MA3209 Mathematical Analysis III AY 2007/2008 Sem 1 NATIONAL UNIVERSITY OF SINGAPORE MATHEMATICS SOCIETY PAST YEAR PAPER SOLUTIONS with credits to Chan Yu Ming, Poh Wei Shan Charlotte MA3209 Mathematical Analysis III AY 2007/2008 Sem 1 Throughout this document, let A denote the closure of A; Nr (x) be the open neighbourhood of x with radius r. Assume that all the metric spaces stated are non-empty. Question 1 (a) Note that for all x1 , x2 ∈ X, y1 , y2 ∈ Y , since dX and dY are metrics on X and Y respectively, so 1 dX (x1 , x2 ) < ∞ and dY (y1 , y2 ) < ∞. Hence, d((x1 , y1 ), (x2 , y2 )) = (dX (x1 , x2 )p + dY (y1 , y2 )p ) p < ∞. 1 Since dX (x1 , x2 ) ≥ 0 and dY (y1 , y2 ) ≥ 0, so d((x1 , y1 ), (x2 , y2 )) = (dX (x1 , x2 )p + dY (y1 , y2 )p ) p ≥ 0. We have 1 d((x1 , y1 ), (x2 , y2 )) = 0 ⇔ (dX (x1 , x2 )p + dY (y1 , y2 )p ) p = 0 ⇔ dX (x1 , x2 )p + dY (y1 , y2 )p = 0 ⇔ dX (x1 , x2 ) = 0 and dY (y1 , y2 ) = 0 ⇔ x1 = x2 and y1 = y2 ⇔ (x1 , y1 ) = (x2 , y2 ). We also have 1 d((x1 , y1 ), (x2 , y2 )) = (dX (x1 , x2 )p + dY (y1 , y2 )p ) p 1 = (dX (x2 , x1 )p + dY (y2 , y1 )p ) p = d((x2 , y2 ), (x1 , y1 )) It suffices to show that d satisfies the triangle inequality. Take any (x1 , y1 ), (x2 , y2 ), (x3 , y3 ) ∈ X × Y . Since dX is a metric, so dX (x1 , x3 ) ≤ dX (x1 , x2 ) + dX (x2 , x3 ). Hence, dX (x1 , x3 )p ≤ [dX (x1 , x2 ) + dX (x2 , x3 )]p Similarly, dY (y1 , y3 )p ≤ [dY (y1 , y2 ) + dY (y2 , y3 )]p Recall the Mink¨ owski’s inequality: " n #1 " n #1 " n #1 p p p X X X |ai + bi |p ≤ |ai |p + |bi |p i=1 i=1 (1) i=1 Putting n = 2, a1 = dX (x1 , x2 ), a2 = dY (y1 , y2 ), b1 = dX (x2 , x3 ) and b2 = dY (y2 , y3 ) into (1), we obtain: 1 d((x1 , y1 ), (x3 , y3 )) = [dX (x1 , x2 )p + dY (y1 , y3 )p ] p 1 ≤ [(dX (x1 , x2 ) + dX (x2 , x3 ))p + (dY (y1 , y2 ) + dY (y2 , y3 ))p ] p 1 1 ≤ [dX (x1 , x2 )p + dY (y1 , y2 )p ] p + [dX (x2 , x3 )p + dY (y2 , y3 )p ] p by (1) = d((x1 , y1 ), (x2 , y2 )) + d((x2 , y2 ), (x3 , y3 )) NUS Math LaTeXify Proj Team Page: 1 of 7 NUS Mathematics Society MA3209 Mathematical Analysis III AY 2007/2008 Sem 1 ∴ d is a metric on X × Y . (b)(i) Let f ∈ C[0, 1] be a limit point of S. Then given any ε > 0, there exists a function gε ∈ S such that d∞ (f, gε ) < ε. Hence, |f (0)| = |f (0) − gε (0)| since gε (0) = 0 ≤ sup{|f (x) − gε (x)| : x ∈ [0, 1]} = d∞ (f, gε ) < ε. Since ε is arbitrary, so |f (0)| = 0, which implies f (0) = 0. So f ∈ S. ∴ S is closed in (C[0, 1], d∞ ). (b)(ii) For any small positive ε, define the function fε (x) : [0, 1] → R, ( 2x if 0 ≤ x < 2ε fε (x) = ε 1 if 2ε ≤ x ≤ 1 Since fε is continuous on [0, 1] and fε (0) = 0, so fε ∈ S. Consider the function g : [0, 1] → R, g(x) ≡ 1. So g ∈ C[0, 1], but g ∈ / S. Claim: g is a limit point of S. Proof: Given any ε > 0, Z d1 (fε , g) = 1 |fε (x) − g(x)| dx Z ε 2 2x dx − 1 ε 0 Z ε 2 2x 1− dx ε 0 ε x2 2 x− ε 0 ε 4 ε 0 = = = = < In other words, for every ε > 0, there exists fε ∈ S such that d1 (fε , g) < ε. So g is a limit point of S. Since g ∈ / S, so S is not closed in (C[0, 