定義
![平坦模](/img/f/3a0/wZwpmL0ATOyUTNxcDN2YjN1UTM1QDN5MjM5ADMwAjMwUzL3QzLzczLt92YucmbvRWdo5Cd0FmLxE2LvoDc0RHa.jpg)
![平坦模](/img/8/4f3/wZwpmL2ATMxADN5IjNwMzM1UTM1QDN5MjM5ADMwAjMwUzLyYzL0UzLt92YucmbvRWdo5Cd0FmLyE2LvoDc0RHa.jpg)
![平坦模](/img/d/33f/wZwpmLyATM5IDO4gDN2YjN1UTM1QDN5MjM5ADMwAjMwUzL4QzL2AzLt92YucmbvRWdo5Cd0FmLzE2LvoDc0RHa.jpg)
![平坦模](/img/8/84f/wZwpmL4czNzUzN1YDN0MTN1UTM1QDN5MjM5ADMwAjMwUzL2QzL0YzLt92YucmbvRWdo5Cd0FmLyE2LvoDc0RHa.jpg)
定義1 平坦模是一類重要的模。右A模M,若函子-是正合的,則稱是 平坦模。類似地,對左A模M,若函子-正合,則稱是平坦模。投射模一定是平坦模,反之不一定成立。環A上每個左A平坦模是投射模的充分必要條件是,環A是左完全環 。
![平坦模](/img/4/a34/wZwpmL4czM5YTM5cTO4kzM0UTMyITNykTO0EDMwAjMwUzL3kzL4czLt92YucmbvRWdo5Cd0FmL0E2LvoDc0RHa.jpg)
![平坦模](/img/8/6d6/wZwpmL0gDNxQjN1cDN2YjN1UTM1QDN5MjM5ADMwAjMwUzL3QzLwEzLt92YucmbvRWdo5Cd0FmLxE2LvoDc0RHa.jpg)
定義2-模叫作平坦的,若對每個單同態
![平坦模](/img/6/bb4/wZwpmLyIjNxQDOxUzN2YjN1UTM1QDN5MjM5ADMwAjMwUzL1czLxMzLt92YucmbvRWdo5Cd0FmL0E2LvoDc0RHa.jpg)
![平坦模](/img/3/75d/wZwpmLyQjN5MTNzgjN2YjN1UTM1QDN5MjM5ADMwAjMwUzL4YzL0IzLt92YucmbvRWdo5Cd0FmLyE2LvoDc0RHa.jpg)
映射也是單同態。
相關性質定理
推論1平坦模的同構像也是平坦的。
![平坦模](/img/8/192/wZwpmL2QzNyADO5kTN2YjN1UTM1QDN5MjM5ADMwAjMwUzL5UzL4QzLt92YucmbvRWdo5Cd0FmLzE2LvoDc0RHa.jpg)
![平坦模](/img/6/c28/wZwpmLyYTM5ADN1UjN2YjN1UTM1QDN5MjM5ADMwAjMwUzL1YzLzUzLt92YucmbvRWdo5Cd0FmLyE2LvoDc0RHa.jpg)
![平坦模](/img/0/a26/wZwpmL1MDO4YjM5cTO4kzM0UTMyITNykTO0EDMwAjMwUzL3kzL3IzLt92YucmbvRWdo5Cd0FmLyE2LvoDc0RHa.jpg)
![平坦模](/img/b/528/wZwpmLwMTOzkDN1ITN2YjN1UTM1QDN5MjM5ADMwAjMwUzLyUzL0IzLt92YucmbvRWdo5Cd0FmLyE2LvoDc0RHa.jpg)
定理2令(或),則有:是平坦的是平坦的。
定理3每個投射模都是平坦的。
![平坦模](/img/4/a34/wZwpmL4czM5YTM5cTO4kzM0UTMyITNykTO0EDMwAjMwUzL3kzL4czLt92YucmbvRWdo5Cd0FmL0E2LvoDc0RHa.jpg)
![平坦模](/img/2/c55/wZwpmL1QDMyUjN0EjN2YjN1UTM1QDN5MjM5ADMwAjMwUzLxYzL2AzLt92YucmbvRWdo5Cd0FmLzE2LvoDc0RHa.jpg)
