Let X be a metric space. (a) Suppose that for some $\epsilon>0$, every $\epsilon$-ball in $X$ has compact closure. Show that $X$ is complete. (b) Suppose that for each $x\in X$ there is an $\epsilon>0$ such that the ball $B(x,\epsilon)$ has compact closure. Show by means of an example that $X$ need not be complete.
Let $X$ be a space. Let $\mathscr D$ be a collection of subsets of $X$ that is maximal with respect to the finite intersection property. (a) Show that $x\in \bar D$ for every $D\in\mathscr D$ iff every open nbhd of $x$ belongs to $\mathscr D$. Which implication uses maximality of $\mathscr D$? (b) Let $D\in\mathscr D$. Show that if $A\supset D$, then $A\in\mathscr D$ (c) Show that if $X$ satisfies the $T_1$ axiom, there is at most one point belonging to $\bigcap_{D\in\mathscr D}\bar D$
I may skip some overly simple problems, such as checking the details of examples in the main text, straightforward verifications of definitions, or direct generalization of theorems from the text.
Some problems in Munkres are incorrect; I will note this where applicable.
I am not interested in topological groups, so I skip all related exercises.
Some challenging problems might remain unfinished for now, and I will gradually add their solutions.
When I refer to a “neighborhood (nbhd)” of a point, I do not necessarily mean an open neighborhood; rather, I mean any set containing some open neighborhood of that point. I will specify the openness if needed.
All solutions are either my own or collected from online sources. I cannot guarantee their correctness. If you spot any mistakes or typo, feel free to comment to correct me, or send me an email at mcwestlifer@gmail.com.
有一句话我的印象很深刻:If you can’t be a poet, be a poem. 我为自己的研究生方向选择了基础数学,而且是一个在基础数学中都算冷门的方向,有时候难免在夜不能寐时反问自己是否把路走太窄了。我一直深知自己在数学方面实在没太多天赋,仅凭年轻的热血就铁着头走到现在,虽还没撞到南墙,却时不时觉着前方有一堵障碍,我始终活在它的阴影之下,不知何时会碰上。
回想这本科这三年的经历,我有一段时间像个中二少年一样,将自己伪装成沉浸在数学中的角色,对周遭的一切漠不关心,冷淡。但逐渐的才意识到自己正值而立之年,却整日在泡影一样的世界中,错过了世界的太多美好,我想到每天摸鱼学几个法语单词的日子里看到的一句话:La vérité ne se trouve d’ailleurs pas dans les livres, mais dans la vie. 真相不在书中,而在生活里。如果错过生活里的细节,那对自己灵魂的理解往往只存在自己大脑编造的幻象里,进行着某种角色扮演。