✣❸■ ❍➴❈ ❚❍⑩■ ◆●❯❨➊◆
❚❘×❮◆● ✣❸■ ❍➴❈ ❙× P❍❸▼
✯✯✯✯✯✯✯✯✯✯✯✯✯✯✯✯
❍❖⑨◆● ❚❍➚ ◆●❹◆
❱➋ ❱❆■ ❚❘➪ ❈Õ❆ ❚❖⑩◆ ❚Û ❈❍■➌❯
❚❘❖◆● ❇⑨■ ❚❖⑩◆ ❇❻❚ ✣➃◆● ❚❍Ù❈ ❇■➌◆ P❍❹◆
▲❯❾◆ ❱❿◆ ❚❍❸❈ ❙➒ ❚❖⑩◆ ❍➴❈
❚❤→✐ ◆❣✉②➯♥ ✲ ♥➠♠ ✷✵✶✺
✣❸■ ❍➴❈ ❚❍⑩■ ◆●❯❨➊◆
❚❘×❮◆● ✣❸■ ❍➴❈ ❙× P❍❸▼
✯✯✯✯✯✯✯✯✯✯✯✯✯✯✯✯
❍❖⑨◆● ❚❍➚ ◆●❹◆
❱➋ ❱❆■ ❚❘➪ ❈Õ❆ ❚❖⑩◆ ❚Û ❈❍■➌❯
❚❘❖◆● ❇⑨■ ❚❖⑩◆ ❇❻❚ ✣➃◆● ❚❍Ù❈ ❇■➌◆ P❍❹◆
▲❯❾◆ ❱❿◆ ❚❍❸❈ ❙➒ ❚❖⑩◆ ❍➴❈
❈❤✉②➯♥ ♥❣➔♥❤✿ ❚❖⑩◆ ●■❷■ ❚➑❈❍
▼➣ sè✿ ✻✵✳✹✻✳✵✶✳✵✷
◆❣÷í✐ ❤÷î♥❣ ❞➝♥ ❦❤♦❛ ❤å❝✿
●❙✳ ❚❙❑❍✳ ▲➊ ❉Ô◆● ▼×❯
❚❤→✐ ◆❣✉②➯♥ ✲ ♥➠♠ ✷✵✶✺
▲í✐ ❝❛♠ ✤♦❛♥
✐
❚æ✐ ①✐♥ ❝❛♠ ✤♦❛♥ t♦➔♥ ❜ë ♥ë✐ ❞✉♥❣ ❧✉➟♥ ✈➠♥ ❧➔ ❝æ♥❣ tr➻♥❤ ♥❣❤✐➯♥
❝ù✉ ❝õ❛ r✐➯♥❣ tæ✐✱ ✤÷ñ❝ tê♥❣ ❤ñ♣ tø ❝→❝ t➔✐ ❧✐➺✉ t❤❛♠ ❦❤↔♦✳ ❈→❝ ❦➳t q✉↔
tr➻♥❤ ❜➔② tr♦♥❣ ❧✉➟♥ ✈➠♥ ❤♦➔♥ t♦➔♥ tr✉♥❣ t❤ü❝✱ ❦❤æ♥❣ s❛♦ ❝❤➨♣✱ trò♥❣
❧➦♣ ✈î✐ ❜➜t ❦➻ t➔✐ ❧✐➺✉ ♥➔♦ ❦❤→❝✳
❍å❝ ✈✐➯♥
❍♦➔♥❣ ❚❤à ◆❣➙♥
ớ ỡ
ữủ t t trữớ ồ sữ
rữợ tr ở ừ tổ ỷ
ớ ỡ t s s tợ ụ ữ t
ữớ trỹ t ữợ t t ú ù ở tổ
tr sốt q tr ự t
ổ ụ t ỡ ỏ s ồ
qỵ t ổ tr ồ ợ ồ
t t ủ ú ù ở tổ tr sốt q
tr ồ t ự t trữớ
tổ tọ ỏ t ỡ s s tợ ữớ t tr
tr ổ ở tổ tr sốt q
tr t ồ
ũ õ õ ữ ổ tr
ọ ỳ t sõt ổ rt ữủ ỳ ỵ
õ õ qỵ ừ t ổ ồ ữủ t
ỡ
tr trồ ỡ
t
ồ
✐✐✐
▼ö❝ ❧ö❝
▲í✐ ❝❛♠ ✤♦❛♥
✐
▲í✐ ❝↔♠ ì♥
✐✐
▼ö❝ ❧ö❝
✐✐✐
▼ð ✤➛✉
✶
✶ ❈→❝ ❦✐➳♥ t❤ù❝ ❝❤✉➞♥ ❜à
✷
✶✳✶✳ ❈→❝ ❦✐➳♥ t❤ù❝ ✈➲ ❦❤æ♥❣ ❣✐❛♥ ❍✐❧❜❡rt
✶✳✶✳✶✳ ❚➼❝❤ ✈æ ❤÷î♥❣ ✳ ✳ ✳ ✳ ✳ ✳ ✳ ✳ ✳
✶✳✶✳✷✳ ❑❤æ♥❣ ❣✐❛♥ t✐➲♥ ❍✐❧❜❡rt ✳ ✳ ✳
✶✳✶✳✸✳ ❑❤æ♥❣ ❣✐❛♥ ❍✐❧❜❡rt ✳ ✳ ✳ ✳ ✳ ✳
✶✳✷✳ ❈→❝ ❦✐➳♥ t❤ù❝ ✈➲ t➟♣ ❧ç✐✱ ❤➔♠ ❧ç✐ ✳ ✳
✶✳✷✳✶✳ ❚➟♣ ❧ç✐ ✳ ✳ ✳ ✳ ✳ ✳ ✳ ✳ ✳ ✳ ✳ ✳ ✳
✶✳✷✳✷✳ ❍➔♠ ❧ç✐ ✳ ✳ ✳ ✳ ✳ ✳ ✳ ✳ ✳ ✳ ✳ ✳
✶✳✷✳✸✳ ❈→❝ ✤à♥❤ ❧➼ t→❝❤ ✳ ✳ ✳ ✳ ✳ ✳ ✳ ✳
✶✳✷✳✹✳ ❉÷î✐ ✈✐ ♣❤➙♥ ✳ ✳ ✳ ✳ ✳ ✳ ✳ ✳ ✳
✳
✳
✳
✳
✳
✳
✳
✳
✳
✳
✳
✳
✳
✳
✳
✳
✳
✳
✳
✳
✳
✳
✳
✳
✳
✳
✳
✳
✳
✳
✳
✳
✳
✳
✳
✳
✳
✳
✳
✳
✳
✳
✳
✳
✳
✳
✳
✳
✳
✳
✳
✳
✳
✳
✳
✳
✳
✳
✳
✳
✳
✳
✳
✳
✳
✳
✳
✳
✳
✳
✳
✳
✳
✳
✳
✳
✳
✳
✳
✳
✳
✳
✳
✳
✳
✳
✳
✳
✳
