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A document containing important theorems and definitions for the course metric spaces for a BSc in Mathematics.
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Metric Spaces – Important Theorems and Definitions Chapter 3: Sets and Functions 𝑫𝒆𝒇𝒊𝒏𝒊𝒕𝒊𝒐𝒏 𝟑.𝟏: 𝑇ℎ𝑒 𝑖𝑚𝑎𝑔𝑒 𝑓 𝐴 𝑜𝑓 𝐴 𝑢𝑛𝑑𝑒𝑟 𝑓 𝑖𝑠 𝑡ℎ𝑒 𝑠𝑢𝑏𝑠𝑒𝑡 𝑜𝑓 𝒴 𝑔𝑖𝑣𝑒𝑛 𝑏𝑦 𝑦 ∈ 𝒴 ∶ 𝑦 = 𝑓 𝑎 𝑓𝑜𝑟 𝑠𝑜𝑚𝑒 𝑎 ∈ 𝐴 . 𝑫𝒆𝒇𝒊𝒏𝒊𝒕𝒊𝒐𝒏 𝟑.𝟐: 𝑇ℎ𝑒 𝑝𝑟𝑒𝑖𝑚𝑎𝑔𝑒 𝑓!! 𝒞 𝑜𝑓 𝒞 𝑢𝑛𝑑𝑒𝑟 𝑓 𝑖𝑠 𝑡ℎ𝑒 𝑠𝑢𝑏𝑠𝑒𝑡 𝑜𝑓 𝒳 𝑔𝑖𝑣𝑒𝑛 𝑏𝑦 𝑥 ∈ 𝒳 ∶ 𝑓 𝑥 ∈ 𝒞 . 𝐴 𝑚𝑎𝑝 𝑓:𝒳 → 𝒴 𝑖𝑠 𝑖𝑛𝑗𝑒𝑐𝑡𝑖𝑣𝑒 𝑖𝑓 𝑓 𝑥 = 𝑓 𝑥! ⟹ 𝑥 = 𝑥!. 𝐴 𝑚𝑎𝑝 𝑓:𝒳 → 𝒴 𝑖𝑠 𝑠𝑢𝑟𝑗𝑒𝑐𝑡𝑖𝑣𝑒 (𝑜𝑟 𝑜𝑛𝑡𝑜) 𝑖𝑓 ∀𝑦 ∈ 𝒴 ∃𝑥 ∈ 𝑋 ∶ 𝑦 = 𝑓 𝑥 . 𝐴 𝑚𝑎𝑝 𝑓:𝒳 → 𝒴 𝑡ℎ𝑎𝑡 𝑖𝑠 𝑖𝑛𝑗𝑒𝑐𝑡𝑖𝑐𝑒 𝑎𝑛𝑑 𝑠𝑢𝑟𝑗𝑒𝑐𝑡𝑖𝑣𝑒 𝑖𝑠 1 − 1 𝑜𝑟 𝑏𝑖𝑗𝑒𝑐𝑡𝑖𝑣𝑒. 𝑇ℎ𝑒 𝑚𝑎𝑝 𝑓:𝒳 → 𝒴 𝑟𝑒𝑠𝑡𝑟𝑖𝑐𝑡𝑒𝑑 𝑡𝑜 𝑎 𝑠𝑢𝑏𝑠𝑒𝑡 𝐴 ∈ 𝒳, 𝑓|!: 𝐴 → 𝒴, 𝑖𝑠 𝑑𝑒𝑓𝑖𝑛𝑒𝑑 𝑠𝑢𝑐ℎ 𝑡ℎ𝑎𝑡 ∀𝑎 ∈ 𝐴 𝑓|! 𝑎 ≔ 𝑓 𝑎 . 𝑷𝒓𝒐𝒑𝒐𝒔𝒊𝒕𝒊𝒐𝒏 𝟑.𝟔: 𝑆𝑢𝑝𝑝𝑜𝑠𝑒 𝑡ℎ𝑎𝑡 𝑓:𝒳 → 𝒴 𝑖𝑠 𝑎 𝑚𝑎𝑝 𝑡ℎ𝑎𝑡 𝐴,𝐵 𝑎𝑟𝑒 𝑠𝑢𝑏𝑠𝑒𝑡𝑠 𝑜𝑓 𝒳 𝑎𝑛𝑑 𝑡ℎ𝑎𝑡 𝐶,𝐷 𝑎𝑟𝑒 𝑠𝑢𝑏𝑠𝑒𝑡𝑠 𝑜𝑓 𝒴. 𝑇ℎ𝑒𝑛:
- 𝑓 𝐴 ∪ 𝐵 = 𝑓 𝐴 ∪ 𝑓 𝐵 , - 𝑓 𝐴 ∩ 𝐵 ⊆ 𝑓 𝐴 ∩ 𝑓 𝐵 , - 𝑓!! 𝐶 ∪ 𝐷 = 𝑓!! 𝐶 ∪ 𝑓!! 𝐷 , 𝑎𝑛𝑑 - 𝑓!! 𝐶 ∩ 𝐷 = 𝑓!! 𝐶 ∩ 𝑓!! 𝐷 .
𝑷𝒓𝒐𝒑𝒐𝒔𝒊𝒕𝒊𝒐𝒏 𝟑.𝟖: 𝑆𝑢𝑝𝑝𝑜𝑠𝑒 𝑡ℎ𝑎𝑡 𝑓:𝒳 → 𝒴 𝑖𝑠 𝑎 𝑚𝑎𝑝 𝑎𝑛𝑑 𝐵 ⊆ 𝒳,𝐷 ⊆ 𝒴. 𝑇ℎ𝑒𝑛
- 𝑓 𝒳 \𝑓 𝐵 ⊆ 𝑓 𝒳\𝐵 , - 𝑓!! 𝒴\𝐷 = 𝒳\𝑓!! 𝐷 .
𝑷𝒓𝒐𝒑𝒐𝒔𝒊𝒕𝒊𝒐𝒏 𝟑.𝟗: 𝑆𝑢𝑝𝑝𝑜𝑠𝑒 𝑡ℎ𝑎𝑡 𝑓:𝒳 → 𝒴 𝑖𝑠 𝑎 𝑚𝑎𝑝 𝑡ℎ𝑎𝑡 𝐴,𝐵 𝑎𝑟𝑒 𝑠𝑢𝑏𝑠𝑒𝑡𝑠 𝑜𝑓 𝒳 𝑎𝑛𝑑 𝑡ℎ𝑎𝑡 𝐶,𝐷 𝑎𝑟𝑒 𝑠𝑢𝑏𝑠𝑒𝑡𝑠 𝑜𝑓 𝒴. 𝑇ℎ𝑒𝑛:
- 𝑓 𝐴 \𝑓 𝐵 ⊆ 𝑓 𝐴\𝐵 , - 𝑓!! 𝐶\𝐷 = 𝑓!!(𝐶)\𝑓!! 𝐷 .