1], d1 ). Question 2 (i) Take any x, y ∈ X. Since f (y) = inf{d(y, a) : a ∈ A}, so given any ε > 0, there exists z ∈ A such that d(y, z) ≤ f (y)+ε. Then f (x) ≤ d(x, z) ≤ d(x, y) + d(y, z) ≤ d(x, y) + f (y) + ε. So f (x) − f (y) ≤ d(x, y) + ε. Since ε is arbitrary, so f (x) − f (y) ≤ d(x, y). Similarly, f (y) − f (x) ≤ d(x, y). ∴ |f (x) − f (y)| ≤ d(x, y) for all x, y ∈ X. Now, given any ε > 0, we let δ = ε. So whenever x, y ∈ X and d(x, y) < δ, we have |f (x) − f (y)| ≤ d(x, y) < δ = ε. So f is uniformly continuous on X. NUS Math LaTeXify Proj Team Page: 2 of 7 NUS Mathematics Society MA3209 Mathematical Analysis III AY 2007/2008 Sem 1 (ii)(a) Since d(K, A) = inf{d(x, a) : x ∈ K, a ∈ A}, so given any ε > 0, there exists k ∈ K and a ∈ A such that d(k, a) ≤ d(K, A) + ε. Thus, inf{f (x) : x ∈ K} ≤ f (k) = d(k, A) ≤ d(k, a) ≤ d(K, A) + ε. Since ε is arbitrary, so inf{f (x) : x ∈ K} ≤ d(K, A). (2) Furthermore, ∃k ∈ K such that f (k) ≤ inf{f (x) : x ∈ K} + ε, so d(K, A) = inf{d(x, a) : a ∈ A, x ∈ K} ≤ inf{d(k, a) : a ∈ A} = d(k, A) = f (k) ≤ inf{f (x) : x ∈ K} + ε Since ε is arbitrary, so d(K, A) ≤ inf{f (x) : x ∈ K} (3) By (2) and (3), we have d(K, A) = inf{f (x) : x ∈ K}. Since f is continuous on the compact set K, so f attains its minimum at some k1 ∈ K, i.e. ∃k1 ∈ K such that f (k1 ) = inf{f (x) : x ∈ K} = d(K, A). Note that d(K, A) ≥ 0. Suppose d(K, A) = 0. Then from above f (k1 ) = inf{d(k1 , a) : a ∈ A} = 0. Therefore, for any ε > 0, there exists aε ∈ A such that d(k1 , aε ) < ε. So k1 is a limit point of A. Since A is closed, so k1 ∈ A. However, K ∩ A = φ from the assumption given in the question, and this contradicts k1 ∈ K ∩ A. ∴ d(K, A) > 0. (ii)(b) From 2(ii)(a), d(K, A) > 0. Let m = d(K, A). Let U := {x ∈ X : d(x, K) < So K ⊆ U and A ⊆ V . m 2 }. Let V := {x ∈ X : d(x, A) < m 2 }. Claim: U ∩ V = φ. Proof: Suppose U ∩ V 6= φ. Take any y ∈ U ∩ V . Since y ∈ U , so d(y, K) < m 2. 0 ∈ K such that d(y, k 0 ) < d(y, K) + [ m − d(y, K)] = Since m − d(y, K) > 0, so there exists k 2 2 Similarly, there exists a0 ∈ A such that d(y, a0 ) < m . 2 m Thus, d(k 0 , a0 ) ≤ d(k 0 , y) + d(y, a0 ) < m 2 + 2 = m = d(K, A) = inf{d(k, a) : k ∈ K, a ∈ A}. This is a contradiction. Therefore, U and V are disjoint. m 2. It remains to show that U and V are open in X. Take any x0 ∈ U . Case 1: x0 ∈ K. For any p in the neighbourhood N m2 (x0 ), we have d(p, K) = inf{d(p, k) : k ∈ K} ≤ d(p, x0 ) m . < 2 So p ∈ U . Therefore, N m2 (x0 ) ⊆ U . Thus, every x0 ∈ K has an open neighbourhood contained in U. Case 2: x0 ∈ / K. NUS Math LaTeXify Proj Team Page: 3 of 7 NUS Mathematics Society MA3209 Mathematical Analysis III Since K is closed, so d(x0 , K) > 0. Let n = d(x0 , K). Consider the neighbourhood N m2 −n (x0 ) := {x ∈ X : d(x0 , x) < Then for all q ∈ N m2 −n (x0 ), d(q, K) = AY 2007/2008 Sem 1 m 2 − n}. inf{d(q, k) : k ∈ K} ≤ inf{d(q, x0 ) + d(x0 , k) : k ∈ K} < inf{( m 2 − n) + d(x0 , k) : k ∈ K} = (m 2 − n) + inf{(d(x0 , k) : k ∈ K} = = (m 2 − n) + d(x0 , K) m 2 −n+n m 2 = Thus, for all q ∈ N m2 −n (x0 ), q ∈ U . Thus, every x0 ∈ U \ K has an open neighbourhood which is contained in U . Combining the two cases, we conclude that every x0 ∈ U is an interior point of U . ∴ U is open in X. Note that in the above proof that U is open in X, we only made use of the assumption that K is closed in X. Since A being compact implies that A is closed in X, so by the same argument as above, we can conclude that V is open in X. Question 3 (1) ⇒ (2): Assume X is connected, and f is locally constant. Fix a point p ∈ X. Define S1 := {x ∈ X : f (x) = f (p)}, S2 := {x ∈ X : f (x) 6= f (p)} Since p ∈ S1 , so S1 is non-empty. Suppose f is not a constant function, then there exists x0 ∈ X such that f (x0 ) 6= f (p). So S2 is non-empty. For any x1 ∈ S1 , there exists an open neighbourhood Ux1 containing x1 such that f (Ux1 ) = {p} since f is locally constant. Note that Ux1 ⊆ S1 . Therefore, S1 is open in X. Similarly S2 is open in X. Note that S1 ∩ S2 = φ and S1 ∪ S2 = X. So X can be written as the union of two non-empty disjoint sets, so X is disconnected. This contradicts our assumption that X is connected. (2) ⇒ (1): Suppose X is not connected, then X is a disjoint union of two non-empty open sets A and B. Define f : X → R, ( 0 if x ∈ A f (x) = 1 if x ∈ B For any x0 ∈ X. x0 ∈ A or x0 ∈ B, but not both. Without loss of generality, assume x0 ∈ A. Since A is open, there exists an open neighbourhood Ux0 of x0 such that Ux0 ⊆ A. So f (Ux0 ) = {0}. Thus, f is locally constant. By assumption, f must be a constant function. However, f is not a constant function from definition. Therefore, X must be connected. Question 4 (i) Since T N is a contraction mapping, so there exists c ∈ (0, 1) such that for all x, y ∈ X, d(T N (x), T N (y)) ≤ c d(x, y) NUS Math LaTeXify Proj Team Page: 4 of 7 (4) NUS Mathematics Society MA3209 Mathematical Analysis III AY 2007/2008 Sem 1 Since X is complete, so by the contraction mapping principle, there exists a unique fixed point x0 ∈ X such that T N (x0 ) = x0 . Using (4), we obtain: d(T N (T (x0 )), T N (x0 )) ≤ c d(T (x0 ), x0 ) d(T (T N (x0 )), T N (x0 )) ≤ c d(T (x0 ), x0 ) d(T (x0 ), x0 ) ≤ c d(T (x0 ), x0 ) This can only happen if d(T (x0 ), x0 ) = 0. Thus, T (x0 ) = x0 . So x0 is a fixed point of T . It remains to show that T has at most one fixed point. Suppose T has at least two different fixed points. Denote two of these by x1 and x2 . Since x1 6= x2 , so d(x1 , x2 ) > 0. We have d(x1 , x2 ) = d(T N (x1 ), T N (x2 )) ≤ c d(x1 , x2 ). Dividing by d(x1 , x2 ), we obtain 1 ≤ c, which is a contradiction. ∴ T has a unique fixed point, namely x0 . (ii)(a) Suppose φ is a contraction mapping on C[0, 1], then there exists c ∈ (0, 1) such that for all f1 , f2 ∈ C[0, 1], d(φ(f1 ), φ(f2 )) ≤ c d(f1 , f2 ) where d stands for the uniform metric. So for all f1 , f2 ∈ C[0, 1], Z x Z x d sin x + f1 (t) dt, sin x + f2 (t) dt ≤ c d(f1 , f2 ) 0 0 Z x Z x sup sin x + f2 (t) dt − sin x + f1 (t) dt ≤ c sup |f1 (x) − f2 (x)| x∈[0,1] 