
❚ã♠ t➽t
❈❤♦
R
❧➭ ♠ét ✈➭♥❤ ◆♦❡t❤❡r ❣✐❛♦ ❤♦➳♥✱
a
❧➭ ♠ét ✐➤➟❛♥ ❝ñ❛
R
✈➭
M
❧➭ ♠ét
R
✲♠➠➤✉♥ ❤÷✉ ❤➵♥ s✐♥❤✳ ▼ô❝ t✐➟✉ ❝❤Ý♥❤ ❝ñ❛ ❧✉❐♥ ➳♥ ❧➭ t×♠ ♥❤÷♥❣ ➤✐Ò✉ ❦✐Ö♥
➤Ó ❝➳❝ ♠➠➤✉♥ ➤è✐ ➤å♥❣ ➤✐Ò✉ ➤Þ❛ ♣❤➢➡♥❣
Hi
a(•)
❝ã tÝ♥❤ ❝❤✃t ❝❤❰ r❛ ✈➭ ➳♣
❞ô♥❣ ♥ã ✈➭♦ ♥❤✐Ò✉ ✈✃♥ ➤Ò ❦❤➳❝ ♥❤❛✉ ❝ñ❛ ➜➵✐ sè ●✐❛♦ ❤♦➳♥✳ ▲✉❐♥ ➳♥ ➤➢î❝
❝❤✐❛ ❧➭♠ ❜è♥ ❝❤➢➡♥❣✳
❚r♦♥❣ ❈❤➢➡♥❣ ✶✱ tr➢í❝ ❤Õt ❝❤ó♥❣ t➠✐ ♥❤➽❝ ❧➵✐ ♠ét sè tÝ♥❤ ❝❤✃t ❝➡ ❜➯♥ ❝ñ❛
♠➠➤✉♥ ➤è✐ ➤å♥❣ ➤✐Ò✉ ➤Þ❛ ♣❤➢➡♥❣ ✈➭ ♣❤Ð♣ t♦➳♥ tr♦♥❣ ♠➠➤✉♥
Ext1
R(•,•)
✳ ➜Ó
❝❤ø♥❣ ♠✐♥❤ ♠ét ❞➲② ❦❤í♣ ♥❣➽♥
0→A→B→C→0
❧➭ ❝❤❰ r❛ ❝❤ó♥❣ t➠✐
❝❤ø♥❣ ♠✐♥❤ ♥ã ➤➵✐ ❞✐Ö♥ ❝❤♦ ♣❤➬♥ tö
0
❝ñ❛
Ext1
R(C, A)
✳ ❈✉è✐ ❝❤➢➡♥❣ ❝❤ó♥❣
t➠✐ ❝❤ø♥❣ ♠✐♥❤ ♠ét ➤Þ♥❤ ❧Ý ❝❤❰ r❛ ❝ñ❛ ➤è✐ ➤å♥❣ ➤✐Ò✉ ➤Þ❛ ♣❤➢➡♥❣ ✈í✐ ➤✐Ò✉
❦✐Ö♥
Hi
a(M)
❧➭ ❤÷✉ ❤➵♥ s✐♥❤ ✈í✐ ♠ä✐
i < t
♥➭♦ ➤ã✳ ▼ét sè ➳♣ ❞ô♥❣ ❝ñ❛ ➤Þ♥❤
❧Ý ❝❤❰ r❛ ♥➭② ✈➭♦ tÝ♥❤ æ♥ ➤Þ♥❤ ❝ñ❛ ❤Ö t❤❛♠ sè ❝ñ❛ ♠➠➤✉♥ ❈♦❤❡♥✲▼❛❝❛✉❧❛②
s✉② ré♥❣ ❝ò♥❣ ➤➢î❝ ➤➢❛ r❛✳
❚r♦♥❣ ❈❤➢➡♥❣ ✷✱ ❝❤ó♥❣ t➠✐ ➳♣ ❞ô♥❣ tÝ♥❤ ❝❤❰ r❛ ❝ñ❛ ➤è✐ ➤å♥❣ ➤✐Ò✉ ➤Þ❛