✳
✳ ✷
✳ ✷
✳ ✸
✳ ✸
✳ ✽
✳ ✽
✳ ✶✵
✳ ✶✷
✳ ✶✷
✷ ❱❛✐ trá ❝õ❛ t♦→♥ tû ❝❤✐➳✉ ✤è✐ ✈î✐ ❜➔✐ t♦→♥ ❜➜t ✤➥♥❣ t❤ù❝
❜✐➳♥ ♣❤➙♥
✐✈
✷✳✶✳ ❚♦→♥ tû ❝❤✐➳✉ ✳ ✳ ✳ ✳ ✳ ✳ ✳ ✳ ✳ ✳ ✳ ✳ ✳ ✳ ✳ ✳ ✳ ✳ ✳ ✳ ✳ ✳ ✳ ✳
✷✳✷✳ ❇➔✐ t♦→♥ ❜➜t ✤➥♥❣ t❤ù❝ ❜✐➳♥ ♣❤➙♥ ✳ ✳ ✳ ✳ ✳ ✳ ✳ ✳ ✳ ✳ ✳ ✳ ✳
✷✳✷✳✶✳ P❤→t ❜✐➸✉ ❜➔✐ t♦→♥ ✳ ✳ ✳ ✳ ✳ ✳ ✳ ✳ ✳ ✳ ✳ ✳ ✳ ✳ ✳ ✳ ✳
✷✳✷✳✷✳ ❙ü tç♥ t↕✐ ♥❣❤✐➺♠ ✳ ✳ ✳ ✳ ✳ ✳ ✳ ✳ ✳ ✳ ✳ ✳ ✳ ✳ ✳ ✳ ✳ ✳
✷✳✸✳ P❤÷ì♥❣ ♣❤→♣ ❝❤✐➳✉ ❣✐↔✐ ❜➔✐ t♦→♥ ❜➜t ✤➥♥❣ t❤ù❝ ❜✐➳♥ ♣❤➙♥
✷✳✸✳✶✳ P❤÷ì♥❣ ♣❤→♣ ❝❤✐➳✉ ❝ì ❜↔♥ ✳ ✳ ✳ ✳ ✳ ✳ ✳ ✳ ✳ ✳ ✳ ✳ ✳
✷✳✸✳✷✳ P❤÷ì♥❣ ♣❤→♣ ✤↕♦ ❤➔♠ t➠♥❣ ❝÷í♥❣ ✳ ✳ ✳ ✳ ✳ ✳ ✳ ✳
✶✻
✶✻
✷✵
✷✵
✷✷
✷✼
✷✽
✸✵
❑➳t ❧✉➟♥
✸✸
❚➔✐ ❧✐➺✉ t❤❛♠ ❦❤↔♦
✸✸
tỷ ởt t ỗ õ ởt ợ q trồ
tr t t t ự ử r ổ rt
tỹ t tỷ ổ tỗ t õ t t tũ õ t
t ự qt t tr ỹ
ữ tr ỵ tt tố ữ t t tự
t
t t tự ởt ợ t q trồ õ
ự ử tr ữỡ tr t t ỵ
tt ụ ữ tr tố ữ ữợ ự tr
t t tự sỹ tỗ t ữỡ
tr õ ữỡ ỹ t tỷ ỵ t
ở tữớ ữủ sỷ ử ữủ tr ừ tr t
ử ợ t trỏ ừ t tỷ
tr ổ rt ử ợ t t
ử t
ỷ ử t tỷ t ủ ợ ỵ t ở rr
ự sỹ tỗ t ừ t t tự
ợ t ữỡ ỹ t tỷ t
t tự õ ữỡ ỡ
t t tự ỡ õ t st
ữỡ t ữớ t t
tự ỡ
✷
❈❤÷ì♥❣ ✶
❈→❝ ❦✐➳♥ t❤ù❝ ❝❤✉➞♥ ❜à
❚r♦♥❣ ❝❤÷ì♥❣ ♥➔②✱ t❛ s➩ ♥❤➢❝ ❧↕✐ ♠ët sè ❦✐➳♥ t❤ù❝ q✉❛♥ trå♥❣ ❧➔♠
♥➲♥ t↔♥❣ ✤➸ ♥❣❤✐➯♥ ❝ù✉ ❝❤÷ì♥❣ s❛✉ ✤â ❧➔ ❝→❝ ❦✐➳♥ t❤ù❝ ❝ì ❜↔♥ ✈➲ ❦❤æ♥❣
❣✐❛♥ ❍✐❧❜❡rt ✈➔ ❣✐↔✐ t➼❝❤ ❧ç✐✳ ❈→❝ ♥ë✐ ❞✉♥❣ tr♦♥❣ ❝❤÷ì♥❣ ✤÷ñ❝ tr➼❝❤ ❞➝♥
tø ❝→❝ t➔✐ ❧✐➺✉ t❤❛♠ ❦❤↔♦ ❬✶❪✱ ❬✷❪✱ ❬✸❪✳
✶✳✶✳ ❈→❝ ❦✐➳♥ t❤ù❝ ✈➲ ❦❤æ♥❣ ❣✐❛♥ ❍✐❧❜❡rt
✶✳✶✳✶✳ ❚➼❝❤ ✈æ ❤÷î♥❣
✣à♥❤ ♥❣❤➽❛ ✶✳✶✳✶✳ ❈❤♦ H ❧➔ ♠ët ❦❤æ♥❣ ❣✐❛♥ ✈❡❝tì tr➯♥ tr÷í♥❣ sè t❤ü❝
R✳
❚➼❝❤ ✈æ ❤÷î♥❣ tr➯♥ H ❧➔ ♠ët →♥❤ ①↕ ①→❝ ✤à♥❤ ♥❤÷ s❛✉✿
., . : H × H → R
(x, y) → x, y
t❤♦↔ ♠➣♥ ❝→❝ ✤✐➲✉ ❦✐➺♥ s❛✉ ✤➙②✿
✶✳
✷✳
✸✳
x, x ≥ 0, ∀x ∈ H, x, x = 0
❦❤✐ ✈➔ ❝❤➾ ❦❤✐ x = 0.
x, y = y, x , ∀x, y ∈ H.
x + y, z = x, z + y, z , ∀x, y, z ∈ H.
✹✳
x, y
✸
λx, y = λ x, y , ∀x, y ∈ H, λ ∈ R.