0 0 Z sup x∈[0,1] x x∈[0,1] Z x f2 (t) dt − 0 0 f1 (t) dt ≤ c sup |f1 (x) − f2 (x)| x∈[0,1] In particular, let f1 ≡ 0 and f2 ≡ 1 on [0, 1]. Then Z x 1 dt ≤ c sup 1 sup x∈[0,1] 0 x∈[0,1] sup x ≤ c x∈[0,1] 1 ≤ c This is a contradiction. Therefore, φ cannot be a contraction mapping. (ii)(b) Claim: φ2 is a contraction mapping on C[0, 1]. Proof: For all f ∈ C[0, 1] and for all x ∈ [0, 1], Z x 2 (φ f )(x) = φ sin x + (φf )(t) dt 0 Z x Z t = sin x + sin t + f (u) du dt 0 0 Z x Z xZ t = sin x + sin t dt + f (u) du dt 0 NUS Math LaTeXify Proj Team Page: 5 of 7 0 0 NUS Mathematics Society MA3209 Mathematical Analysis III AY 2007/2008 Sem 1 So for all f1 , f2 ∈ C[0, 1] and for all x ∈ [0, 1], Z x Z t 2 2 (φ f2 )(x) − (φ f1 )(x) = f2 (u) − f1 (u) du dt 0 0 Z x Z t ≤ f2 (u) − f1 (u) du dt Z0 x 0 t sup |f2 (u) − f1 (u)| dt ≤ 0 u∈[0,1] !Z = ≤ t dt 0 u∈[0,1] = x sup |f2 (u) − f1 (u)| x2 d(f1 , f2 ) 2 1 d(f1 , f2 ) 2 Therefore, sup (φ2 f2 )(x) − (φ2 f1 )(x) ≤ x∈[0,1] d(φ2 f1 , φ2 f2 ) ≤ 1 d(f1 , f2 ) 2 1 d(f1 , f2 ) 2 So φ2 is a contraction mapping on C[0, 1]. Since C[0, 1] is complete, so by part (i), φ has a unique fixed point. Question 5 (a)(i) Let (zi ) be a limit point of c. We want to show that (zi ) ∈ c. Let ε > 0 be given. ε Then we can choose a convergent sequence (xi ) such that d((xi ), (zi )) = sup|xi − zi | < . 3 i∈N Since (xi ) is Cauchy, so there exists N0 ∈ N such that for all m, n ≥ N0 , we have |xm − xn | < 3ε . So for all m, n ≥ N0 , |zm − zn | = |(zm − xm ) + (xm − xn ) + (xn − zn )| ≤ |zm − xm | + |xm − xn | + |xn − zn | ε ≤ sup |xi − zi | + + sup |xi − zi | 3 i∈N i∈N = ε Thus, (zi ) is a Cauchy sequence. Since (zi ) is a real sequence, so it is also a convergent sequence, i.e. (zi ) ∈ c. ∴ c is closed in (`∞ , d). (a)(ii) Since a closed subset of a complete metric space is complete, so c being a closed subset of the complete metric space (`∞ , d) is complete. (b) We need to show that ∀ε > 0, ∃δ > 0 such that ∀f ∈ F and ∀x1 , x2 ∈ K, d(x1 , x2 ) < δ ⇒ |f (x1 ) − f (x2 )| < ε. Let ε > 0 be given. Since F is totally bounded, there exists a finite subset S = {f1 , f2 , ..., fn } ⊆ C(K) such that F⊆ n [ Nε (fi ) (5) i=1 NUS Math LaTeXify Proj Team Page: 6 of 7 NUS Mathematics Society MA3209 Mathematical Analysis III AY 2007/2008 Sem 1 where Nε (fi ) = {f ∈ C(K) : kf − fi k < ε}. Since continuous functions on compact sets are uniformly continuous, so f1 , f2 , ..., fn are all uniformly continuous on K. So for all i ∈ {1, 2, ..., n}, ∃δi > 0 such that for all x1 , x2 ∈ K with d(x1 , x2 ) < δi , we have |fi (x1 ) − fi (x2 )| < ε. Let δ = min{δ1 , δ2 , ..., δn }. Given any f ∈ F, by (5), f ∈ Nε (fi ) for some i ∈ {1, 2, ..., n}, i.e. sup |f (x) − fi (x)| < ε. x∈K Then ∀x1 , x2 ∈ K with d(x1 , x2 ) < δ, we have |f (x1 ) − f (x2 )| ≤ |f (x1 ) − fi (x1 )| + |fi (x1 ) − fi (x2 )| + |fi (x2 ) − f (x2 )| < ε+ε+ε = 3ε. ∴ F is equicontinuous on K. NUS Math LaTeXify Proj Team Page: 7 of 7 NUS Mathematics Society
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