𝑫𝒆𝒇𝒊𝒏𝒊𝒕𝒊𝒐𝒏 𝟑.𝟏𝟕: 𝐴 𝑚𝑎𝑝 𝑓:𝒳 → 𝒴 𝑖𝑠 𝑖𝑛𝑣𝑒𝑟𝑡𝑖𝑏𝑙𝑒 𝑖𝑓 ∃ 𝑎 𝑚𝑎𝑝 𝑔:𝒴 → 𝒳 𝑠. 𝑡 𝑡ℎ𝑒 𝑐𝑜𝑚𝑝𝑜𝑠𝑖𝑠𝑡𝑖𝑜𝑛 𝑔 ∘ 𝑓 𝑖𝑠 𝑡ℎ𝑒 𝑖𝑑𝑒𝑛𝑡𝑖𝑡𝑦 𝑚𝑎𝑝 𝑜𝑓 𝒳 𝑎𝑛𝑑 𝑡ℎ𝑒 𝑐𝑜𝑚𝑝𝑜𝑠𝑖𝑡𝑖𝑜𝑛 𝑓 ∘ 𝑔 𝑖𝑠 𝑡ℎ𝑒 𝑖𝑑𝑒𝑛𝑡𝑖𝑡𝑦 𝑚𝑎𝑝 𝑜𝑓 𝒴. 𝑷𝒓𝒐𝒑𝒐𝒔𝒊𝒕𝒊𝒐𝒏 𝟑.𝟏𝟖:𝐴 𝑚𝑎𝑝 𝑓:𝒳 → 𝒴 𝑖𝑠 𝑖𝑛𝑣𝑒𝑟𝑡𝑖𝑏𝑙𝑒 ⇔ 𝑓 𝑖𝑠 𝑏𝑖𝑗𝑒𝑐𝑖𝑡𝑣𝑒. 𝑷𝒓𝒐𝒑𝒐𝒔𝒊𝒕𝒊𝒐𝒏 𝟑.𝟐𝟎: 𝑆𝑢𝑝𝑝𝑜𝑠𝑒 𝑡ℎ𝑎𝑡 𝑓:𝒳 → 𝒴 𝑖𝑠 𝑎 𝑜𝑛𝑒 − 𝑜𝑛𝑒 𝑐𝑜𝑟𝑟𝑒𝑠𝑝𝑜𝑛𝑑𝑒𝑛𝑐𝑒 𝑜𝑓 𝑠𝑒𝑡𝑠 𝒳𝑎𝑛𝑑 𝒴 𝑎𝑛𝑑 𝑡ℎ𝑎𝑡 𝒱 ⊆ 𝒳.𝑇ℎ𝑒𝑛 𝑡ℎ𝑒 𝑖𝑛𝑣𝑒𝑟𝑠𝑒 𝑖𝑚𝑎𝑔𝑒 𝑜𝑓 𝒱 𝑢𝑛𝑑𝑒𝑟 𝑡ℎ𝑒 𝑖𝑛𝑣𝑒𝑟𝑠𝑒 𝑚𝑎𝑝 𝑓!!:𝒴 → 𝒳 𝑒𝑞𝑢𝑎𝑙𝑠 𝑡ℎ𝑒 𝑖𝑚𝑎𝑔𝑒 𝑠𝑒𝑡 𝑓 𝒱 .
Chapter 4: Real Analysis 𝑫𝒆𝒇𝒊𝒏𝒊𝒕𝒊𝒐𝒏: 𝐴𝑛 𝑢𝑝𝑝𝑒𝑟 𝑏𝑜𝑢𝑛𝑑 𝑓𝑜𝑟 𝑎 𝑠𝑒𝑡 𝒮 ⊆ ℝ ,𝑤ℎ𝑒𝑟𝑒 𝒮 ≠ ∅, 𝑖𝑠 𝑥 ∈ ℝ 𝑠. 𝑡.∀𝑦 ∈ 𝒮, 𝑦 ≤ 𝑥. 𝑫𝒆𝒇𝒊𝒏𝒊𝒕𝒊𝒐𝒏: 𝐴 𝑙𝑜𝑤𝑒𝑟 𝑏𝑜𝑢𝑛𝑑 𝑓𝑜𝑟 𝑎 𝑠𝑒𝑡 𝒮 ⊆ ℝ ,𝑤ℎ𝑒𝑟𝑒 𝒮 ≠ ∅, 𝑖𝑠 𝑥 ∈ ℝ 𝑠. 𝑡.∀𝑦 ∈ 𝒮, 𝑦 ≥ 𝑥. 𝑫𝒆𝒇𝒊𝒏𝒊𝒕𝒊𝒐𝒏 𝟒.𝟐: 𝑇ℎ𝑒 𝑟𝑒𝑎𝑙 𝑛𝑢𝑚𝑏𝑒𝑟 𝑏 𝑖𝑠 𝑎 𝑙𝑒𝑎𝑠𝑡 𝑢𝑝𝑝𝑒𝑟 𝑏𝑜𝑢𝑛𝑑 𝑜𝑓 𝑡ℎ𝑒 𝑠𝑒𝑡 𝒮 ≠ ∅ 𝑖𝑓:
- 𝑏 𝑖𝑠 𝑎𝑛 𝑢𝑝𝑝𝑒𝑟 𝑏𝑜𝑢𝑛𝑑, - 𝑓𝑜𝑟 𝑎𝑛𝑦 𝑜𝑡ℎ𝑒𝑟 𝑢𝑝𝑝𝑒𝑟 𝑏𝑜𝑢𝑛𝑑, 𝑏!, 𝑏 ≤ 𝑏!,
𝑇ℎ𝑒𝑛 𝑏 = 𝑠𝑢𝑝𝒮. 𝑵𝒐𝒕𝒆: 𝑏 = sup 𝒮 𝑖𝑠 𝑢𝑛𝑖𝑞𝑢𝑒. 𝑳𝒆𝒎𝒎𝒂 𝟏: 𝑏 = 𝑠𝑢𝑝𝒮 ⇔ 𝑏 𝑖𝑠 𝑎𝑛 𝑢𝑝𝑝𝑒𝑟 𝑏𝑜𝑢𝑛𝑑 𝑓𝑜𝑟 𝒮 𝑎𝑛𝑑 ∀𝑎 < 𝑏 ∃𝑥 ∈ 𝒮 𝑠. 𝑡. 𝑎 < 𝑥. 𝑫𝒆𝒇𝒊𝒏𝒊𝒕𝒊𝒐𝒏 𝟒.𝟏𝟐: 𝑇ℎ𝑒 𝑠𝑒𝑞𝑢𝑒𝑛𝑐𝑒 𝑠! → 𝑙 ∈ ℝ 𝑖𝑓 ∀𝜖 > 0 ∃𝑁 ∈ ℕ 𝑠. 𝑡. 𝑠! − 𝑙 < 𝜖 ∀𝑛 ≥ 𝑁. 𝑇ℎ𝑒𝑛 𝑙 = lim
!→!𝑠!.
𝑷𝒓𝒐𝒑𝒐𝒔𝒊𝒕𝒊𝒐𝒏 𝟒.𝟏𝟑: 𝐿𝑒𝑡 lim
!→!𝑠! = 𝑙, 𝑡ℎ𝑒𝑛 𝑙 𝑖𝑠 𝑢𝑛𝑖𝑞𝑢𝑒.
𝑫𝒆𝒇𝒊𝒏𝒊𝒕𝒊𝒐𝒏 𝟒.𝟏𝟓: 𝐴 𝑠𝑒𝑞𝑢𝑒𝑛𝑐𝑒 𝑠! 𝑖𝑠 𝑚𝑜𝑛𝑜𝑡𝑖𝑐
- 𝑖𝑛𝑐𝑟𝑒𝑎𝑠𝑖𝑛𝑔 𝑛𝑜𝑛𝑑𝑒𝑐𝑟𝑒𝑎𝑠𝑖𝑛𝑔 𝑖𝑓 𝑠! ≤ 𝑠!!! - 𝑑𝑒𝑐𝑟𝑒𝑎𝑠𝑖𝑛𝑔 𝑛𝑜𝑛𝑖𝑛𝑐𝑟𝑒𝑎𝑠𝑖𝑛𝑔 𝑖𝑓 𝑠! ≥ 𝑠!!! .
