正合函子 目录 阿貝爾範疇間的正合函子 一般範疇中的正合函子 例子 文獻 导航菜单

同调代数函子加法范畴


範疇論極限函子阿貝爾範疇正合序列阿貝爾範疇阿貝爾範疇正合序列




在範疇論中,正合函子(或譯作恰當函子)是保存有限極限的函子。在阿貝爾範疇中,這就相當於保存正合序列的函子。




目录





  • 1 阿貝爾範疇間的正合函子


  • 2 一般範疇中的正合函子


  • 3 例子


  • 4 文獻




阿貝爾範疇間的正合函子


C,C′displaystyle mathcal C,mathcal C' 為阿貝爾範疇,F:C→C′displaystyle F:mathcal Cto mathcal C' 為加法函子。若對每個正合序列


⋯⟶Xi⟶Xi−1⟶⋯displaystyle cdots longrightarrow X_ilongrightarrow X_i-1longrightarrow cdots

Fdisplaystyle F 後得到的序列


⋯⟶F(Xi)⟶F(Xi−1)⟶⋯displaystyle cdots longrightarrow F(X_i)longrightarrow F(X_i-1)longrightarrow cdots

仍為正合序列,則稱 Fdisplaystyle F正合函子


由於正合序列總能拆解為短正合序列,在定義中僅須考慮短正合序列即可。


此外,若對每個短正合序列 0→X′→X→X″→0displaystyle 0to X'to Xto X''to 0,其像截去尾端零對象後 0→F(X′)→F(X)→F(X″)displaystyle 0to F(X')to F(X)to F(X'') 為正合序列,則稱 Fdisplaystyle F左正合函子;類似地,若 F(X′)→F(X)→F(X″)→0displaystyle F(X')to F(X)to F(X'')to 0 為正合序列,則稱 Fdisplaystyle F右正合函子。正合性等價於左正合性+右正合性。



一般範疇中的正合函子


考慮一個函子 F:C→C′displaystyle F:mathcal Crightarrow mathcal C'


  • Cdisplaystyle mathcal C裡存在任意的有限射影極限,且Fdisplaystyle F與有限射影極限交換(即:F(lim←i⁡Xi)→∼lim←i⁡F(Xi)displaystyle F(varprojlim _iX_i)stackrel sim to varprojlim _iF(X_i)),則稱Fdisplaystyle F左正合

  • Cdisplaystyle mathcal C裡存在任意的有限歸納極限,且Fdisplaystyle F與有限歸納極限交換(即:lim→i⁡F(Xi)→∼F(lim→i⁡Xi)displaystyle varinjlim _iF(X_i)stackrel sim to F(varinjlim _iX_i)),則稱Fdisplaystyle F右正合

  • 若上述條件同時被滿足,則稱Fdisplaystyle F正合

在阿貝爾範疇中,由於任意有限射影(或歸納)極限可以由核(或上核)與有限積(或上積)生成,此時的定義遂回歸到正合序列的定義。



例子


  • 根據極限的泛性質,Hom(−,−)displaystyle mathrm Hom (-,-)函子無論對哪個變數都是左正合的,這是左正合函子的基本例子。

  • (F,G)displaystyle (F,G)是一對伴隨函子。若Cdisplaystyle mathcal C存在任意有限歸納極限,則Fdisplaystyle F右正合;若存在任意有限射影極限,Gdisplaystyle G左正合。此法可建立許多函子的正合性。

  • Xdisplaystyle X 為拓撲空間,阿貝爾群數學範疇上的整體截面函子 X↦F(X)displaystyle Xmapsto F(X) 是左正合函子。

  • Rdisplaystyle R 為環,Tdisplaystyle T 為右 Rdisplaystyle R-模,則左 Rdisplaystyle R-模範疇上的張量積函子 T⊗R−displaystyle Totimes _R- 是右正合函子。

  • A,Bdisplaystyle mathcal A,;mathcal B 為兩個阿貝爾範疇,考慮函子範疇 BAdisplaystyle mathcal B^mathcal A,固定一對象 A∈Adisplaystyle Ain mathcal A,對 Adisplaystyle A 的「求值」是正合函子。


文獻


  • Masaki Kashiwara and Pierre Schapira, Categories and Sheaves, Springer. ISBN 3540279490

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