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Friday, July 29, 2011

暑假。學

最近都喺到學 real analysis,學習方法係:睇課文,然後完成果個 section/chapter 最少一半問題。基本上 Royden 第 4 edition 嘅 2, 3, 4, 5, 6, 7, 17, 18(.1, .2, .3, .4), 19(.1, .2), 20(.1, .2) 課都俾我``做"左。唔急於學新嘅野,只求確定自己已學嘅野唔會學得太表面同有果方面嘅解難能力 (當然會驚唔識做 d 問題...但 no pain, no gain)。本來想開始 chapter 21,但,賣割...!因為佢 study locally compact Hausdorff space,有 d result on normal space 要學返──例如 Urysohn's lamma,偏偏就係嚴民冇教果 d。我其實好想佢唔教第 8 課,即講 surface 果課,而講多 d point-set topology ...。

可能有同學問點解我明明已經讀左 math 204 仲要走去讀返 Lebesgue measure 嘅野呢?其實正正係因為讀 204,我發覺有非常大嘅必要去學返好 Lebesgue measure 知識。嚴民教授嘅 math204 課程範圍大得非常之有問題 (個人角度),要我地短時間由少少 Lebesgue measure (少少,係少少,所有引理/命題/定理全部都只係關注有界集),然後證完 Carath\'{e}odory 就直接跳去 general measure。冇錯我咁樣識左 general measure integration product measure signed measure, Randon-Nykodym,但學得非常之唔實在。單單 Lebesgue measure 仲有好多課題可以講,Borel σ-algebra、dense subspace of Lp(E)、Egoroff, Lusin's theorem (我記得變左做 exercise)、approximation of measurable function by simple functions (結合 Lebesgue dominated convergence theorem,解 integration 題目嘅利器)、approximation of measurable set by Gδ,Fσ (簡單應用:前者可以用返喺 integration;後者可以證 Lipschitz function take measurable set 去 measurable set),等等。好可惜,我喺 204 冇機會學到呢 d 基本野。

而喺我學緊呢 d 基本野嘅同時,有一班 year 1 想搞 workshop,我就即刻諗:「仲唔上馬?」順便 pre 一 present 我解過嘅題 (大部份 presentation problem 嘅來源都係 royden,仲有好多 exercise 我未放落 presentation 到,有 d 係 LCM notes 標住 ``difficult" 嘅問題,有 d 係胡繼善份 notes 嘅題且有 d 答案用左三頁紙,但我有方法只做一頁多少少)。

Workshop 嘅 notes 唔打算 upload 上黎 wordpress 住……,等到改好哂之後,確定冇乜錯漏先再放上黎。

其實呢個假唔單止睇 Royden,有 d American Mathematical Society 出版嘅書都寫得非常之好,我抽左一兩個 topic 黎睇。例如,一年前學左 Randon-Nikodym theorem,前幾日知道佢嘅應用──證明當 1p< 時,對於 σ-finite 嘅 measure space (X,μ),有 (Lp(X,μ))=Lq(X,μ),其中 qp 的 conjugate。我打算睇睇同樣嘅 theorem 其他書有冇更好嘅證法。Inder K. Rana 所寫嘅 An Introduction To Measure and Integration 都有同樣嘅證法,不過當 p>1 時佢做多一小步,將結果即刻推廣到任意 measure space (X,μ),兩頁紙內證完。

A simple problem for entertainment.

Problem. Let f:RR be a continuous function. A point x is called a shadow point if there exists a point yR with y>x such that f(y)>f(x). Let a<b be real numbers and suppose that
  • All the points of the open interval I=(a,b) are shadow points;

  • a and b are not shadow points.
Prove that
  1. f(x)f(b) for all a<x<b;

  2. f(a)=f(b).

Workshop 已暫停兩個星期 (原因係班人要做人口普查),下個星期五開始照常繼續。黎緊兩個星期,一日講返多 d measure,一日開始講 approximation of measurable function,換句話說,Littlewood's 3 principle (我譯為``小木三律" =w=) 其中兩條 。

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