♣❤➢➡♥❣ ➤Ó ❝❤ø♥❣ ♠✐♥❤ ♠ét sè tÝ♥❤ ❝❤✃t æ♥ ➤Þ♥❤ ❝ñ❛ ❤Ö t❤❛♠ sè tèt ❝ñ❛ ❝➳❝
♠➠➤✉♥ ❈♦❤❡♥✲▼❛❝❛✉❧❛② s✉② ré♥❣ ❞➲②✳
❚r♦♥❣ ❈❤➢➡♥❣ ✸✱ ❝❤ó♥❣ t➠✐ ❧✉➠♥ ①Ðt ✈➭♥❤ ❝➡ së
(R, m)
❧➭ ➯♥❤ ➤å♥❣ ❝✃✉
❝ñ❛ ♠ét ✈➭♥❤ ❈♦❤❡♥✲▼❛❝❛✉❧❛② ➤Þ❛ ♣❤➢➡♥❣✳ ❈❤ó♥❣ t➠✐ ❝❤ø♥❣ ♠✐♥❤ tÝ♥❤ ❝❤❰
r❛ ❝ñ❛ ➤è✐ ➤å♥❣ ➤✐Ò✉ ➤Þ❛ ♣❤➢➡♥❣ t❤❡♦ ❝➳❝ ♣❤➬♥ tö t❤❛♠ sè
x∈b(M)3
✱ ë ➤➞②
b(M) = ∩d
x;i=1Ann(0 : xi)M/(x1,...,xi−1)M,
✈í✐
x=x1, ..., xd
❝❤➵② tr♦♥❣ t✃t ❝➯ ❝➳❝ ❤Ö t❤❛♠ sè ❝ñ❛
M
✳ ▼ét ➳♣ ❞ô♥❣ ➤➳♥❣
❝❤ó ý ❝ñ❛ ➤Þ♥❤ ❧Ý ❝❤❰ r❛ ♥➭② ❧➭ ❝❤ó♥❣ t➠✐ ➤➲ ①➞② ❞ù♥❣ ➤➢î❝ ♠ét ❧♦➵✐ ❜❐❝ ♠ë
ré♥❣ t❤❡♦ ♥❣❤Ü❛ ❝ñ❛ ❲✳ ❱❛s❝♦♥❝❡❧♦s ✈➭ ❣ä✐ ➤ã ❧➭ ❜❐❝ ❦❤➠♥❣ tré♥ ❧➱♥✳
❚r♦♥❣ ❈❤➢➡♥❣ ✹✱ ❝❤ó♥❣ t➠✐ ❝❤ø♥❣ ♠✐♥❤ ♠ét sè tÝ♥❤ ❝❤✃t ❤÷✉ ❤➵♥ ❝ñ❛ t❐♣
✐➤➟❛♥ ♥❣✉②➟♥ tè ❧✐➟♥ ❦Õt ❝ñ❛ ♠➠➤✉♥ ➤è✐ ➤å♥❣ ➤✐Ò✉ ➤Þ❛ ♣❤➢➡♥❣ ➤➬✉ t✐➟♥ ❦❤➠♥❣
❤÷✉ ❤➵♥ s✐♥❤ ✈➭ ❝ã t❐♣ ❣✐➳ ✈➠ ❤➵♥✳ ❈❤ó♥❣ t➠✐ ❝ò♥❣ ❝❤ø♥❣ ♠✐♥❤ tÝ♥❤ ❤÷✉ ❤➵♥
❝ñ❛ ♠ét sè t❐♣ ✐➤➟❛♥ ♥❣✉②➟♥ tè ❧✐➟♥ ❦Õt ❧✐➟♥ q✉❛♥ ✈í✐ ❝❤✐Ò✉ ❤÷✉ ❤➵♥ ❝ñ❛
M
t➢➡♥❣ ø♥❣ ✈í✐ ♠ét ✐➤➟❛♥
a
✳