![平坦模](/img/8/6d6/wZwpmL0gDNxQjN1cDN2YjN1UTM1QDN5MjM5ADMwAjMwUzL3QzLwEzLt92YucmbvRWdo5Cd0FmLxE2LvoDc0RHa.jpg)
引理4設有-模和,
![平坦模](/img/4/014/wZwpmL3UDNwYTOxkTN2YjN1UTM1QDN5MjM5ADMwAjMwUzL5UzL3EzLt92YucmbvRWdo5Cd0FmLwE2LvoDc0RHa.jpg)
![平坦模](/img/5/1ae/wZwpmL4MTOwQDM3EjN2YjN1UTM1QDN5MjM5ADMwAjMwUzLxYzL2gzLt92YucmbvRWdo5Cd0FmLzE2LvoDc0RHa.jpg)
![平坦模](/img/a/df7/wZwpmLxETOzIzMycDN2YjN1UTM1QDN5MjM5ADMwAjMwUzL3QzL2UzLt92YucmbvRWdo5Cd0FmLxE2LvoDc0RHa.jpg)
![平坦模](/img/7/09d/wZwpmLwUzM4MTO1gjN2YjN1UTM1QDN5MjM5ADMwAjMwUzL4YzLyMzLt92YucmbvRWdo5Cd0FmL0E2LvoDc0RHa.jpg)
![平坦模](/img/4/014/wZwpmL3UDNwYTOxkTN2YjN1UTM1QDN5MjM5ADMwAjMwUzL5UzL3EzLt92YucmbvRWdo5Cd0FmLwE2LvoDc0RHa.jpg)
(1) 若,則存在有限生成子模與()使得。
![平坦模](/img/6/266/wZwpmL3MTM4UzNwATN2YjN1UTM1QDN5MjM5ADMwAjMwUzLwUzL1czLt92YucmbvRWdo5Cd0FmLwE2LvoDc0RHa.jpg)
![平坦模](/img/1/5c1/wZwpmLwQjMwUzNzQjN2YjN1UTM1QDN5MjM5ADMwAjMwUzL0YzLyQzLt92YucmbvRWdo5Cd0FmLxE2LvoDc0RHa.jpg)
(2)設,則。
![平坦模](/img/8/6d6/wZwpmL0gDNxQjN1cDN2YjN1UTM1QDN5MjM5ADMwAjMwUzL3QzLwEzLt92YucmbvRWdo5Cd0FmLxE2LvoDc0RHa.jpg)
推論5 設是一個模且使它的每個有限生成子模包含在一個平坦子模內,則M是平坦的。
![平坦模](/img/e/e44/wZwpmLycTNzYjNyAjN2YjN1UTM1QDN5MjM5ADMwAjMwUzLwYzL4gzLt92YucmbvRWdo5Cd0FmLyE2LvoDc0RHa.jpg)
![平坦模](/img/8/6d6/wZwpmL0gDNxQjN1cDN2YjN1UTM1QDN5MjM5ADMwAjMwUzL3QzLwEzLt92YucmbvRWdo5Cd0FmLxE2LvoDc0RHa.jpg)
推論6若對一個同態和一個模,
![平坦模](/img/a/009/wZwpmL0QzN4AjNxQTN2YjN1UTM1QDN5MjM5ADMwAjMwUzL0UzL2EzLt92YucmbvRWdo5Cd0FmLxE2LvoDc0RHa.jpg)
![平坦模](/img/e/1a1/wZwpmLxQTO4gjM3QjN2YjN1UTM1QDN5MjM5ADMwAjMwUzL0YzL4UzLt92YucmbvRWdo5Cd0FmLyE2LvoDc0RHa.jpg)
![平坦模](/img/2/d27/wZwpmL0ADO2ATN4YTN2YjN1UTM1QDN5MjM5ADMwAjMwUzL2UzLyUzLt92YucmbvRWdo5Cd0FmL0E2LvoDc0RHa.jpg)
不是單同態,則存在有限生成子模使得也不是單同態。
![平坦模](/img/8/6d6/wZwpmL0gDNxQjN1cDN2YjN1UTM1QDN5MjM5ADMwAjMwUzL3QzLwEzLt92YucmbvRWdo5Cd0FmLxE2LvoDc0RHa.jpg)
定理7對模,下列敘述是等價的:
![平坦模](/img/8/6d6/wZwpmL0gDNxQjN1cDN2YjN1UTM1QDN5MjM5ADMwAjMwUzL3QzLwEzLt92YucmbvRWdo5Cd0FmLxE2LvoDc0RHa.jpg)
(1)是平坦的;
![平坦模](/img/4/09a/wZwpmL1YzMwETO4MzM3QTN1UTM1QDN5MjM5ADMwAjMwUzLzMzLwIzLt92YucmbvRWdo5Cd0FmLwE2LvoDc0RHa.jpg)
![平坦模](/img/8/866/wZwpmL1YTO1EDOxUzN2YjN1UTM1QDN5MjM5ADMwAjMwUzL1czL0YzLt92YucmbvRWdo5Cd0FmLzE2LvoDc0RHa.jpg)
![平坦模](/img/3/0eb/wZwpmL3MDO2IDMycTN2YjN1UTM1QDN5MjM5ADMwAjMwUzL3UzL0QzLt92YucmbvRWdo5Cd0FmLwE2LvoDc0RHa.jpg)
(2) 對每個有限生成右理想,及包含映射,則是單同態;
![平坦模](/img/2/d22/wZwpmLzAjM0UjM4YzN2YjN1UTM1QDN5MjM5ADMwAjMwUzL2czLyIzLt92YucmbvRWdo5Cd0FmLyE2LvoDc0RHa.jpg)
(3)是內射的。
![平坦模](/img/4/a34/wZwpmL4czM5YTM5cTO4kzM0UTMyITNykTO0EDMwAjMwUzL3kzL4czLt92YucmbvRWdo5Cd0FmL0E2LvoDc0RHa.jpg)
定理8對於環,下列條件是等價的:
![平坦模](/img/8/6d6/wZwpmL0gDNxQjN1cDN2YjN1UTM1QDN5MjM5ADMwAjMwUzL3QzLwEzLt92YucmbvRWdo5Cd0FmLxE2LvoDc0RHa.jpg)
(1) 每個模是平坦的;
![平坦模](/img/8/063/wZwpmLycjMxUjN5IDN0MTN1UTM1QDN5MjM5ADMwAjMwUzLyQzLyIzLt92YucmbvRWdo5Cd0FmLzE2LvoDc0RHa.jpg)
![平坦模](/img/a/98a/wZwpmLxYTNwEDM2MjN2YjN1UTM1QDN5MjM5ADMwAjMwUzLzYzL3gzLt92YucmbvRWdo5Cd0FmLwE2LvoDc0RHa.jpg)
![平坦模](/img/3/0e5/wZwpmLwgjM2kjMwQzN2YjN1UTM1QDN5MjM5ADMwAjMwUzL0czL2EzLt92YucmbvRWdo5Cd0FmLzE2LvoDc0RHa.jpg)
(2) 對每個元素,存在一個元素使得;
![平坦模](/img/4/a34/wZwpmL4czM5YTM5cTO4kzM0UTMyITNykTO0EDMwAjMwUzL3kzL4czLt92YucmbvRWdo5Cd0FmL0E2LvoDc0RHa.jpg)
![平坦模](/img/b/83b/wZwpmLzQzM1AzN4gDM3QTN1UTM1QDN5MjM5ADMwAjMwUzL4AzL4MzLt92YucmbvRWdo5Cd0FmLxE2LvoDc0RHa.jpg)
(3)的每個循環右理想是的直和項;
![平坦模](/img/4/a34/wZwpmL4czM5YTM5cTO4kzM0UTMyITNykTO0EDMwAjMwUzL3kzL4czLt92YucmbvRWdo5Cd0FmL0E2LvoDc0RHa.jpg)
![平坦模](/img/b/83b/wZwpmLzQzM1AzN4gDM3QTN1UTM1QDN5MjM5ADMwAjMwUzL4AzL4MzLt92YucmbvRWdo5Cd0FmLxE2LvoDc0RHa.jpg)
(4)的每個有限生成有理想是的直和項。
![平坦模](/img/8/6d6/wZwpmL0gDNxQjN1cDN2YjN1UTM1QDN5MjM5ADMwAjMwUzL3QzLwEzLt92YucmbvRWdo5Cd0FmLxE2LvoDc0RHa.jpg)