𝑻𝒉𝒆𝒐𝒓𝒆𝒎 𝟒.𝟏𝟔: 𝐴𝑛𝑦 𝑏𝑜𝑢𝑛𝑑𝑒𝑑 𝑚𝑜𝑛𝑜𝑡𝑜𝑛𝑖𝑐 𝑠𝑒𝑞𝑢𝑛𝑒𝑛𝑐𝑒 𝑖𝑠 𝑐𝑜𝑛𝑣𝑒𝑟𝑔𝑒𝑛𝑡. 𝑻𝒉𝒆𝒐𝒓𝒆𝒎 𝟒.𝟏𝟗: 𝐵𝑜𝑙𝑧𝑎𝑛𝑜 𝑊𝑒𝑖𝑒𝑟𝑠𝑡𝑟𝑎𝑠𝑠 𝐹𝑜𝑟 𝑎𝑛𝑦 𝑏𝑜𝑢𝑛𝑑𝑒𝑑 𝑠𝑒𝑞𝑢𝑛𝑐𝑒 𝑠! ⊆ ℝ, 𝑡ℎ𝑒𝑟𝑒 𝑒𝑥𝑖𝑠𝑡𝑠 𝑎𝑡 𝑙𝑒𝑎𝑠𝑡 𝑜𝑛𝑒 𝑐𝑜𝑛𝑣𝑒𝑟𝑔𝑒𝑛𝑡 𝑠𝑢𝑏𝑠𝑒𝑞𝑢𝑒𝑛𝑐𝑒 ((𝑠!!), 𝑘 ∈ ℕ). Lots more theorems and definition covered in the book that are not in the lecture notes.
Chapter 5: Metric Spaces and Open sets and Open balls. 𝑫𝒆𝒇𝒊𝒏𝒊𝒕𝒊𝒐𝒏: 𝑓:ℝ → ℝ 𝑖𝑠 𝑐𝑜𝑛𝑡𝑖𝑛𝑢𝑜𝑢𝑠 𝑎𝑡 𝑎 ∈ ℝ 𝑖𝑓 ∀𝜖 > 0 ∃𝛿 𝜖 > 0 𝑠. 𝑡.∀𝑥 𝑖𝑓 𝑥 − 𝑎 < 𝛿 𝑡ℎ𝑒𝑛 𝑓 𝑥 − 𝑓 𝑎 < 𝜖. 𝑫𝒆𝒇𝒊𝒏𝒊𝒕𝒊𝒐𝒏:𝐴 𝑚𝑒𝑡𝑟𝑖𝑐 𝑓𝑢𝑛𝑐𝑡𝑖𝑜𝑛 𝑑𝒳:𝒳×𝒳 → ℝ 𝑚𝑢𝑠𝑡 𝑠𝑎𝑡𝑖𝑠𝑓𝑦 𝑡ℎ𝑒 𝑓𝑜𝑙𝑙𝑜𝑤𝑖𝑛𝑔 𝑎𝑥𝑖𝑜𝑚𝑠:
𝑨𝒙 𝟏 :∀𝑥, 𝑦 ∈ 𝒳, 𝑑𝒳 𝑥, 𝑦 ≥ 0 𝑤𝑖𝑡ℎ 𝑑𝒳 𝑥, 𝑦 = 0⇔ 𝑥 = 𝑦. 𝑨𝒙 𝟐 :∀𝑥, 𝑦 ∈ 𝒳, 𝑑𝒳 𝑥, 𝑦 = 𝑑𝒳 𝑦, 𝑥 . 𝑨𝒙 𝟑 :∀𝑥, 𝑦, 𝑥 ∈ 𝒳, 𝑑𝒳 𝑥, 𝑧 ≤ 𝑑𝒳 𝑥, 𝑦 + 𝑑𝒳 𝑦, 𝑧 .
𝑫𝒆𝒇𝒊𝒏𝒊𝒕𝒊𝒐𝒏 𝟓.𝟐:𝐴 𝑚𝑒𝑡𝑟𝑖𝑐 𝑠𝑝𝑎𝑐𝑒 𝑐𝑜𝑛𝑠𝑖𝑠𝑡𝑠 𝑜𝑓 𝑎 𝑛𝑜𝑛 − 𝑒𝑚𝑝𝑡𝑦 𝑠𝑒𝑡 𝒳 𝑡𝑜𝑔𝑒𝑡ℎ𝑒𝑟 𝑤𝑖𝑡ℎ 𝑎 𝑓𝑢𝑛𝑐𝑡𝑖𝑜𝑛 𝑑:𝒳×𝒳 → ℝ 𝑠𝑢𝑐ℎ 𝑡ℎ𝑎𝑡 𝐴𝑥 1 ,𝐴𝑥 2 𝑎𝑛𝑑 𝐴𝑥 3 ℎ𝑜𝑙𝑑. 𝑫𝒆𝒇𝒊𝒏𝒊𝒕𝒊𝒐𝒏 𝟓.𝟑: 𝑆𝑢𝑝𝑝𝑜𝑠𝑒 𝒳,𝑑𝒳 𝑎𝑛𝑑 𝒴,𝑑𝒴 𝑎𝑟𝑒 𝑚𝑒𝑡𝑟𝑖𝑐 𝑠𝑝𝑎𝑐𝑒𝑠 𝑎𝑛𝑑 𝑙𝑒𝑡 𝑓:𝒳 → 𝒴 𝑏𝑒 𝑎 𝑚𝑎𝑝. 𝑇ℎ𝑒𝑛 𝑓 𝑖𝑠 𝑐𝑜𝑛𝑡𝑖𝑛𝑢𝑜𝑢𝑠 𝑎𝑡 𝑥! ∈ 𝒳 𝑖𝑓 ∀𝜖 > 0 ∃𝛿 > 0 𝑠. 𝑡. 𝑑𝒳 𝑥, 𝑥! < 𝛿 ⟹ 𝑑𝒴 𝑓 𝑥 , 𝑓 𝑥! < 𝜖. 𝑫𝒆𝒇𝒊𝒏𝒊𝒕𝒊𝒐𝒏:𝑇ℎ𝑒 𝑚𝑎𝑝 𝑓 𝑖𝑠 𝑐𝑜𝑛𝑡𝑖𝑛𝑢𝑜𝑢𝑠 𝑖𝑓 𝑓 𝑖𝑠 𝑐𝑜𝑛𝑡𝑖𝑛𝑢𝑜𝑢𝑠 𝑎𝑡 𝑒𝑣𝑒𝑟𝑦 𝑥! ∈ 𝒳. 𝑷𝒓𝒐𝒑𝒐𝒔𝒊𝒕𝒊𝒐𝒏 𝟓.𝟏𝟕: 𝑆𝑢𝑝𝑝𝑜𝑠𝑒 𝑡ℎ𝑎𝑡 𝑓,𝑔:𝒳 → ℝ 𝑎𝑟𝑒 𝑐𝑜𝑛𝑡𝑖𝑛𝑢𝑜𝑢𝑠 𝑟𝑒𝑎𝑙 𝑣𝑎𝑙𝑢𝑒𝑑 𝑓𝑢𝑛𝑐𝑡𝑖𝑜𝑛𝑠 𝑜𝑛 𝑎 𝑚𝑒𝑡𝑟𝑖𝑐 𝑠𝑝𝑎𝑐𝑒 𝒳,𝑑 . 𝑇ℎ𝑒𝑛 𝑠𝑜 𝑎𝑟𝑒: (i) 𝑓 (ii) 𝑓 + 𝑔 (iii) 𝑓 ⋅ 𝑔 (iv) !
! ; 𝑝𝑟𝑜𝑣𝑖𝑑𝑒𝑑 𝑔 𝑖𝑠 𝑛𝑒𝑣𝑒𝑟 𝑧𝑒𝑟𝑜 𝑜𝑛 𝒳.