✤÷ñ❝ ❣å✐ ❧➔ t➼❝❤ ✈æ ❤÷î♥❣ ❝õ❛ ❤❛✐ ✈❡❝tì x ✈➔ y tr➯♥ H✳
✶✳✶✳✷✳ ❑❤æ♥❣ ❣✐❛♥ t✐➲♥ ❍✐❧❜❡rt
✣à♥❤ ♥❣❤➽❛ ✶✳✶✳✷✳ ❈➦♣ H, ., . ✱ tr♦♥❣ ✤â H ❧➔ ♠ët ❦❤æ♥❣ ❣✐❛♥ ✈❡❝tì
tr➯♥ tr÷í♥❣ sè t❤ü❝ R✱ ✳✱✳ ❧➔ ♠ët t➼❝❤ ✈æ ❤÷î♥❣ tr➯♥ H ✤÷ñ❝ ❣å✐ ❧➔
❦❤æ♥❣ ❣✐❛♥ t✐➲♥ ❍✐❧❜❡rt ✭❤❛② ❝á♥ ❣å✐ ❧➔ ❦❤æ♥❣ ❣✐❛♥ ❯♥✐t❛✮✳
✣à♥❤ ❧þ ✶✳✶ ✭❇➜t ✤➥♥❣ t❤ù❝ ❈❛✉❝❤②✲ ❙❝❤✇❛rt③✮✳ ❚r♦♥❣ ❦❤æ♥❣ ❣✐❛♥ t✐➲♥
❍✐❧❜❡rt H✱ ✈î✐ ♠å✐ x, y ∈ H t❛ ❧✉æ♥ ❝â ❜➜t ✤➥♥❣ t❤ù❝ s❛✉✿
| x, y |2 ≤ x, x y, y .
(1.1)
❉➜✉ ❜➡♥❣ ❝õ❛ ❜➜t ✤➥♥❣ t❤ù❝ ①↔② r❛ ❦❤✐ ✈➔ ❝❤➾ ❦❤✐ x, y ♣❤ö t❤✉ë❝
t✉②➳♥ t➼♥❤✳
▼è✐ q✉❛♥ ❤➺ ❣✐ú❛ ❝❤✉➞♥ ✈➔ t➼❝❤ ✈æ ❤÷î♥❣ ✤÷ñ❝ t❤➸ ❤✐➸♥ q✉❛ ✤à♥❤ ❧➼ s❛✉✳
✣à♥❤ ❧þ ✶✳✷✳ ▼å✐ ❦❤æ♥❣ ❣✐❛♥ t✐➲♥ ❍✐❧❜❡rt H ✤➲✉ ❧➔ ❦❤æ♥❣ ❣✐❛♥ t✉②➳♥
t➼♥❤ ✤à♥❤ ❝❤✉➞♥✱ ✈î✐ ❝❤✉➞♥ ✤÷ñ❝ ①→❝ ✤à♥❤ ❜ð✐ ❝æ♥❣ t❤ù❝
x =
x, x
∀x ∈ H.
(1.2)
❈❤✉➞♥ ♥➔② ✤÷ñ❝ ❣å✐ ❧➔ ❝❤✉➞♥ ❝↔♠ s✐♥❤ tø t➼❝❤ ✈æ ❤÷î♥❣✳ ❚❤❡♦ ✤à♥❤
❧þ tr➯♥✱ ❦❤æ♥❣ ❣✐❛♥ t✐➲♥ ❍✐❧❜❡rt ❧➔ ❦❤æ♥❣ ❣✐❛♥ t✉②➳♥ t➼♥❤ ✤à♥❤ ❝❤✉➞♥✱ ❝â
t❤➸ ✤➛② ✤õ ❤♦➦❝ ❦❤æ♥❣ ✤➛② ✤õ✳
✶✳✶✳✸✳ ❑❤æ♥❣ ❣✐❛♥ ❍✐❧❜❡rt
✣à♥❤ ♥❣❤➽❛ ✶✳✶✳✸✳ ◆➳✉ H ❧➔ ♠ët ❦❤æ♥❣ ❣✐❛♥ t✐➲♥ ❍✐❧❜❡rt ✈➔ ✤➛② ✤õ ✤è✐
✈î✐ ❝❤✉➞♥ ❝↔♠ s✐♥❤ tø t➼❝❤ ✈æ ❤÷î♥❣ t❤➻ ✤÷ñ❝ ❣å✐ ❧➔ ❦❤æ♥❣ ❣✐❛♥ ❍✐❧❜❡rt✳
❱➼ ❞ö ✶✳✶✳
ổ R ổ rt tỹ ợ t ổ ữợ
n
n
x, y =
xi yi ,
i=1
tr õ x, y Rn, x = (x1, ã ã ã , xn),
s tứ t ổ ữợ
y = (y1 , ã ã ã , yn ) Rn ,
n
n
x = x, x =
2
xi xi =
x .
i=1
i=1
ổ
n
2
2
l = x = {xn }n K :
x < +
n=1
ởt ổ rt ợ t ổ ữợ
x, y =
x n yn ,
n=1
s
x
x =
2
n=1
ợ x, y l2, x = (xn),
y = (yn ), n N
ữ t õ ổ rt ởt ổ
õ ổ õ t t ừ ởt ổ
õ t ởt số t t ợ s
ỵ sỷ H ởt ổ t rt õ t ổ
ữợ ởt số tử tr H ì H.
ỵ ợ ồ x, y tở ổ t rt H t ổ õ
tự s
x+y
2
+ xy
2
= 2( x
2
2
+ y ).
(1.3)
ử tự tỡ x y x z
t õ q s
q sỷ H ởt ổ t rt x, y, z H
õ t õ tự s
2( x y
2
2
+ xz )=4 x
t
y+z
2
+ yz .
2
tự qt ởt t t q tở tr ồ
tờ ữỡ ữớ ừ ởt
tờ ữỡ ừ ỵ õ õ õ t
tự
tự ụ ừ ữ ữủ t ổ
ữợ ổ ữủ H ổ
tr õ tự ữủ t ợ
ồ tỷ tở H t tr H s tỗ t ởt t ổ ữợ s
r tr ổ H ữủ t
q ỵ s
ỵ sỷ H, . ởt ổ tr trữớ
K, tr õ tự ú ợ ồ x, y H
tự
2
2
2
2
x+y
+ xy
=2
x
+ y
.
2
xy
2
õ ợ trữớ R t t
x, y = p(x, y) =
t
., .
1
4
x+y
,
(1.4)
ởt t ổ ữợ tr H t õ
2
x, x = x ,
x H.
ợ ờ t ừ ổ rt s ợ ổ
tr ổ rt t ổ ữợ
t trỹ trỹ r s ú t s
ởt số t t ỡ ởt số ử q
✻
✣à♥❤ ♥❣❤➽❛ ✶✳✶✳✹✳ ❈❤♦ H ❧➔ ♠ët ❦❤æ♥❣ ❣✐❛♥ t✐➲♥ ❍✐❧❜❡rt✳
✶✳ ❍❛✐ ♣❤➛♥ tû x, y ∈ H ✤÷ñ❝ ❣å✐ ❧➔ trü❝ ❣✐❛♦ ✈î✐ ♥❤❛✉✱ ❦➼ ❤✐➺✉ ❧➔ x⊥y
♥➳✉ x, y = 0.
