] is an inner product on R m n. Consequently, ( ) 1 A = tr [A A], A R m n, (.6) defines a norm in R m n.
22 Optimization Theory Lecture Notes page Optimization Theory A Concise Introduction From the above, we see that under (A, B) tr [A B], R m n is an inner product space, and therefore, it is also a normed linear space. Further, one can directly check that {E(i, j) 1 i m, 1 j n} forms an orthonormal basis of R m n, under the abovedefined inner product. Next, for any A R m n, we introduce three elementary row operations: (i) Exchange the ith and the jth rows; (ii) Multiply the ith row by λ R, λ 0; and (iii) Add the µ multiple of the jth row to the ith row, µ R. We now translate these three operations into products of matrices. To this end, we recall the definition of E(i, j) R m m, 1 i, j m. Define three types of elementary matrices as follows: T 1 (i, j) = E(k, k) + E(i, j) + E(j, i), i j, k i,j T (i ; λ) = k i E(k, k) + λe(i, i), λ 0, T 3 (i, j ; µ) = I m + µe(i, j), i j. We see that T 1 (i, j) is the matrix obtain from I m by exchange the ith and the jth columns; T (i ; λ) is the matrix obtained from I m by replacing the ith diagonal 1 by λ; and T 3 (i, j ; µ) is the matrix obtained from I m by replacing the 0 entry at (i, j) position by µ R. Direct computations show that the products T 1 (i, j)a, T (i ; λ)a, and T 3 (i, j ; µ)a are exactly the three elementary row operations applied on A, respectively. Likewise, we may introduce elementary column operations, which can be achieved as follows: First make a transpose to the matrix, followed by an elementary row operation, then make a transpose back. We leave the details to the interested readers. Now, for a given A = (a ij ) R m n, let 1 i 1 < < i k m and 1 j 1 < < j l n. Then B = (b αβ ) R k l is called a submatrix of A where b αβ = a iαj β, 1 α k, 1 β l. When k = l, we call the above B a square submatrix of A. We now introduce an important notion for square matrices, which will be useful
23 Optimization Theory Lecture Notes page 3 Mathematical Preparations 3 below. For any A = (a ij ) R n n we define its determinant det(a) by the following (inductively): a 11, n = 1, det(a) = n (.7) ( 1) i+j a ij det(a ij ), n > 1, j=1 where A ij R (n 1) (n 1) is the submatrix of A obtained by deleting the ith row and the jth column of A. We also define adj (A) = ( ( 1) i+j det(a j i ) ). This is called the adjugate of A. One should note that the entry of adj (A) at (i, j) is ( 1) i+j det(a j i ) (not ( 1) i+j det(a ij )). The following proposition collects basic properties of determinants. Proposition.3. Let A, B R n n. Then (i) det(a ) = det(a); (ii) If two rows or two columns of A are the same, then det(a) = 0; (iii) If B is obtained by exchanging two rows (or two columns) of A, then det(b) = det(a); (iv) If B is obtained by adding a multiple of a row (or a column) to another row (column) of A, then det(b) = det(a). (v) det(ab) = det(a) det(b). (vi) A adj(a) = det(a)i. The following result presents a very interesting usage of determinant. Proposition.4. Let ξ 1,, ξ k R n and let A = (ξ 1,, ξ k ) R n k. Then ξ 1,, ξ k are linearly independent if and only if there exists a (k k) square submatrix B of A such that det(b) 0. In particular, if k = n, then the column vectors of A are linearly independent if and only if det(a) 0. In this case, A must be invertible and A 1 = 1 adj (A). (.8) det(a)