𝑫𝒆𝒇𝒊𝒏𝒊𝒕𝒊𝒐𝒏 𝟓.𝟐𝟎:𝑇ℎ𝑒 𝑝𝑟𝑜𝑗𝑒𝑐𝑡𝑖𝑜𝑛𝑠 𝑝𝒳:𝒳×𝒳 → 𝒳, 𝑎𝑛𝑑 𝑝𝒴:𝒴×𝒴 → 𝒴 𝑑𝑒𝑓𝑖𝑛𝑒𝑑 𝑏𝑦 𝑝𝒳 𝑥, 𝑦 = 𝑥 𝑎𝑛𝑑 𝑝𝒴 𝑥, 𝑦 = 𝑦 𝑎𝑟𝑒 𝑐𝑜𝑛𝑡𝑖𝑛𝑢𝑜𝑢𝑠. 𝑫𝒆𝒇𝒊𝒏𝒊𝒕𝒊𝒐𝒏 𝟓.𝟐𝟏:𝑇ℎ𝑒 𝑑𝑖𝑎𝑔𝑜𝑛𝑎𝑙 𝑚𝑎𝑝 𝛥:𝒳 → 𝒳×𝒳 𝑜𝑓 𝑎𝑛𝑦 𝒳 𝑖𝑠 𝑡ℎ𝑒 𝑚𝑎𝑝 𝑑𝑒𝑓𝑖𝑛𝑒𝑑 𝑏𝑦 𝛥 𝑥 = 𝑥, 𝑥 . 𝑫𝒆𝒇𝒊𝒏𝒊𝒕𝒊𝒐𝒏 𝟓.𝟐𝟑:𝐴 𝑠𝑢𝑏𝑠𝑒𝑡 𝒮 𝑜𝑓 𝑎 𝑚𝑒𝑡𝑟𝑖𝑐 𝑠𝑝𝑎𝑐𝑒 𝒳,𝑑 𝑖𝑠 𝑏𝑜𝑢𝑛𝑑𝑒𝑑 𝑖𝑓 ∃𝑥! ∈ 𝒳 𝑎𝑛𝑑 𝑅 ∈ ℝ 𝑠. 𝑡. ∀𝑥 ∈ 𝒮, 𝑑𝒳 𝑥!, 𝑥 < 𝑅. 𝑵𝒐𝒕𝒆:∅ 𝑖𝑠 𝑏𝑜𝑢𝑛𝑑𝑒𝑑 𝑏𝑦 𝑑𝑒𝑓𝑖𝑛𝑖𝑡𝑜𝑛. 𝑫𝒆𝒇𝒊𝒏𝒊𝒕𝒊𝒐𝒏 𝟓.𝟐𝟒: 𝐼𝑓 𝒮 𝑖𝑠 𝑎 𝑛𝑜𝑛𝑒𝑚𝑝𝑡𝑦 𝑏𝑜𝑢𝑛𝑑𝑒𝑑 𝑠𝑢𝑏𝑠𝑒𝑡 𝑜𝑓 𝑎 𝑚𝑒𝑡𝑟𝑖𝑐 𝑠𝑝𝑎𝑐𝑒 𝒳,𝑑𝒳 , 𝑡ℎ𝑒𝑛 𝑡ℎ𝑒 𝑑𝑖𝑎𝑚𝑒𝑡𝑒𝑟 𝑜𝑓 𝒮 𝑖𝑠: 𝑑𝑖𝑎𝑚𝒮 = sup 𝑑𝒳 𝑥, 𝑦 . 𝑵𝒐𝒕𝒆:𝑑𝑖𝑎𝑚∅ = 0 (𝑏𝑢𝑡 𝑑𝑖𝑎𝑚𝐴 = 0 ⇏ 𝐴 = ∅. 𝑫𝒆𝒇𝒊𝒏𝒊𝒕𝒊𝒐𝒏 𝟓.𝟐𝟓: 𝐼𝑓 𝑓: 𝒮 → 𝒳 𝑖𝑠 𝑎 𝑚𝑎𝑝 𝑓𝑟𝑜𝑚 𝑎 𝑠𝑒𝑡 𝒮 𝑡𝑜 𝑎 𝑚𝑒𝑡𝑟𝑖𝑐 𝑠𝑝𝑎𝑐𝑒 𝒳, 𝑡ℎ𝑒𝑛 𝑓 𝑖𝑠 𝑏𝑜𝑢𝑛𝑑𝑒𝑑 𝑖𝑓 𝑡ℎ𝑒 𝑠𝑢𝑏𝑠𝑒𝑡 𝑓 𝒮 𝑜𝑓 𝒳 𝑖𝑠 𝑏𝑜𝑢𝑛𝑑𝑒𝑑.
𝑷𝒓𝒐𝒑𝒐𝒔𝒊𝒕𝒊𝒐𝒏 𝟓.𝟐𝟔: 𝐼𝑓 𝒮! 𝑖𝑠 𝑏𝑜𝑢𝑛𝑑𝑒𝑑 ∀𝑖 = 1,2,… ,𝑁, 𝑡ℎ𝑒𝑛 𝒮 = 𝒮!
!!!
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𝑖𝑠 𝑏𝑜𝑢𝑛𝑑𝑒𝑑.
𝑵𝒐𝒕𝒆:𝑇ℎ𝑖𝑠 𝑑𝑜𝑒𝑠 𝑛𝑜𝑡 ℎ𝑜𝑙𝑑 𝑓𝑜𝑟 𝑖𝑛𝑓𝑖𝑛𝑖𝑡𝑒𝑙𝑦 𝑚𝑎𝑛𝑦 𝑠𝑒𝑡𝑠 𝒮.
𝑫𝒆𝒇𝒊𝒏𝒊𝒕𝒊𝒐𝒏 𝟓.𝟐𝟕:𝐹𝑜𝑟 𝑎 𝑚𝑒𝑡𝑟𝑖𝑐 𝑠𝑝𝑎𝑐𝑒 𝒳,𝑑𝒳 , 𝑥! ∈ 𝒳 𝑎𝑛𝑑 𝑟 ∈ ℝ 𝑤𝑖𝑡ℎ 𝑟 > 0, 𝑎𝑛 𝑜𝑝𝑒𝑛 𝑏𝑎𝑙𝑙, 𝐵, 𝑐𝑒𝑛𝑡𝑟𝑒𝑑 𝑎𝑡 𝑥! 𝑤𝑖𝑡ℎ 𝑟𝑎𝑑𝑖𝑢𝑠 𝑟 𝑖𝑠 𝑡ℎ𝑒 𝑠𝑒𝑡
𝐵! 𝑥! = 𝑥 ∈ 𝒳 ∶ 𝑑𝒳 𝑥, 𝑥! < 𝑟 . 𝑷𝒓𝒐𝒑𝒐𝒔𝒊𝒕𝒊𝒐𝒏 𝟓.𝟑𝟎: 𝑆𝑢𝑝𝑝𝑜𝑠𝑒 𝒳,𝑑𝒳 𝑎𝑛𝑑 𝒴,𝑑𝒴 𝑎𝑟𝑒 𝑚𝑒𝑡𝑟𝑖𝑐 𝑠𝑝𝑎𝑐𝑒𝑠 𝑎𝑛𝑑 𝑙𝑒𝑡 𝑓:𝒳 → 𝒴 𝑏𝑒 𝑎 𝑚𝑎𝑝. 𝑇ℎ𝑒𝑛 𝑓 𝑖𝑠 𝑐𝑜𝑛𝑡𝑖𝑛𝑢𝑜𝑢𝑠 𝑎𝑡 𝑥! ∈ 𝒳 ⟺ ∀𝜖 > 0 ∃𝛿 > 0 𝑠. 𝑡. 𝑓(𝐵!
!𝒳 𝑥!) ⊆ 𝐵!!𝒴 𝑓(𝑥!) .