✷✳ ❍❛✐ t➟♣ A, B ⊂ H ✤÷ñ❝ ❣å✐ ❧➔ trü❝ ❣✐❛♦ ✈î✐ ♥❤❛✉✱ ❦➼ ❤✐➺✉ ❧➔ A⊥B
♥➳✉ ✈î✐ ♠é✐ x ∈ A, y ∈ B t❛ ❝â x⊥y ✱ tù❝ ❧➔ x, y = 0.
✸✳ ❚❛ ♥â✐ ♣❤➛♥ tû x ❝õ❛ H trü❝ ❣✐❛♦ ✈î✐ t➟♣ ❝♦♥ A ❝õ❛ H ♥➳✉ ✈î✐
∀y ∈ A t❛ ❝â x, y = 0 ✈➔ ✤÷ñ❝ ❦➼ ❤✐➺✉ ❧➔ x⊥A✳
✹✳ P❤➛♥ ❜ò trü❝ ❣✐❛♦ ❝õ❛ A ∈ H ❧➔ t➟♣ ❤ñ♣ ❝→❝ ♣❤➛♥ tû t❤♦↔ ♠➣♥✿
A⊥ = x ∈ H : x⊥y, ∀y ∈ A ,
♥➳✉ A = {x}, t❛ ✈✐➳t x⊥ t❤❛② ❝❤♦ {x}⊥✳
✺✳ ❍å O ⊂ H ✤÷ñ❝ ❣å✐ ❧➔ ❤➺ trü❝ ❣✐❛♦ ♥➳✉ ❝→❝ ♣❤➛♥ tû ❝õ❛ O ✤æ✐ ♠ët
trü❝ ❣✐❛♦ ✈î✐ ♥❤❛✉✱ tù❝ ❧➔ ∀x, y ∈ O, x = y, t❤➻ x, y = 0✳
✻✳ ❍å E = {ei}i∈∧ ⊂ H ✤÷ñ❝ ❣å✐ ❧➔ ❤➺ trü❝ ❝❤✉➞♥ ♥➳✉ E ❧➔ ♠ët ❤➺
trü❝ ❣✐❛♦ ✈➔ ei = 1, ∀ei ∈ E✳ ◆❤÷ ✈➟② E = {ei}i∈∧ ❧➔ ♠ët ❤➺ trü❝
❝❤✉➞♥ ♥➳✉
0 ♥➳✉ i = j
ei , ej =
1 ♥➳✉ i = j.
✣à♥❤ ❧þ ✶✳✻✳ ◆➳✉ A ❧➔ ♠ët t➟♣ ❤ñ♣ tr♦♥❣ ❦❤æ♥❣ ❣✐❛♥ t✐➲♥ ❍✐❧❜❡rt H t❤➻
A⊥ ❧➔ ♠ët ❦❤æ♥❣ ❣✐❛♥ ❝♦♥ ✤â♥❣ ❝õ❛ H.
✣à♥❤ ❧þ ✶✳✼✳ ❍➺ trü❝ ❝❤✉➞♥ ❧➔ ♠ët ❤➺ ✤ë❝ ❧➟♣ t✉②➳♥ t➼♥❤✱ tù❝ ❧➔ ♠å✐ ❤å
❝♦♥ ❤ú✉ ❤↕♥ ❝õ❛ ❤➺ ❧➔ ✤ë❝ ❧➟♣ t✉②➳♥ t➼♥❤✳
◆❣÷ñ❝ ❧↕✐✱ tø ♠ët ❤➺ ✤➳♠ ✤÷ñ❝ ❝→❝ ♣❤➛♥ tû ✤ë❝ ❧➟♣ t✉②➳♥ t➼♥❤✱ t❛
❝â t❤➸ ①➙② ❞ü♥❣ ✤÷ñ❝ ♠ët ❤➺ trü❝ ❣✐❛♦ t❤❡♦ ♣❤÷ì♥❣ ♣❤→♣ trü❝ ❣✐❛♦ ❤♦→
❙❝❤♠✐❞t✳
▼➺♥❤ ✤➲ ✶✳✶✳ ❚r♦♥❣ ❦❤æ♥❣ ❣✐❛♥ t✐➲♥ ❍✐❧❜❡rt H t❛ ❧✉æ♥ ❝â✿
✶✳ ◆➳✉ x⊥y t❤➻ y⊥x✱ x⊥x ❦❤✐ ✈➔ ❝❤➾ ❦❤✐ x = 0.
✷✳ ◆➳✉ x⊥y ✈î✐ ♠å✐ y ∈ H t❤➻ x = 0.
✼
✸✳ ◆➳✉ x⊥yi✱ ✈î✐ ♠é✐ i ∈ {1, · · · , n} t❤➻ x⊥(λ1y1, · · · , λnyn).
✹✳ ◆➳✉ x⊥yn ✈î✐ ♠å✐ n ✈➔ limn→∞ yn = y t❤➻ x⊥y.
✺✳ ◆➳✉ A trò ♠➟t tr♦♥❣ H t❤➻ M ⊥ = {0}✳ ❚ù❝ ❧➔ x⊥M ❦❤✐ ✈➔ ❝❤➾ ❦❤✐
x = 0.
✻✳ ◆➳✉ x⊥y t❤➻ x + y 2 = x 2 + y 2✱ tê♥❣ q✉→t ❤ì♥✱ ♥➳✉ x1, · · · , xn
✤æ✐ ♠ët trü❝ ❣✐❛♦ ✈î✐ ♥❤❛✉ t❤➻ t❛ ❝â ✤➥♥❣ t❤ù❝ P②t❤❛❣♦r❡
x1 + · · · + xn
2
= x1
2
2
+ · · · + xn
▼ð rë♥❣ ✤➥♥❣ t❤ù❝ P②t❤❛❣♦r❡ t❛ ❝â ✤à♥❤ ❧➼ s❛✉✳
✣à♥❤ ❧þ ✶✳✽✳
❈❤♦ {xn, n ∈ N } ❧➔ ❤➺ trü❝ ❣✐❛♦ tr♦♥❣ H t❤➻ ❝❤✉é✐
∞
∗
xn
❤ë✐ tö
n=1
✳
❦❤✐ ✈➔ ❝❤➾ ❦❤✐ ❝❤✉é✐
∞
xn
2
❤ë✐ tö. ❑❤✐ ✤â
n=1
∞
∞
xn
2
2
=
n=1
xn .