24 Optimization Theory Lecture Notes page 4 4 Optimization Theory A Concise Introduction By some straightforward calculations, we have det ( T 1 (i, j) ) = 1, det ( T (i ; λ) ) = λ, det ( T 3 (i, j ; µ) ) = 1. (.9) Therefore, by Proposition.4, T 1 (i, j), T (i ; λ) and T 3 (i, j ; µ) are all invertible, for any λ 0 and µ R. Actually, one has T 1 (i, j) 1 = T 1 (i, j), T (i ; λ) 1 = T (i ; λ 1 ), which are still elementary matrices. T 3 (i, j ; µ) 1 = T 3 (i, j ; µ), Now, we return to the product of matrices. As an important special case, for any A = (a ij ) R m n and x = (x 1,, x n ) R n, Ax makes sense: a 11 a 1 a 1n x 1 b 1 a 1 a a n x Ax =.... = b. = b Rm, a m1 a m a mn with b j = x n b m n a ik x k, 1 j m. k=1 Thus, matrix A R m n induces a map from R n to R m. Moreover, this map has the following property: A(λx + µy) = λax + µay, x, y R n, λ, µ R. (.10) The above property is referred to as the linearity. Hence, we say that the map induced by A (hereafter, it will be identified with A) is a linear map (or a linear transformation). Further, if we write A = (ξ 1,, ξ n ), ξ i R m, 1 i n, and x = (x 1,, x n ) R n, then Ax = n x i ξ i, i=1 which is a linear combination of ξ 1,, ξ n. We now introduce some more relevant notions. The kernel N (A) of a matrix A R m n is defined by N (A) = {x R } n Ax = 0. (.11)
25 Optimization Theory Lecture Notes page 5 Mathematical Preparations 5 The range R(A) of a matrix A R m n is defined by { R(A) = Ax x R n}. (.1) We have the following result. Proposition.5. (i) Let A R m n. Then N (A) and R(A) are linear subspaces of R n and R m, respectively. Moreover, (ii) Let A R n n. Then R(A) = N (A ), N (A) = R(A ). (.13) A 1 exists N (A) = {0} R(A) = R n det(a) 0 A = E 1 E, E k, with E l (1 l k) being elementary matrices. Let us prove (.13). Note that y R(A) 0 = y (Ax) = (A y) x, x R n A y = 0 y N (A ). This proves the first relation in (.13). Applying the first relation in (.13) to A, we have R(A ) = N (A). Then taking orthogonal complimentary on both sides, we obtain the second relation in (.13). The proofs of the other conclusions are left to the readers. Next, for any A = (a 1,, a n ) R m n with a i R m, we define the rank of A by the following: ( ) rank (A) = dim R(A) = dim span {a 1,, a n }. (.14) Thus, rank (A) is the maximum number of linearly independent vectors in the set {a 1,, a n }. According to Proposition.4, this is also the maximum order of the square submatrix B of A so that det(b) 0. Such an observation leads to a proof of the following proposition, part (i). Proposition.6. (i) For any A R m n, rank (A) = rank (A ). (.15) (ii) Let B R m m and C R n n be invertible. Then for any A R m n, rank (BA) = rank (AC) = rank (A). (.16)
26 Optimization Theory Lecture Notes page 6 6 Optimization Theory A Concise Introduction In particular, for any λ 0, µ R, rank ( AT 1 (i, j) ) = rank ( AT (i ; λ) ) = rank ( AT 1 (i, j ; µ) ) = rank (A). (.17) One can usually use the above to find the rank of a given matrix A by performing elementary row operations. Also, this can be used to find the inverse of A as follows: Assume A is invertible. Then by (ii) of Proposition.5, A = E 1 E E k, with E 1,, E k being elementary matrices. This leads to A 1 = E 1 k E 1 k 1 E 1 E 1 1. Hence, by performing elementary row operations E1 1, E 1,, E 1 k 1, E 1 k consecutively to the augmented matrix (A, I), we have E 1 1 E 1 E 1 k (A, I) = (A 1 A, A 1 ) = (I, A 1 ). Practically, the above method is very effective. The following result is about linear equation of form Ax = b. Proposition.7. Let A R m n. (i) Let b R m. Linear equation Ax = b has at least one solution if and only if b R(A), which is also equivalent to the following: rank (A) = rank (A, b). (.18) (ii) Linear equation Ax = b has at least one solution for every b R m if and only if R(A) = R m. (iii) Linear equation Ax = b has at most one solution for some b R m if and only if N (A) = {0}, which is also equivalent to the fact that the equation Ax = 0 only admits the zero solution. If this happens, then it is necessary that m n. (iv) Linear equation Ax = b admits a unique solution for any b R m if and only if m = n and det(a) 0; in this case, A 1 exists and for any b R n, the unique solution of equation Ax = b admits the following representation: x = A 1 b. From the above, we obtain a very important corollary. Corollary.8. Let A R m n. Linear equation Ax = 0 admits a nonzero solution if and only if either m < n, or m = n with det(a) = 0.