𝑷𝒓𝒐𝒑𝒐𝒔𝒊𝒕𝒊𝒐𝒏 𝟓.𝟑𝟏:𝐺𝑖𝑣𝑒𝑛 𝑎𝑛 𝑜𝑝𝑒𝑛 𝑏𝑎𝑙𝑙 𝐵! 𝑥 𝑖𝑛 𝑎 𝑚𝑒𝑡𝑟𝑖𝑐 𝑠𝑝𝑎𝑐𝑒 𝒳,𝑑𝒳 𝑎𝑛𝑑 𝑎 𝑝𝑜𝑖𝑛𝑡 𝑦 ∈ 𝐵! 𝑥 , ∃𝜖 > 0 𝑠. 𝑡.𝐵! 𝑦 ⊆ 𝐵! 𝑥 . 𝑫𝒆𝒇𝒊𝒏𝒊𝒕𝒊𝒐𝒏 𝟓.𝟑𝟐: 𝐿𝑒𝑡 𝒳,𝑑𝒳 𝑏𝑒 𝑎 𝑚𝑒𝑡𝑟𝑖𝑐 𝑠𝑝𝑎𝑐𝑒 𝑎𝑛𝑑 𝒰 ⊆ 𝒳. 𝒰 𝑖𝑠 𝑜𝑝𝑒𝑛 𝑖𝑛 𝒳 𝑖𝑓 ∀𝑥 ∈ 𝒰 ∃𝜖! > 0 𝑠. 𝑡.𝐵!! 𝑥 ⊆ 𝒰. 𝑵𝒐𝒕𝒆: 𝐼𝑛 𝑜𝑡ℎ𝑒𝑟 𝑤𝑜𝑟𝑑𝑠, 𝑒𝑣𝑒𝑟𝑦 𝑝𝑜𝑖𝑛𝑡 𝑜𝑓 𝒰 𝑐𝑎𝑛 𝑏𝑒 𝑒𝑛𝑐𝑖𝑟𝑐𝑙𝑒𝑑 𝑏𝑦 𝑎𝑛 𝑜𝑝𝑒𝑛 𝑠𝑝ℎ𝑒𝑟𝑖𝑐𝑎𝑙 𝑛𝑒𝑖𝑔ℎ𝑏𝑜𝑢𝑟ℎ𝑜𝑜𝑑. 𝑷𝒓𝒐𝒑𝒐𝒔𝒊𝒕𝒊𝒐𝒏 𝟓.𝟑𝟕: 𝑆𝑢𝑝𝑝𝑜𝑠𝑒 𝑡ℎ𝑎𝑡 𝑓:𝒳 → 𝒴 𝑖𝑠 𝑎 𝑚𝑎𝑝 𝑏𝑒𝑡𝑤𝑒𝑒𝑛 𝑡𝑤𝑜 𝑚𝑒𝑡𝑟𝑖𝑐 𝑠𝑝𝑎𝑐𝑒𝑠,𝒳 𝑎𝑛𝑑 𝒴, 𝑎𝑛𝑑 𝒰 ⊆ 𝒴. 𝑇ℎ𝑒𝑛 𝑓 𝑖𝑠 𝑐𝑜𝑛𝑡𝑖𝑛𝑢𝑜𝑢𝑠 ⟺ 𝑓!! 𝒰 𝑖𝑠 𝑜𝑝𝑒𝑛 𝑖𝑛 𝒳 𝑤ℎ𝑒𝑛𝑒𝑣𝑒𝑟 𝒰 𝑖𝑠 𝑜𝑝𝑒𝑛 𝑖𝑛 𝒴. 𝑵𝒐𝒕𝒆:𝑃𝑟𝑒𝑖𝑚𝑎𝑔𝑒 𝑜𝑓 𝑜𝑝𝑒𝑛 𝑖𝑠 𝑜𝑝𝑒𝑛. 𝑵𝒐𝒕𝒆: 𝑓 𝑚𝑎𝑦 𝑏𝑒 𝑐𝑜𝑛𝑡𝑖𝑛𝑢𝑜𝑢𝑠 𝑎𝑛𝑑 𝑉 𝑖𝑠 𝑜𝑝𝑒𝑛 𝑏𝑢𝑡 𝑓 𝑉 𝑖𝑠 𝑛𝑜𝑡 𝑛𝑒𝑐𝑒𝑠𝑠𝑎𝑟𝑖𝑙𝑦 𝑜𝑝𝑒𝑛.
𝑷𝒓𝒐𝒑𝒐𝒔𝒊𝒕𝒊𝒐𝒏 𝟓.𝟑𝟗: 𝐼𝑓 𝒰!,𝒰!,… ,𝒰! 𝑎𝑟𝑒 𝑜𝑝𝑒𝑛 𝑖𝑛 𝑎 𝑚𝑒𝑡𝑟𝑖𝑐 𝑠𝑝𝑎𝑐𝑒 𝒳 𝑡ℎ𝑒𝑛 𝑠𝑜 𝑖𝑠 𝒰!
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.
𝑷𝒓𝒐𝒑𝒐𝒔𝒊𝒕𝒊𝒐𝒏 𝟓.𝟒𝟏: 𝑇ℎ𝑒 𝑢𝑛𝑖𝑜𝑛 𝑜𝑓 𝑎𝑛𝑦 𝑐𝑜𝑙𝑙𝑒𝑐𝑡𝑖𝑜𝑛 𝑜𝑓 𝑜𝑝𝑒𝑛 𝑠𝑒𝑡𝑠 𝑖𝑛 𝑎 𝑚𝑒𝑡𝑟𝑖𝑐 𝑠𝑝𝑎𝑐𝑒 𝒳 𝑖𝑠 𝑜𝑝𝑒𝑛 𝑖𝑛 𝒳.