n=1
✣➦❝ ❜✐➺t✱ ♥➳✉ {en, n ∈ N } ❧➔ ❤➺ trü❝ ❝❤✉➞♥ tr♦♥❣ H t❤➻ ❝❤✉é✐
∞
∗
λn en
n=1
❤ë✐ tö ❦❤✐ ✈➔ ❝❤➾ ❦❤✐ ❝❤✉é✐
∞
λn
2
❤ë✐ tö ✈➔
∞
∞
λn en
2
n=1
n=1
2
=
en .
n=1
✣à♥❤ ❧þ ✶✳✾✳ ❈❤♦ {e1, · · · , en} ❧➔ ♠ët ❤➺ trü❝ ❝❤✉➞♥ tr♦♥❣ ❦❤æ♥❣ ❣✐❛♥
❍✐❧❜❡rt H✱ ❦➼ ❤✐➺✉ A ❧➔ ❦❤æ♥❣ ❣✐❛♥ ❝♦♥ s✐♥❤ ❜ð✐ ❤➺ ✈❡❝tì {e1, · · · , en}✳
❑❤✐ ✤â ✈î✐ ♠é✐ x ∈ H t❛ ❝â y =
n
xi , ei ei
❧➔ ❤➻♥❤ ❝❤✐➳✉ trü❝ ❣✐❛♦
i=1
❝õ❛ x ❧➯♥ ❦❤æ♥❣ ❣✐❛♥ ❝♦♥ A ✈➔
n
xi , ei
i=1
2
2
≤ x .
✽
✶✳✷✳ ❈→❝ ❦✐➳♥ t❤ù❝ ✈➲ t➟♣ ❧ç✐✱ ❤➔♠ ❧ç✐
❚❛ ♥❤➢❝ ❧↕✐ ♠ët sè ❦✐➳♥ t❤ù❝ ❝ì ❜↔♥ ❝õ❛ ❣✐↔✐ t➼❝❤ ❧ç✐ ♥❤÷ t➟♣ ❧ç✐✱
❤➔♠ ❧ç✐✱ ❞÷î✐ ✈✐ ♣❤➙♥✱. . . tr♦♥❣ ❦❤æ♥❣ ❣✐❛♥ ❍✐❧❜❡rt H✳
✶✳✷✳✶✳ ❚➟♣ ❧ç✐
✣à♥❤ ♥❣❤➽❛ ✶✳✷✳✶✳ ❈❤♦ a ∈ H✱ ❦❤✐ ✤â t❛ ❝â ❝→❝ ✤à♥❤ ♥❣❤➽❛ s❛✉✿
✶✳ ❚➟♣ A ✤÷ñ❝ ❣å✐ ❧➔ t➟♣ ❛❢❢✐♥❡ ♥➳✉
(1 − λ)x + λy ∈ A
∀x, y ∈ A, ∀λ ∈ R.
✷✳ ●✐❛♦ ❝õ❛ t➜t ❝↔ ❝→❝ t➟♣ ❛❢❢✐♥❡ ❝❤ù❛ t➟♣ ❆ ✤÷ñ❝ ❣å✐ ❧➔ ❜❛♦ ❛❢❢✐♥❡ ❝õ❛
❆✱ ✈➔ ✤÷ñ❝ ❦➼ ❤✐➺✉ ❧➔ af f A.
✸✳ P❤➛♥ tr♦♥❣ ❝õ❛ t➟♣ A✱ ❦➼ ❤✐➺✉ ❧➔ intA = x ∈ H : ∃ > 0, x + B ⊂
A ✳
P❤➛♥ tr♦♥❣ t÷ì♥❣ ✤è✐ ❝õ❛ t➟♣ A ❧➔ ♣❤➛♥ tr♦♥❣ ❝õ❛ A tr♦♥❣ af f A✱
✤÷ñ❝ ❦➼ ❤✐➺✉ ❧➔ riA = x ∈ af f A : ∃ > 0, (x + B) ∩ af f A ⊂ A .
✣à♥❤ ♥❣❤➽❛ ✶✳✷✳✷✳ ▼ët t➟♣ A ⊂ H ✤÷ñ❝ ❣å✐ ❧➔ t➟♣ ❧ç✐ ♥➳✉
∀a, b ∈ A, ∀λ ∈ [0, 1] t❛ ❝â λa + (1 − λ)b ∈ A.
✣à♥❤ ♥❣❤➽❛ ✶✳✷✳✸✳ ●✐↔ sû A ⊂ H, a, b ∈ A✳ ✣♦↕♥ t❤➥♥❣ ♥è✐ ❤❛✐ ✤✐➸♠
a, b ✤÷ñ❝ ✤à♥❤ ♥❣❤➽❛ ♥❤÷ s❛✉✿
[a, b] = x ∈ A : x = λa + (1 − λ)b, λ ∈ [0, 1] .
◆❤➟♥ ①➨t ✶✳✷✳ ❱➲ ♠➦t ❤➻♥❤ ❤å❝✱ ✤à♥❤ ♥❣❤➽❛ t➟♣ ❧ç✐ ❝â ♥❣❤➽❛ r➡♥❣✱ ♥➳✉
❤❛✐ ✤✐➸♠ ❜➜t ❦➻ t❤✉ë❝ ❆✱ ❝↔ ✤♦↕♥ t❤➥♥❣ ♥è✐ ❤❛✐ ✤✐➸♠ ➜② ❝ô♥❣ ♥➡♠ trå♥
tr♦♥❣ ❆✳
❱➼ ❞ö ✶✳✷✳
✶✳ ❚➟♣ ré♥❣ ❧➔ t➟♣ ❧ç✐✳
✷✳ ❚➟♣ ❝❤➾ ❝❤ù❛ ♠ët ✤✐➸♠ ❞✉② ♥❤➜t ❧➔ t➟♣ ❧ç✐✳
r t tr ổ ồ q tở
ữ t t ỳ t ở ỳ t
trỏ . . . ỳ t ỗ
õ
ỗ õ ợ ở ợ ởt số tỹ
ừ t ỗ t ỗ
t ừ t ỗ q t t t ỗ
x H ữủ ồ tờ ủ ỗ ừ x1, ã ã ã , xn H tỗ t
n
i > 0, i = 1, ã ã ã , n,
n
i = 1 sao cho x =
i=1
i xi .
i=1
sỷ A H ừ tt t ỗ ự A ữủ ồ
ỗ ừ t A coA ừ tt t ỗ õ
ự A ữủ ồ ỗ õ ừ t A coA
K H ữủ ồ õ õ t
x K, > 0, x K.
K ữủ ồ õ õ t x0 K x0 õ õ t
õ K õ t ữủ ồ õ ỗ K ởt t ỗ tự
x, y K, , à > 0 a + ày K.
tỡ x H ữủ ồ t ừ t ỗ A
t x0 A
x , x x0 0,
x A.
tt tỡ t ừ t ỗ A t x0 A ữủ ồ õ
t ừ A t x0
NA (x0 ) = x H : x , x x0 0, x A .