27 Optimization Theory Lecture Notes page 7 Mathematical Preparations 7 We also note that by performing the elementary row operations, one can solve linear equation Ax = b. More precisely, suppose A is invertible. Then with elementary row operations, say, E 1, E,, E k, one will end up with E k E k 1 E 1 A = I. Therefore, applying such a sequence of operations, we will have x = E k E k 1 E 1 Ax = E k E k 1 E 1 b A 1 b, which gives the solution. This procedure is called the Gaussian elimination method. Even when A is not invertible, this method will still work. In general, after a sequence of elementary row operations, we end up with an equation A 0 x = b 0 with A 0 being a simplified form, called a reduced echelon form of A. Since elementary row operations are reversible, the resulting equation A 0 x = b 0 is equivalent to the original one, in the sense that they have the same set of solutions. Next, we introduce the following definition. Definition.9. Let A R n n. (i) The polynomial det(λi A) is called the characteristic polynomial of A. (ii) Any root of the characteristic equation det(λi A) = 0, which could be complex, is called an eigenvalue of A. The set of all eigenvalues of A is denoted by σ(a) and is called the spectrum of A. (iii) If λ C is an eigenvalue of A and ξ, η R n such that ξ + iη 0 and A(ξ + iη) = λ(ξ + iη), then ξ + iη is called an eigenvector of A. Now, for any A S n, we define a map x x Ax which is called a quadratic form. This is a map from R n to R. We introduce the following notions. Definition.10. Let A S n. (i) A is said to be positive semidefinite, denoted by A 0, if x Ax 0, x R n. (ii) A is said to be positive definite, denoted by A > 0, if x Ax > 0, x R n \ {0}. (iii) A is said to be negative semidefinite, denoted by A 0, if A is positive semidefinite; and A is said to be negative definite, denoted by A < 0, if A is positive definite.
28 Optimization Theory Lecture Notes page 8 8 Optimization Theory A Concise Introduction We have the following criterion for positive definiteness of symmetric matrices. Theorem.11. (Sylvester) Let A = (a ij ) S n. Define ( ) a11 a d 1 = a 11, d = det 1,, d n = det(a). a 1 a Then A is positive definite if and only if d 1, d,, d n > 0. (.19) Note that we do not have a similar criterion for positive semidefiniteness of matrices. Namely, if instead of (.19), one has d 1, d,, d n 0, the matrix A is not necessarily positive semidefinite. For example, for ( ) 0 0 A =, 0 1 we have d 1 = d = 0. But the above matrix A is not positive semidefinite. However, we have the following result. Proposition.1. Let A S n. Then σ(a) R. Moreover, A is positive semidefinite if and only if σ(a) [0, ), and A is positive definite if and only if σ(a) (0, ). Sometimes, the following extension of Definition.10 is useful. Definition.13. Let A S n and X R n be a subspace. (i) A is said to be positive semidefinite on X if x Ax 0, x X. (.0) (ii) A is said to be positive definite on X if x Ax > 0, x X \ {0}. (.1) (iii) A is said to be negative semidefinite on X if A is positive semidefinite on X; and A is said to be negative definite on X if A is positive definite.