![平坦模](/img/7/de0/wZwpmLzcjN0MjM1gjN2YjN1UTM1QDN5MjM5ADMwAjMwUzL4YzLyMzLt92YucmbvRWdo5Cd0FmLzE2LvoDc0RHa.jpg)
引理9 令是平坦的,表示包含映射,則下列是等價的:
![平坦模](/img/0/308/wZwpmLzETN5UzMwMzN2YjN1UTM1QDN5MjM5ADMwAjMwUzLzczL4MzLt92YucmbvRWdo5Cd0FmLyE2LvoDc0RHa.jpg)
(1)是單同態;
![平坦模](/img/5/8ff/wZwpmL0cjN5cjN5kDN2YjN1UTM1QDN5MjM5ADMwAjMwUzL5QzLygzLt92YucmbvRWdo5Cd0FmLwE2LvoDc0RHa.jpg)
(2)。
![平坦模](/img/8/6d6/wZwpmL0gDNxQjN1cDN2YjN1UTM1QDN5MjM5ADMwAjMwUzL3QzLwEzLt92YucmbvRWdo5Cd0FmLxE2LvoDc0RHa.jpg)
![平坦模](/img/9/776/wZwpmL3MTNyYDOwATN2YjN1UTM1QDN5MjM5ADMwAjMwUzLwUzLzczLt92YucmbvRWdo5Cd0FmLxE2LvoDc0RHa.jpg)
定理10令是平坦的,,則下列是等價的:
![平坦模](/img/f/83e/wZwpmL0EDM0ATOxYzN2YjN1UTM1QDN5MjM5ADMwAjMwUzL2czL1YzLt92YucmbvRWdo5Cd0FmLxE2LvoDc0RHa.jpg)
(1)是平坦的;
![平坦模](/img/5/8ff/wZwpmL0cjN5cjN5kDN2YjN1UTM1QDN5MjM5ADMwAjMwUzL5QzLygzLt92YucmbvRWdo5Cd0FmLwE2LvoDc0RHa.jpg)
![平坦模](/img/4/09a/wZwpmL1YzMwETO4MzM3QTN1UTM1QDN5MjM5ADMwAjMwUzLzMzLwIzLt92YucmbvRWdo5Cd0FmLwE2LvoDc0RHa.jpg)
(2)對每個有限生成理想成立。
![平坦模](/img/f/e45/wZwpmL2IzN5MjNzMjN2YjN1UTM1QDN5MjM5ADMwAjMwUzLzYzL0IzLt92YucmbvRWdo5Cd0FmL0E2LvoDc0RHa.jpg)
![平坦模](/img/f/8cb/wZwpmL2UTMzATN5QzN2YjN1UTM1QDN5MjM5ADMwAjMwUzL0czLyMzLt92YucmbvRWdo5Cd0FmLxE2LvoDc0RHa.jpg)
![平坦模](/img/a/b77/wZwpmL3EDM3YTMwkDN2YjN1UTM1QDN5MjM5ADMwAjMwUzL5QzLwEzLt92YucmbvRWdo5Cd0FmLzE2LvoDc0RHa.jpg)
![平坦模](/img/4/49c/wZwpmLzYDOzkTO1MjN2YjN1UTM1QDN5MjM5ADMwAjMwUzLzYzLyUzLt92YucmbvRWdo5Cd0FmL0E2LvoDc0RHa.jpg)
定理11令是投射的,,如果是平坦的,則。
![平坦模](/img/f/e45/wZwpmL2IzN5MjNzMjN2YjN1UTM1QDN5MjM5ADMwAjMwUzLzYzL0IzLt92YucmbvRWdo5Cd0FmL0E2LvoDc0RHa.jpg)
![平坦模](/img/4/55d/wZwpmLyEzNyAzM3gTN2YjN1UTM1QDN5MjM5ADMwAjMwUzL4UzL0YzLt92YucmbvRWdo5Cd0FmLzE2LvoDc0RHa.jpg)
![平坦模](/img/9/e91/wZwpmLwIjNyQzN0MTN0YzM1UTM1QDN5MjM5ADMwAjMwUzLzUzL3IzLt92YucmbvRWdo5Cd0FmL0E2LvoDc0RHa.jpg)
推論12令是投射模,且,則。