✶✵
✶✳✷✳✷✳ ❍➔♠ ❧ç✐
●✐↔ sû A ∈ H ❧➔ t➟♣ ❧ç✐ ❦❤→❝ ré♥❣ ✈➔ →♥❤ ①↕ f : A → R ∪ {+∞}✳
✣à♥❤ ♥❣❤➽❛ ✶✳✷✳✼✳
✶✳ ❚r➯♥ ✤ç t❤à ❝õ❛ ❤➔♠ f ✱ ❦➼ ❤✐➺✉ ❧➔ epif ✱ ✤÷ñ❝ ✤à♥❤ ♥❣❤➽❛ ♥❤÷ s❛✉✿
epif = (x, r) ∈ A × R : f (x) ≤ r .
✷✳ ▼✐➲♥ ❤ú✉ ❤✐➺✉ ✭♠✐➲♥ ①→❝ ✤à♥❤✮ ❝õ❛ ❤➔♠ f ✱ ❦➼ ❤✐➺✉ ❧➔ domf ✱ ✤÷ñ❝
✤à♥❤ ♥❣❤➽❛ ♥❤÷ s❛✉✿
domf = x ∈ A : f (x) < +∞ .
✸✳ ❍➔♠ f ✤÷ñ❝ ❣å✐ ❧➔ ❝❤➼♥❤ t❤÷í♥❣ ♥➳✉ domf = ∅ ✈➔ f (x) > −∞✱ ✈î✐
♠å✐ x ∈ A✳
✹✳ ❍➔♠ f ✤÷ñ❝ ❣å✐ ❧➔ ❤➔♠ ❧ç✐ tr➯♥ A ♥➳✉
f (λx + (1 − λ)y) ≤ λf (x) + (1 − λ)f (y), ∀x, y ∈ A, ∀λ ∈ [0, 1].
✺✳ ❍➔♠ f ✤÷ñ❝ ❣å✐ ❧➔ ❤➔♠ ❧ç✐ ♥❣➦t tr➯♥ A ♥➳✉
f (λx + (1 − λ)y) < λf (x) + (1 − λ)f (y), ∀x, y ∈ A, ∀λ ∈ [0, 1].
✻✳ ❍➔♠ f ✤÷ñ❝ ❣å✐ ❧➔ ❤➔♠ ❧ç✐ ♠↕♥❤ tr➯♥ A ✈î✐ ❤➺ sè α > 0 ♥➳✉
1
f (λx + (1 − λ)y) ≤ λf (x) + (1 − λ)f (y) − λ(1 − λ)α x − y
2
2
✈î✐ ♠å✐ x, y ∈ A, ♠å✐ λ ∈ [0, 1].
✼✳ ❍➔♠ f ✤÷ñ❝ ❣å✐ ❧➔ ❤➔♠ ❧ã♠ ✭t÷ì♥❣ ù♥❣ ❧ã♠ ♥❣➦t✱ ❧ã♠ ♠↕♥❤ ✈î✐ ❤➺
sè α > 0✮ tr➯♥ A ♥➳✉ −f ❧➔ ❤➔♠ ❧ç✐ ✭t÷ì♥❣ ù♥❣ ❧ç✐ ♥❣➦t✱ ❧ç✐ ♠↕♥❤
✈î✐ ❤➺ sè α > 0✮ tr➯♥ A✳
❱➼ ❞ö ✶✳✸✳
✶✳ ❍➔♠ ❛❢❢✐♥❡ f (x) =
♠ët ❤➔♠ ❧ç✐✳
a, b + α,
✈î✐ ∀ a ∈ H∗, ∀ b ∈ H, ∀α ∈ R ❧➔
f (x) =
x =
x, x , x Rn
ởt ỗ
õ t ố ợ ỗ P ự
ữủ s r trỹ t tứ
f1, ã ã ã , fn ỗ tữớ õ
s ỗ
n
fi (x) = f1 + ã ã ã + fn (x).
i=1
n
n
fi = f1 (x1 ) + ã ã ã + fn (xn ) : xi H,
xi = x .
x=1
i=1
tr t ỗ ừ ởt số t õ s
f (x) ừ f t x
sỷ A H f : A H f
tử õ f ỗ
f (x) f (y) f (x), x y , ợ ồ x, y A.
sỷ f, I ỗ tr H õ
tr ừ f ỗ I f (x) = sup f(x)
I
f : H R ữủ ồ st ữỡ
t x0 H
tỗ t U ừ x0 tỗ t số K > 0 s
|f (x) f (x )| K x x
x, y U.
(1.6)
f ữủ ồ st ữỡ tr t A H f st
ữỡ t ồ x A
f ữủ ồ st ợ số st K tr t A H
ú ợ ồ x, x A.
✶✷
▼è✐ q✉❛♥ ❤➺ ❣✐ú❛ ❤➔♠ ❧ç✐ ✈➔ ❤➔♠ ▲✐♣s❝❤✐t③ ✤à❛ ♣❤÷ì♥❣ ✤÷ñ❝ t❤➸
❤✐➺♥ q✉❛ ✤à♥❤ ❧➼ s❛✉✳
✣à♥❤ ❧þ ✶✳✶✵✳ ◆➳✉ f ❧➔ ❤➔♠ ❧ç✐ ✈➔ ❜à ❝❤➦♥ tr➯♥ tr♦♥❣ ♠ët ❧➙♥ ❝➟♥ ❝õ❛
x ∈ H t❤➻ f ❧➔ ▲✐♣s❝❤✐t③ ✤à❛ ♣❤÷ì♥❣ t↕✐ x✳
✶✳✷✳✸✳ ❈→❝ ✤à♥❤ ❧➼ t→❝❤
✣à♥❤ ♥❣❤➽❛ ✶✳✷✳✾✳ ●✐↔ sû H ❧➔ ❦❤æ♥❣ ❣✐❛♥ ❍✐❧❜❡rt✱ H∗ ❧➔ ❦❤æ♥❣ ❣✐❛♥
❧✐➯♥ ❤ñ♣ ❝õ❛ H✳ ▲➜② x∗ ∈ H∗,
x∗ = 0, α ∈ R✱
t❛ ❝â
H(x∗ , α) = x ∈ H : x∗ , x = α
✤÷ñ❝ ❣å✐ ❧➔ ♠ët s✐➯✉ ♣❤➥♥❣ tr♦♥❣ H✳
✣à♥❤ ♥❣❤➽❛ ✶✳✷✳✶✵✳ ❈❤♦ ❤❛✐ t➟♣ A, B
∗
∈ H✱
t❛ ♥â✐ r➡♥❣ s✐➯✉ ♣❤➥♥❣
H(x , α) :
✶✳ t→❝❤ ❤❛✐ t➟♣ A ✈➔ B ♥➳✉ x∗, a ≤ α ≤ x∗, b , ∀a ∈ A, b ∈ B.