29 Optimization Theory Lecture Notes page 9 Mathematical Preparations 9 Let us look at two typical cases of the above. (i) X = R(B), for some B R n m. In this case, x X if and only if x = By for some y R m. Thus, x Ax = y (B AB)y. Consequently, A is positive semidefinite (resp. positive definite, negative semidefinite, negative definite) on X if and only if so is B AB. (ii) X = N (B), for some B R m n. In this case, we let N (B) = span {ξ 1,, ξ k } R n and let C = (ξ 1,, ξ k ) R n k. Then X = R(C). Thus, by (i), we see that A is positive semidefinite (resp. positive definite, negative semidefinite, negative definite) on X if and only if so is C AC. Note that N (B) can be found by solving linear equation Bx = 0. Exercises 1. Show that R m n is a linear space under the addition and scalar multiplication defined by (.).. Show that (A, B) tr [A B] is an inner product of R m n, and {E(i, j) 1 i m, 1 j n} is an orthonormal basis of R m n. 3. Calculate the following determinants: (i) 1 1 ; (ii) cos θ sin θ sin θ cos θ ; (iii) ; (iv) sin ϕ cos θ sin ϕ sin θ cos ϕ ρ cos ϕ cos θ ρ cos ϕ sin θ ρ sin ϕ ρ sin ϕ sin θ ρ sin ϕ cos θ Let ξ 1,, ξ k R n and let A = (ξ 1,, ξ k ) R n k. Show that ξ 1,, ξ k are linearly independent if and only if there exists a (k k) square submatrix B of A such that det(b) Let A R m n. Show that N (A) and R(A) are linear subspaces of R n and R m, respectively. 6. Let A be given by the following. Find N (A), N (A ), R(A), and R(A ):
30 Optimization Theory Lecture Notes page Optimization Theory A Concise Introduction 1 ( ) (i) A = ; (ii) A = Compare N (A) with R(A ), and N (A ) with R(A). 7. Find examples showing that AB and BA have the same order, but AB BA. 8. Let A R n n such that AB = BA for all B R n n. Show that A = λi for some λ R. 9. Calculate eigenvalues and corresponding eigenvectors of A: ( ) ( ) 1 1 (i) A = ; (ii) A = Let A S n. Show that every eigenvalue of A must be a real number. 11. Let A S n be positive definite. Define x, y A = x Ay, x, y R n. Show the above is an inner product in R n. ( ) 1 1. Let A =. 1 1 (i) Determine if A is positive definite. { ( ) 1 } (ii) Let X = span. Is A positive definite on X? Give your 1 reasons. 13. For any p [1, ], let A p = sup Ax p, A R m n. x p=1 Show that p is a norm on R m n. (Recall x p defined by Problem 7 of Exercises in Section 1.) 3 Functions: Limits and Continuity In what follows, for notational simplicity, will stand for the Euclidean norm of R n (unless otherwise stated). Any map f : R n R is called a (scalarvalued) function. If n = 1, such a function is called a singlevariable function and if n, such a function is called a multivariable function.
31 Optimization Theory Lecture Notes page 31 Mathematical Preparations 31 We make the following convention: For general case n 1, the vectors in R n are denoted by x = (x 1,, x n ), y = (y 1,, y n ), etc. However, traditionally, for n = 1,, 3, the vectors in R n are denoted by x, (x, y), and (x, y, z), respectively. The following definition collects several relevant notions. Definition 3.1. Let G R n, and f : G R. (i) Let x Ḡ. If for any ε > 0, there exists a δ = δ( x, ε) > 0, such that f(x) L < ε, x x < δ, x G \ { x}, (3.1) then we say that f(x) admits a limit L as x approaches to x, denoted by lim f(x) = L. x x In the case that x G and f( x) = L, we say that f( ) is continuous at x. If f( ) is continuous at every point x G, we say that f( ) is continuous on G. Further, if f( ) is continuous on G with δ( x, ε) independent of x, then f( ) is uniformly continuous on G. (ii) Suppose for x G, there exist L, δ > 0 such that f(x) f(y) L x y, x, y G B δ ( x). Then f( ) is said to be Lipschitz continuous near x. If f( ) is Lipschitz continuous near any x G, we say that f( ) is locally Lipschitz continuous on G. Further, if f( ) is locally Lipschitz continuous on G with constant(s) L and δ uniformly in x G, then f( ) is said to be globally Lipschitz continuous on G. Definition 3.. Let G R n and f : G R. (i) A point x 0 G is called a global minimum (respectively, global maximum) of f( ) over G if ( ) f(x 0 ) f(x), x G. resp. f(x 0 ) f(x), x G. When the above inequalities are strict for any x G \ {x 0 }, we call x 0 a strict global minimum (respectively, strict global maximum) of f( ) over G. (ii) A point x 0 G is called a (strict) local minimum (respectively, (strict) local maximum) of f( ) if there exists a δ > 0 such that x 0 is a (strict) global minimum (respectively, (strict) global maximum) of f( ) over G B δ (x 0 ).