Chapter 6: Metric Spaces and Closed Sets, Closure and Limit Points 𝑫𝒆𝒇𝒊𝒏𝒊𝒕𝒊𝒐𝒏 𝟔.𝟏: 𝐴 𝑠𝑢𝑏𝑠𝑒𝑡 𝒱 𝑜𝑓 𝑎 𝑚𝑒𝑡𝑟𝑖𝑐 𝑠𝑝𝑎𝑐𝑒 𝒳 𝑖𝑠 𝑐𝑙𝑜𝑠𝑒𝑑 𝑖𝑛 𝒳 𝑖𝑓 𝒳\𝒱 𝑖𝑠 𝑜𝑝𝑒𝑛 𝑖𝑛 𝒳. 𝑷𝒓𝒐𝒑𝒐𝒔𝒊𝒕𝒊𝒐𝒏 𝟔.𝟒: 𝐴𝑛𝑦 𝑖𝑛𝑡𝑒𝑟𝑠𝑒𝑐𝑡𝑖𝑜𝑛 𝑜𝑓 𝑐𝑙𝑜𝑠𝑒𝑑 𝑠𝑒𝑡𝑠 𝑖𝑛 𝑎 𝑚𝑒𝑡𝑟𝑖𝑐 𝑠𝑝𝑎𝑐𝑒 𝒳 𝑖𝑠 𝑐𝑙𝑜𝑠𝑒𝑑 𝑖𝑛 𝒳. 𝑷𝒓𝒐𝒑𝒐𝒔𝒊𝒕𝒊𝒐𝒏 𝟔.𝟓: 𝐹𝑜𝑟 𝑎𝑛𝑦 𝑚𝑒𝑡𝑟𝑖𝑐 𝑠𝑝𝑎𝑐𝑒 𝒳, 𝑡ℎ𝑒 𝑒𝑚𝑝𝑡𝑦 𝑠𝑒𝑡 ∅ 𝑎𝑛𝑑 𝑡ℎ𝑒 𝑤ℎ𝑜𝑙𝑒 𝑠𝑒𝑡 𝒳 𝑎𝑟𝑒 𝑏𝑜𝑡ℎ 𝑜𝑝𝑒𝑛 𝑎𝑛𝑑 𝑐𝑙𝑜𝑠𝑒𝑑 𝑖𝑛 𝒳. 𝑷𝒓𝒐𝒑𝒐𝒔𝒊𝒕𝒊𝒐𝒏 𝟔.𝟔: 𝐿𝑒𝑡 𝒳,𝒴 𝑏𝑒 𝑚𝑒𝑡𝑟𝑖𝑐 𝑠𝑝𝑎𝑐𝑒𝑠 𝑎𝑛𝑑 𝑙𝑒𝑡 𝑓:𝒳 → 𝒴 𝑏𝑒 𝑎 𝑚𝑎𝑝. 𝑇ℎ𝑒𝑛 𝑓 𝑖𝑠 𝑐𝑜𝑛𝑡𝑖𝑛𝑢𝑜𝑢𝑠 ⇔ 𝑓!! 𝑉 𝑖𝑠 𝑐𝑙𝑜𝑠𝑒𝑑 𝑖𝑛 𝒳 𝑤ℎ𝑒𝑛𝑒𝑣𝑒𝑟 𝑉 𝑖𝑠 𝑐𝑙𝑜𝑠𝑒𝑑 𝑖𝑛 𝑌. 𝑫𝒆𝒇𝒊𝒏𝒊𝒕𝒊𝒐𝒏 𝟔.𝟕: 𝑆𝑢𝑝𝑝𝑜𝑠𝑒 𝑡ℎ𝑎𝑡 𝐴 𝑖𝑠 𝑎 𝑠𝑢𝑏𝑠𝑒𝑡 𝑜𝑓 𝑎 𝑚𝑒𝑡𝑟𝑖𝑐 𝑠𝑝𝑎𝑐𝑒 𝒳, 𝑎𝑛𝑑 𝑥 ∈ 𝒳. 𝑇ℎ𝑒𝑛 𝑥 𝑖𝑠 𝑎 𝑝𝑜𝑖𝑛𝑡 𝑜𝑓 𝑐𝑙𝑜𝑠𝑢𝑟𝑒 𝑜𝑓 𝐴 𝑖𝑛 𝒳 𝑖𝑓 ∀𝜖 > 0 ,𝐵! 𝑥 ∩ 𝐴 ≠ ∅. 𝑫𝒆𝒇𝒊𝒏𝒊𝒕𝒊𝒐𝒏: 𝑇ℎ𝑒 𝑐𝑙𝑜𝑠𝑢𝑟𝑒 𝐴 𝑜𝑓 𝐴 ⊆ 𝒳, 𝑖𝑠 𝑡ℎ𝑒 𝑠𝑒𝑡 𝑜𝑓 𝑎𝑙𝑙 𝑝𝑜𝑖𝑛𝑡𝑠 𝑜𝑓 𝑐𝑙𝑜𝑠𝑢𝑟𝑒 𝑜𝑓 𝐴 𝑖𝑛 𝒳. 𝑫𝒆𝒇𝒊𝒏𝒊𝒕𝒊𝒐𝒏 𝟔.𝟗: 𝐴 𝑠𝑢𝑏𝑠𝑒𝑡 𝐴 𝑜𝑓 𝑎 𝑚𝑒𝑡𝑟𝑖𝑐 𝑠𝑝𝑎𝑐𝑒 𝒳 𝑖𝑠 𝑠𝑎𝑖𝑑 𝑡𝑜 𝑏𝑒 𝑑𝑒𝑛𝑠𝑒 𝑖𝑛 𝒳 𝑖𝑓 𝐴 = 𝒳. 𝑷𝒓𝒐𝒑𝒐𝒔𝒊𝒕𝒊𝒐𝒏 𝟔.𝟏𝟏: 𝐿𝑒𝑡 𝐴,𝐵 𝑏𝑒 𝑠𝑢𝑏𝑠𝑒𝑡𝑠 𝑜𝑓 𝑎 𝑚𝑒𝑡𝑟𝑖𝑐 𝑠𝑝𝑎𝑐𝑒 𝒳.𝑇ℎ𝑒𝑛, (a) 𝐴 ⊆ 𝐴; (b) 𝐴 ⊆ 𝐵⟹ 𝐴 ⊆ 𝐵; (c) 𝐴 𝑖𝑠 𝑐𝑙𝑜𝑠𝑒𝑑 𝑖𝑛 𝒳 ⟺ 𝐴 = 𝐴; (d) 𝐴 = 𝐴; (e) 𝐴 𝑖𝑠 𝑐𝑙𝑜𝑠𝑒𝑑 𝑖𝑛 𝒳; (f) 𝐴 𝑖𝑠 𝑡ℎ𝑒 𝑠𝑚𝑎𝑙𝑙𝑒𝑠𝑡 𝑐𝑙𝑜𝑠𝑒𝑑 𝑠𝑢𝑏𝑠𝑒𝑡 𝑜𝑓 𝒳 𝑐𝑜𝑛𝑡𝑎𝑖𝑛𝑖𝑛𝑔 𝐴.
𝑷𝒓𝒐𝒑𝒐𝒔𝒊𝒕𝒊𝒐𝒏 𝟔.𝟏𝟐:𝐴 𝑚𝑎𝑝 𝑓:𝒳 → 𝒴 𝑜𝑓 𝑚𝑒𝑡𝑟𝑖𝑐 𝑠𝑝𝑎𝑐𝑒𝑠 𝑖𝑠 𝑐𝑜𝑛𝑡𝑖𝑛𝑢𝑜𝑢𝑠⟺ 𝑓 𝐴 ⊆ 𝑓 𝐴 ∀𝐴 ⊆ 𝒳. 𝑷𝒓𝒐𝒑𝒐𝒔𝒊𝒕𝒊𝒐𝒏 𝟔.𝟏𝟑: 𝐿𝑒𝑡 𝐴!,𝐴!,… ,𝐴! 𝑏𝑒 𝑠𝑢𝑏𝑠𝑒𝑡𝑠 𝑜𝑓 𝑎 𝑚𝑒𝑡𝑟𝑖𝑐 𝑠𝑝𝑎𝑐𝑒 𝒳. 𝑇ℎ𝑒𝑛
𝐴!
!
!!!
= 𝐴!
!
!!!
.
𝑷𝒓𝒐𝒑𝒐𝒔𝒊𝒕𝒊𝒐𝒏 𝟔.𝟏𝟒: 𝐿𝑒𝑡 𝐴! 𝑏𝑒 𝑎 𝑠𝑢𝑏𝑠𝑒𝑡 𝑜𝑓 𝑎 𝑚𝑒𝑡𝑟𝑖𝑐 𝑠𝑝𝑎𝑐𝑒 𝒳 𝑓𝑜𝑟 𝑒𝑎𝑐ℎ 𝑖 𝑖𝑛 𝑎𝑛 𝑖𝑛𝑑𝑒𝑥𝑖𝑛𝑔 𝑠𝑒𝑡 𝐼. 𝑇ℎ𝑒𝑛
𝐴!!∈!
⊆ 𝐴! .!∈!