✷✳ t→❝❤ ♥❣➦t ❤❛✐ t➟♣ A ✈➔ B ♥➳✉ x∗, a < α < x∗, b , ∀a ∈ A, b ∈ B.
✸✳ t→❝❤ ♠↕♥❤ ❤❛✐ t➟♣ A ✈➔ B ♥➳✉
sup x∗ , a < α < inf x∗ , b .
b∈B
a∈A
✣à♥❤ ❧þ ✶✳✶✶ ✭✣à♥❤ ❧➼ t→❝❤ t❤ù ♥❤➜t✮✳ ❈❤♦ A ✈➔ B ❧➔ ❤❛✐ t➟♣ ❧ç✐ ❦❤→❝
ré♥❣ tr♦♥❣ ❦❤æ♥❣ ❣✐❛♥ ❍✐❧❜❡rt H✱ A ∩ B = ∅✳ ❑❤✐ ✤â ❝â ♠ët s✐➯✉ ♣❤➥♥❣
t→❝❤ A ✈➔ B ✳
✣à♥❤ ❧þ ✶✳✶✷ ✭✣à♥❤ ❧➼ t→❝❤ t❤ù ❤❛✐✮✳ ❈❤♦ A ✈➔ B ❧➔ ❤❛✐ t➟♣ ❧ç✐ ✤â♥❣
❦❤→❝ ré♥❣ tr♦♥❣ ❦❤æ♥❣ ❣✐❛♥ ❍✐❧❜❡rt H ✈➔ A ∩ B = ∅✳ ❚r♦♥❣ ✤â ❝â ➼t ♥❤➜t
♠ët t➟♣ ❧➔ ❝♦♠♣❛❝t✳ ❑❤✐ ✤â✱ ♠ët s✐➯✉ ♣❤➥♥❣ ❝â t❤➸ t→❝❤ ♠↕♥❤ ❤❛✐ t➟♣ A
✈➔ B ✳
✶✳✷✳✹✳ ❉÷î✐ ✈✐ ♣❤➙♥
✣à♥❤ ♥❣❤➽❛ ✶✳✷✳✶✶✳ ●✐↔ sû f ❧➔ ❤➔♠ ❧ç✐ tr➯♥ H✳ ✣↕♦ ❤➔♠ ❝õ❛ ❤➔♠ f
t❤❡♦ ♣❤÷ì♥❣ d t↕✐ x0✱ ❦➼ ❤✐➺✉ ❧➔
f (x0 , d) = lim
λ↓0
f (x0 + λd) − f (x0 )
λ
✶✸
♥➳✉ ❣✐î✐ ❤↕♥ ♥➔② tç♥ t↕✐ ✭❝â t❤➸ ❤ú✉ ❤↕♥ ❤♦➦❝ ❜➡♥❣ ±∞✮✳
◆❤➟♥ ①➨t ✶✳✸✳ ❍➔♠ f (x0, d) ❧➔ ❤➔♠ t❤✉➛♥ ♥❤➜t ❞÷ì♥❣✳
✣à♥❤ ♥❣❤➽❛ ✶✳✷✳✶✷✳ ●✐↔ sû f ❧➔ ❤➔♠ ❧ç✐ tr➯♥ H✳P❤✐➳♠ ❤➔♠ x∗ ∈ H
✤÷ñ❝ ❣å✐ ❧➔ ❞÷î✐ ❣r❛❞✐❡♥t ❝õ❛ ❤➔♠ f t↕✐ x ∈ H ♥➳✉
f (x) − f (x) ≥ x∗ , x − x
∀x ∈ H.
✣à♥❤ ♥❣❤➽❛ ✶✳✷✳✶✸✳ ❚➟♣ t➜t ❝↔ ❞÷î✐ ❣r❛❞✐❡♥t ❝õ❛ f t↕✐ x ✤÷ñ❝ ❣å✐ ❧➔
❞÷î✐ ✈✐ ♣❤➙♥
❝õ❛ f t↕✐ x✱ ❦➼ ❤✐➺✉ ❧➔ ∂f (x)✱ tù❝ ❧➔
∂f (x) = x∗ ∈ H : f (x) − f (x) ≥ x∗ , x − x , ∀x ∈ H}.
✣à♥❤ ♥❣❤➽❛ ✶✳✷✳✶✹✳ ❍➔♠ f ✤÷ñ❝ ❣å✐ ❧➔ ❦❤↔ ❞÷î✐ ✈✐ ♣❤➙♥ t↕✐ x✱ ♥➳✉
∂f (x) = 0.
❱➼ ❞ö ✶✳✹✳ ❈❤♦ A ❧➔ ♠ët t➟♣ ❧ç✐ ❦❤→❝ ré♥❣ ❝õ❛ ❦❤æ♥❣ ❣✐❛♥ ❍✐❧❜❡rt H✳
❳➨t ❤➔♠ ❝❤➾ tr➯♥ t➟♣ A
∂A(x) =
0
+∞
♥➳✉ x ∈ A
♥➳✉ x ∈ A.
❑❤✐ ✤â ❞÷î✐ ✈✐ ♣❤➙♥ ❝õ❛ ❤➔♠ ❝❤➾ ❝õ❛ A t↕✐ x ∈ A ❧➔ ♥â♥ ♣❤→♣ t✉②➳♥ ❝õ❛
A t↕✐ x✳
✣à♥❤ ❧þ ✶✳✶✸✳ ●✐↔ sû f ❧➔ ❤➔♠ ❧ç✐ ❝❤➼♥❤ t❤÷í♥❣ tr➯♥ H ✈➔ λ > 0✳ ❑❤✐
✤â ✈î✐ ♠å✐ x ∈ H
∂(λf )(x) = λ∂f (x).
✣à♥❤ ❧þ ✶✳✶✹ ✭▼♦r❡❛✉✲ ❘♦❝❦❛❢❡❧❧❛r✮✳ ❈❤♦ f ✈➔ g ❧➔ ❤❛✐ ❤➔♠ ❧ç✐ ❝❤➼♥❤
t❤÷í♥❣ tr➯♥ A✱ ❣✐↔ sû r➡♥❣ f ❧✐➯♥ tö❝ t↕✐ x ∈ dom g✳ ❑❤✐ ✤â t❛ ❝â
∂(f + g)(x) = ∂f (x) + ∂g(x), ✈î✐ ♠å✐ x ∈ A.
❈❤ù♥❣ ♠✐♥❤✳ • ∂(f + g)(x) ⊇ ∂f (x) + ∂g(x).