32 Optimization Theory Lecture Notes page 3 3 Optimization Theory A Concise Introduction Global minimum and global maximum are also called absolute minimum and absolute maximum, respectively. Further, both are simply called global or absolute extremes. Likewise, local minimum and local maximum are also called relative minimum and relative maximum, respectively. And, both local minimum and local maximum are called local extremes. It is clear that if x 0 G is a global minimum (respectively, global maximum) of f( ) over G, then f(x 0 ) = min f(g), ( ) resp. f(x 0 ) = max f(g), and if x 0 G is a local minimum (respectively, local maximum), then for some δ > 0, f(x 0 ) = min f ( G B δ (x 0 ) ) (. resp. f(x 0 ) = max f ( G B δ (x 0 ) ) ). The following result is concerned with the existence of extremes. Theorem 3.3. Let G R n and f : G R be continuous. (i) If G is compact, so is f(g) R. Therefore, inf f(g) = min f(g) f(g), sup f(g) = max f(g) f(g). (3.) (ii) If G is connected, so is f(g) R. Proof. (i) We show that f(g) is bounded and closed. Suppose f(g) were not bounded. Then there would be a sequence x k G such that f(x k ). On the other hand, since G is compact, {x k } admits a convergent subsequence. We may assume that x k itself is convergent to some x G. Then by the continuity of f( ), we obtain f( x) = lim f(x k) =, k which is a contradiction. Hence, f(g) is bounded. To show the closedness of f(g), let y k f(g) such that y k ȳ (as k ). We can find x k G such that f(x k ) = y k. By the compactness of G, we may assume that x k x. Then by the continuity of f( ), one has ȳ = f( x) f(g). Hence, f(g) is compact. Then (3.) follows from Proposition (ii) Since f(g) R, to prove its connectedness, it suffices to prove that for any x 1, x G with f(x 1 ) < f(x ), [f(x 1 ), f(x )] f(g). (3.3) If this were not the case, then there would be some number, say, a, such that f(x 1 ) < a < f(x ), a / f(g).