𝑫𝒆𝒇𝒊𝒏𝒊𝒕𝒊𝒐𝒏 𝟔.𝟏𝟓:𝐴 𝑝𝑜𝑖𝑛𝑡 𝑥 𝑖𝑛 𝑎 𝑚𝑒𝑡𝑟𝑖𝑐 𝑠𝑝𝑎𝑐𝑒 𝒳 𝑖𝑠 𝑠𝑎𝑖𝑑 𝑡𝑜 𝑏𝑒 𝑎 𝑙𝑖𝑚𝑖𝑡 𝑝𝑜𝑖𝑛𝑡 𝑜𝑓 𝑎 𝑠𝑒𝑡 𝐴 ⊆ 𝒳 𝑖𝑓, ∀𝜖 > 0 𝑡ℎ𝑒𝑟𝑒 𝑒𝑥𝑖𝑠𝑡𝑠 𝑎 𝑝𝑜𝑖𝑛𝑡 𝑖𝑛 𝐵! 𝑥 ∩ 𝐴 𝑜𝑡ℎ𝑒𝑟 𝑡ℎ𝑎𝑛 𝑥 𝑖𝑡𝑠𝑒𝑙𝑓, 𝑖. 𝑒. (𝐵! 𝑥 \{𝑥}) ∩ 𝐴 ≠ ∅. 𝑷𝒓𝒐𝒑𝒐𝒔𝒊𝒕𝒊𝒐𝒏 𝟔.𝟏𝟕:𝐴 𝑠𝑢𝑏𝑠𝑒𝑡 𝐴 ⊆ 𝒳 𝑖𝑠 𝑐𝑙𝑜𝑠𝑒𝑑 ⇔ 𝑖𝑡 𝑐𝑜𝑛𝑡𝑎𝑖𝑛𝑠 𝑎𝑙𝑙 𝑖𝑡𝑠 𝑙𝑖𝑚𝑖𝑡 𝑝𝑜𝑖𝑛𝑡𝑠 𝑖𝑛 𝒳. 𝑷𝒓𝒐𝒑𝒐𝒔𝒊𝒕𝒊𝒐𝒏 𝟔.𝟏𝟖: 𝐿𝑒𝑡 𝐴 ⊆ 𝒳. 𝑇ℎ𝑒𝑛 𝐴 𝑖𝑠 𝑡ℎ𝑒 𝑢𝑛𝑖𝑜𝑛 𝑜𝑓 𝐴 𝑤𝑖𝑡ℎ 𝑎𝑙𝑙 𝑖𝑡𝑠 𝑙𝑖𝑚𝑖𝑡 𝑝𝑜𝑖𝑛𝑡𝑠 𝑖𝑛 𝒳. 𝑫𝒆𝒇𝒊𝒏𝒊𝒕𝒊𝒐𝒏 𝟔.𝟏𝟗:𝑇ℎ𝑒 𝑖𝑛𝑡𝑒𝑟𝑖𝑜𝑟, 𝑖𝑛𝑡 𝐴 , 𝑜𝑓 𝑎 𝑠𝑒𝑡 𝐴 ⊆ 𝒳 𝑖𝑠 𝑡ℎ𝑒 𝑠𝑒𝑡 𝑜𝑓 𝑝𝑜𝑖𝑛𝑡𝑠 𝑎 ∈ 𝐴 𝑠𝑢𝑐ℎ 𝑡ℎ𝑎𝑡
𝐵! 𝑥 ⊆ 𝐴 𝑓𝑜𝑟 𝑠𝑜𝑚𝑒 𝜖 > 0. 𝑷𝒓𝒐𝒑𝒐𝒔𝒊𝒕𝒊𝒐𝒏 𝟔.𝟐𝟏: 𝐿𝑒𝑡 𝐴,𝐵 𝑏𝑒 𝑠𝑢𝑏𝑠𝑒𝑡𝑠 𝑜𝑓 𝑎 𝑚𝑒𝑡𝑟𝑖𝑐 𝑠𝑝𝑎𝑐𝑒 𝒳.𝑇ℎ𝑒𝑛, (a) 𝑖𝑛𝑡(𝐴) ⊆ 𝐴; (b) 𝐴 ⊆ 𝐵⟹ 𝑖𝑛𝑡(𝐴) ⊆ 𝑖𝑛𝑡(𝐵); (c) 𝐴 𝑖𝑠 𝑜𝑝𝑒𝑛 𝑖𝑛 𝒳 ⟺ 𝑖𝑛𝑡(𝐴) = 𝐴; (d) 𝑖𝑛𝑡(𝑖𝑛𝑡 𝐴 ) = 𝑖𝑛𝑡(𝐴); (e) 𝑖𝑛𝑡(𝐴) 𝑖𝑠 𝑜𝑝𝑒𝑛 𝑖𝑛 𝒳; (f) 𝑖𝑛𝑡 𝐴 𝑖𝑠 𝑡ℎ𝑒 𝑙𝑎𝑟𝑔𝑒𝑠𝑡 𝑜𝑝𝑒𝑛 𝑠𝑢𝑏𝑠𝑒𝑡 𝑜𝑓 𝒳 𝑐𝑜𝑛𝑡𝑎𝑖𝑛𝑒𝑑 𝑖𝑛 𝐴.
𝑫𝒆𝒇𝒊𝒏𝒊𝒕𝒊𝒐𝒏 𝟔.𝟐𝟐:𝑇ℎ𝑒 𝑏𝑜𝑢𝑛𝑑𝑎𝑟𝑦 𝜕𝐴 𝑜𝑓 𝑎 𝑠𝑢𝑏𝑠𝑒𝑡 𝐴 𝑖𝑛 𝑎 𝑚𝑒𝑡𝑟𝑖𝑐 𝑠𝑝𝑎𝑐𝑒 𝒳 𝑖𝑠 𝑡ℎ𝑒 𝑠𝑒𝑡 𝐴∫ 𝐴 . 𝑷𝒓𝒐𝒑𝒐𝒔𝒊𝒕𝒊𝒐𝒏 𝟔.𝟐𝟒:𝐹𝑜𝑟 𝑎 𝑠𝑢𝑏𝑠𝑒𝑡 𝐴 ⊆ 𝒳, 𝑎 𝑝𝑜𝑖𝑛𝑡 𝑥 ∈ 𝒳 𝑖𝑠 𝑖𝑛 𝜕𝐴⇔ ∀𝜖 > 0 𝑏𝑜𝑡ℎ 𝐴 ∩ 𝐵! 𝑥 𝑎𝑛𝑑 𝒳\𝐴 ∩ 𝐵! 𝑥 𝑎𝑟𝑒 𝑛𝑜𝑛𝑒𝑚𝑝𝑡𝑦. Convergence in Metric Spaces 𝑫𝒆𝒇𝒊𝒏𝒊𝒕𝒊𝒐𝒏 𝟔.𝟐𝟓: 𝐴 𝑠𝑒𝑞𝑢𝑒𝑛𝑐𝑒 𝑥! 𝑖𝑛 𝑎 𝑚𝑒𝑡𝑟𝑖𝑐 𝑠𝑝𝑎𝑐𝑒 𝒳 𝑐𝑜𝑛𝑣𝑒𝑟𝑔𝑒𝑠 𝑡𝑜 𝑎 𝑝𝑜𝑖𝑛𝑡 𝑥 ∈ 𝒳 𝑖𝑓 ∀𝜖 > 0 ∃𝑁 𝑠. 𝑡. 𝑥! ∈ 𝐵! 𝑥 𝑤ℎ𝑒𝑛𝑒𝑣𝑒𝑟 𝑛 ≥ 𝑁. 