33 Optimization Theory Lecture Notes page 33 Mathematical Preparations 33 Define G 1 = {x G f(x) < a}, G = {x G f(x) > a}. Clearly, G 1 G = G. On the other hand, if there exists a point x G 1 G, then f( x) > a, and one must have a sequence x k G 1 converging to x. It follows from x k G 1 that f(x k ) < a. Hence, by the continuity of f( ), we have a < f( x) = lim k f(x k) a, which is a contradiction. Thus, G 1 G =. Likewise, G 1 G =. Hence, G is disconnected, contradicting to the connectedness of G. Part (i) of the above theorem is called Weierstrass Theorem. Note that (3.) means that when G is compact and f : G R is continuous, then f( ) attains its minimum and maximum over G. Part (ii) of the above is called the (generalized) Intermediate Value Theorem, which says that if a, b f(g), then for every c (a, b), there exists some x 0 G such that f(x 0 ) = c. Let us now look at other types of continuity for functions. It is known that for a given function f( ) and x, the limit lim f(x) does x x not necessarily exist. Therefore, we would like to look at a little weaker notions. For any bounded function f : G R, and x Ḡ, define f ( x ; δ) = inf f ( G B δ ( x) \ { x} ), f + ( x ; δ) = sup f ( G B δ ( x) \ { x} ). (3.4) It is easy to see that δ f ( x ; δ) is nonincreasing and δ f + ( x ; δ) is nondecreasing. Hence, the limits on the righthands of the following exist: lim f(x) = lim f ( x ; δ), x x δ 0 lim f(x) = lim f + ( x ; δ). (3.5) x x δ 0 They are called limit inferior and limit superior (or simply called liminf and limsup), respectively, of f( ) as x approaches to x. Note that for any bounded function f( ), liminf and limsup always exist. Here is a very typical example. Let Then it is straightforward that f(x) = sin 1 x, x 0. lim f(x) = 1, x 0 lim f(x) = 1. x 0
34 Optimization Theory Lecture Notes page Optimization Theory A Concise Introduction We have the following result which is comparable with Proposition Proposition 3.4. Let G R n be a domain and x 0 Ḡ. Let f : G R. Then Further, In this case, lim f(x) lim f(x). (3.6) x x 0 x x 0 lim f(x) exists lim f(x) = lim f(x). (3.7) x x 0 x x 0 x x 0 lim f(x) = lim f(x) = lim f(x). (3.8) x x 0 x x 0 x x 0 Next, we introduce the following notions which are generalizations of the continuity. Definition 3.5. Let f : G R and x G. (i) f( ) is lower semicontinuous at x if f( x) lim f(x). x x (ii) f( ) is upper semicontinuous at x if f( x) lim x x f(x). The following example is worthy of remembering: Let { { ϕ (x) = 1, x = 0, ϕ + (x) = 1, x = 0, 0, x 0, 0, x 0. Then ϕ ( ) is lower semicontinuous and ϕ + ( ) is upper semicontinuous. (3.9) It is clear that f( ) is lower semicontinuous if and only if f( ) is upper semicontinuous. The following result reveals some further interesting relations among lower semicontinuity, upper semicontinuity and continuity of functions. Theorem 3.6. Let G R n and f : G R. (i) f( ) is lower semicontinuous on G if and only if for every a R, is open in G. {f > a} {x G f(x) > a}
35 Optimization Theory Lecture Notes page 35 Mathematical Preparations 35 (ii) f( ) is upper semicontinuous on G if and only if for every a R, is open in G. {f < a} {x G f(x) < a} (iii) f( ) is continuous on G if and only if for every a R, both {f < a} and {f > a} are open in G. (iv) f( ) is continuous on G if and only if for every open set V R, the set f 1 (V ) {x G f(x) V } is open in G, i.e., there exists an open set U R n such that f 1 (V ) = G U. The proof of the above result follows from the definitions of the continuity, the lower and upper semicontinuity. Let us present a case for which the above result plays an interesting role. Proposition 3.7. Let G R n be a compact set and let f λ : G R be a family of functions, with λ Λ, for some index set Λ. (i) Suppose each f λ ( ) is upper semicontinuous on G, so is f + ( ) where f + (x) inf λ Λ f λ(x), x G. (ii) Suppose each f λ ( ) is lower semicontinuous on G, so is f ( ) where f (x) sup f λ (x), x G. λ Λ (iii) In particular, if each f λ ( ) is continuous on G, then f + ( ) is upper semicontinuous and f + ( ) is lower semicontinuous. Note that in the above part (iii), functions f ± ( ) are not necessarily continuous. Let us look at the following examples. Example 3.8. Let f k (x) = (1 x ) k, x [ 1, 1], k 1. Then each f k ( ) is continuous and is nonincreasing in k. Hence, inf k 1 f k(x) = lim k f k(x) = ϕ + (x), x [ 1, 1],
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