𝑷𝒓𝒐𝒑𝒐𝒔𝒊𝒕𝒊𝒐𝒏 𝟔.𝟐𝟔:𝐴 𝑙𝑖𝑚𝑖𝑡 𝑜𝑓 𝑎 𝑠𝑒𝑞𝑢𝑒𝑛𝑐𝑒 𝑖𝑛 𝑎 𝑚𝑒𝑡𝑟𝑖𝑐 𝑠𝑝𝑎𝑐𝑒 𝑖𝑠 𝑢𝑛𝑖𝑞𝑢𝑒. 𝑫𝒆𝒇𝒊𝒏𝒊𝒕𝒊𝒐𝒏 𝟔.𝟐𝟕:𝐴 𝑠𝑒𝑞𝑢𝑒𝑛𝑐𝑒 𝑥! 𝑖𝑛 𝑎 𝑚𝑒𝑡𝑟𝑖𝑐 𝑠𝑝𝑎𝑐𝑒 𝒳,𝑑 𝑖𝑠 𝑎 𝐶𝑎𝑢𝑐ℎ𝑦 𝑠𝑒𝑞𝑢𝑒𝑛𝑐𝑒 𝑖𝑓 ∀𝜖 > 0 ∃𝑁 ∈ ℕ 𝑠. 𝑡.𝑑 𝑥!, 𝑥! < 𝜖 𝑤ℎ𝑒𝑛𝑒𝑣𝑒𝑟 𝑚, 𝑛 ≥ 𝑁. 𝑷𝒓𝒐𝒑𝒐𝒔𝒊𝒕𝒊𝒐𝒏 𝟔.𝟐𝟖:𝐴𝑛𝑦 𝑐𝑜𝑛𝑣𝑒𝑟𝑔𝑒𝑛𝑡 𝑠𝑒𝑞𝑢𝑒𝑛𝑐𝑒 𝑖𝑛 𝑎 𝑚𝑒𝑡𝑟𝑖𝑐 𝑠𝑝𝑎𝑐𝑒 𝑖𝑠 𝐶𝑎𝑢𝑐ℎ𝑦. 𝑷𝒓𝒐𝒑𝒐𝒔𝒊𝒕𝒊𝒐𝒏 𝟔.𝟐𝟗: 𝑆𝑢𝑝𝑝𝑜𝑠𝑒 𝑡ℎ𝑎𝑡 𝑌 ⊆ 𝒳 𝑎𝑛𝑑 𝑦! 𝑖𝑠 𝑎 𝑠𝑒𝑞𝑢𝑒𝑛𝑐𝑒 𝑖𝑛 𝑌 𝑤ℎ𝑖𝑐ℎ 𝑐𝑜𝑛𝑣𝑒𝑟𝑔𝑒𝑠 𝑡𝑜 𝑎 𝑝𝑜𝑖𝑛𝑡 𝑎 ∈ 𝒳. 𝑇ℎ𝑒𝑛 𝑎 ∈ 𝑌. 𝑪𝒐𝒓𝒐𝒍𝒍𝒂𝒓𝒚 𝟔.𝟑𝟎: 𝐼𝑓 𝑌 𝑖𝑠 𝑎 𝑐𝑙𝑜𝑠𝑒𝑑 𝑠𝑢𝑏𝑠𝑒𝑡 𝑜𝑓 𝑎 𝑚𝑒𝑡𝑟𝑖𝑐 𝑠𝑝𝑎𝑐𝑒 𝒳 𝑎𝑛𝑑 𝑦! 𝑖𝑠 𝑎 𝑠𝑒𝑞𝑢𝑒𝑛𝑐𝑒 𝑜𝑓 𝑝𝑜𝑖𝑛𝑡𝑠 𝑖𝑛 𝑌 𝑐𝑜𝑛𝑣𝑒𝑟𝑔𝑖𝑛𝑔 𝑡𝑜 𝑎 ∈ 𝒳 𝑡ℎ𝑒𝑛 𝑎 ∈ 𝑌.
Chapter 12: Connected Spaces 𝑫𝒆𝒇𝒊𝒏𝒊𝒕𝒊𝒐𝒏 𝟏𝟐.𝟐:𝐴 𝑝𝑎𝑟𝑡𝑖𝑡𝑖𝑜𝑛 𝐴,𝐵 𝑜𝑓 𝑎 𝑡𝑜𝑝𝑜𝑙𝑜𝑔𝑖𝑐𝑎𝑙 𝑠𝑝𝑎𝑐𝑒 𝒳 𝑖𝑠 𝑎 𝑝𝑎𝑖𝑟 𝑜𝑓 𝑛𝑜𝑛𝑒𝑚𝑝𝑡𝑦 𝑠𝑢𝑏𝑠𝑒𝑡𝑠 𝐴,𝐵 𝑜𝑓 𝒳 𝑠𝑢𝑐ℎ 𝑡ℎ𝑎𝑡 𝒳 = 𝐴 ∪ 𝐵,𝐴 ∩ 𝐵 ≠ ∅, 𝑎𝑛𝑑 𝑏𝑜𝑡ℎ 𝐴 𝑎𝑛𝑑 𝐵 𝑎𝑟𝑒 𝑜𝑝𝑒𝑛 𝑖𝑛 𝒳. 𝑷𝒓𝒐𝒑𝒐𝒔𝒊𝒕𝒊𝒐𝒏 𝟏𝟐.𝟑:𝐴 𝑡𝑜𝑝𝑜𝑙𝑜𝑔𝑖𝑐𝑎𝑙 𝑠𝑝𝑎𝑐𝑒 𝑖𝑠 𝑐𝑜𝑛𝑛𝑒𝑐𝑡𝑒𝑑 ⇔ 𝑖𝑡 𝑎𝑑𝑚𝑖𝑡𝑠 𝑛𝑜 𝑝𝑎𝑟𝑡𝑖𝑡𝑖𝑜𝑛. 𝑻𝒉𝒆𝒐𝒓𝒆𝒎 𝟏𝟐.𝟖,𝟏𝟎:𝐴𝑛𝑦 𝑐𝑜𝑛𝑛𝑒𝑐𝑡𝑒𝑑 𝑠𝑢𝑏𝑠𝑝𝑎𝑐𝑒 𝑆 ⊆ ℝ 𝑖𝑠 𝑎𝑛 𝑖𝑛𝑡𝑒𝑟𝑣𝑎𝑙, 𝑎𝑛𝑑 ℎ𝑒𝑛𝑐𝑒 𝑖𝑠 𝑐𝑜𝑛𝑛𝑒𝑐𝑡𝑒𝑑. 𝑷𝒓𝒐𝒑𝒐𝒔𝒊𝒕𝒊𝒐𝒏 𝟏𝟐.𝟏𝟏: 𝐼𝑓 𝑓:𝒳 → 𝒴 𝑖𝑠 𝑐𝑜𝑛𝑡𝑖𝑛𝑢𝑜𝑢𝑠 𝑎𝑛𝑑 𝒳 𝑖𝑠 𝑐𝑜𝑛𝑛𝑒𝑐𝑡𝑒𝑑 𝑡ℎ𝑒𝑛 𝑓 𝒳 𝑖𝑠 𝑐𝑜𝑛𝑛𝑒𝑐𝑡𝑒𝑑. 𝑻𝒉𝒆𝒐𝒓𝒆𝒎 𝟏𝟐.𝟏𝟖:𝑇ℎ𝑒 𝑡𝑜𝑝𝑜𝑙𝑜𝑔𝑖𝑐𝑎𝑙 𝑝𝑟𝑜𝑑𝑢𝑐𝑡 𝒳×𝒴 𝑖𝑠 𝑐𝑜𝑛𝑛𝑒𝑐𝑡𝑒𝑑 ⇔ 𝒳 𝑎𝑛𝑑 𝒴 𝑎𝑟𝑒 𝑐𝑜𝑛𝑛𝑒𝑐𝑡𝑒𝑑. 𝑷𝒓𝒐𝒑𝒐𝒔𝒊𝒕𝒊𝒐𝒏 𝟏𝟐.𝟐𝟑:𝐴𝑛𝑦 𝑝𝑎𝑡ℎ 𝑐𝑜𝑛𝑛𝑒𝑐𝑡𝑒𝑑 𝑠𝑝𝑎𝑐𝑒 𝑖𝑠 𝑐𝑜𝑛𝑛𝑒𝑐𝑡𝑒𝑑.