❚❛ ❧➜② x∗1 ∈ ∂f (x)✱ x∗2 ∈ ∂g(x)✳ ●✐↔ sû x∗1 + x∗2 ∈ ∂f (x) + ∂g(x)✳ ❑❤✐
✤â ✈î✐ ♠å✐ y ∈ A t❛ ❝â
x∗1 , y − x ≤ f (y) − f (x),
✶✹
x∗2 , y
− x ≤ g(y) − g(x).
❈ë♥❣ tø♥❣ ✈➳ ❝õ❛ ❤❛✐ ❜➜t ✤➥♥❣ t❤ù❝ tr➯♥ t❛ ❝â
x∗1 + x∗2 , y − x ≤ f (y) + g(y) − (f (x) + g(x)).
❙✉② r❛ x∗1 + x∗2 ∈ ∂(f + g)(x).
• ∂(f + g)(x) ⊆ ∂f (x) + ∂g(x).
❚❛ ❝â int(epif ) = ∅✳ ❚❤➟t ✈➟②✱
♠ð ❝õ❛ x s❛♦ ❝❤♦
✈î✐ ♠å✐
f (x) − f (x) <
>0
tç♥ t↕✐ ♠ët ❧➙♥ ❝➟♥
✈î✐ ♠å✐ x ∈ U.
❙✉② r❛ B := {(x, θ) ∈ A × H : θ > f (x) + , x ∈ U } ⊂ epif
✈➔ B ❧➔ ♠ð s✉② r❛ int(epif ) = ∅.
●✐↔ sû x∗ ∈ ∂(f + g)(x)✳ ❚❛ ①➨t ❝→❝ t➟♣ s❛✉✿
H1 = {(y, θ) ∈ A × H : θ ≥ f (x + y) − f (x)}
H2 = {(y, θ) ∈ A × H : θ < x∗ , y − g(x + y) + g(x)}.
❑❤✐ ✤â H1 = epif − (x, f (x)) s✉② r❛ H1 ❧➔ ❧ç✐ ✈➔ intH1 = ∅.
▲➜② (y1, θ1)✱ (y2, θ2) ∈ H2 ✈➔ λ ∈ [0, 1]✱ t❛ ❝â✿
θ1 < x∗ , y1 − g(x + y1 ) + g(x),
θ2 < x∗ , y2 − g(x + y2 ) + g(x).
❱➻ g ❧➔ ❤➔♠ ❧ç✐ ♥➯♥
g(λ(x + y1 ) + (1 − λ)(x + y2 )) ≤ λg(x + y1 ) + (1 − λ)g(x + y2 ).
❚ø ❤❛✐ ❜➜t ✤➥♥❣ t❤ù❝ tr➯♥ t❛ ❝â
λθ1 + (1 − λ)θ2 < x∗ , λy1 + (1 − λ)y2
−g(λ(x+y1 )+(1−λ)(x+y2 ))+g(x)⇒ λ(y1 , θ1 )+(1−λ)(y2 , θ2 ) ∈ H2 ✳
❱➟② H2 ❧ç✐✳
●✐↔ sû (x0, y0) ∈ H1 ∩ H2 t❤➻ x∗, x0 > −g(x + x0) + g(x) >
f (x + x0 ) − f (x)
⇒ x∗ , x0 > f (x + x0 ) + g(x + x0 ) − (f (x) + g(x))✳ ✣✐➲✉ ♥➔② ♠➙✉
t❤✉➝♥ ✈î✐ x∗ ∈ ∂(f + g)(x)✳ ❱➟② H1 ∩ H2 = ∅.
✶✺
❚❤❡♦ ✣à♥❤ ❧➼ t→❝❤ t❤ù ♥❤➜t✱ tç♥ t↕✐ x∗1 ∈ H ✈➔ β ∈ R ✈î✐ (x∗1, β) = ∅
s❛♦ ❝❤♦
sup { x∗1 , y + βθ} ≤
(y,θ)∈H1
inf { x∗1 , y + βθ}.
(y,θ)∈H2
❘ã r➔♥❣ β < 0✱ ❦❤æ♥❣ ♠➜t t➼♥❤ tê♥❣ q✉→t t❛ ❝â t❤➸ ❣✐↔ sû β = −1✳
◆❤÷ ✈➟② H1 ✈➔ H2 t→❝❤ ✤÷ñ❝ ❜ð✐ s✐➯✉ ♣❤➥♥❣
H = {(y, θ) ∈ A × H : x∗1 , y > −θ = 0}.
❱➟② t❛ ❝â
sup{ x∗1 , y −f (x+y)+f (x)} ≤ inf {< x∗1 −x∗ , y +g(x+y)−g(x)}.
y
y
✣➦t x∗2 = x∗ − x∗1 t❛ ❝â
f (x + y) − f (x) ≥ x∗1 , y
f (x + y) − g(x) ≥ x∗2 , y
⇒ x∗1 ∈ ∂f (x), x∗2 ∈ ∂g(x).
⇒ x∗ = x∗1 + x∗2 ∈ ∂f (x) + ∂g(x).
✈î✐ ♠å✐ y ∈ A,
✈î✐ ♠å✐ y ∈ A.
ữỡ
trỏ ừ t tỷ ố ợ
t t tự
tỷ
r t ởt số t t ừ
t tỷ ởt t ỗ õ tr ổ rt ỳ
tự ừ ữủ tr tứ t
A t ỗ õ rộ tr ổ
rt H sỷ x H, p A tứ x t A
dA (x) ởt ữủ
dA (x) := inf x p , p A.
pA
r trữớ ủ õ ởt tỷ p A x p = dA(x) ợ ồ
x A t t õ r p ừ x t A PA : x
PA (x) ữủ ồ t tỷ t A
ỵ A t ỗ õ rộ tr ổ rt
H sỷ x H, p A t t õ
p = PA (x) p x, y p 0 ợ ồ y A.
(2.1)
ự
sỷ x H,
p A t õ
p x, y p 0 ợ ồ y A
p x, y x + x p 0
p x + p x, y x 0
2
p x p x, y x .
t2 tự rt t õ
px px
p = PA (x).
yx px yx .
ứ õ t õ
y A, (0, 1) t y := y + (1 )p tt
t õ A t ỗ y, p A y A.
p = PA(x) t õ p x x y
2
2
p x x y
2
2
p x x [y + (1 )p]
2
2
p x (p x) (y p)
2
2
2
p x 2 y p + p x + 2 y p, x p
2
2 y p + 2 p x, y p 0.
(0, 1) > 0 t õ y p 2 + 2 p x, y p 0
ú ợ ồ y A ồ (0, 1) tợ t ữủ
p x, y p 0 ợ ồ y A
q A t ỗ õ rộ tr ổ rt
H
õ ợ ồ x H p A t õ p = PA(x) x p NA(p).
A t ỗ õ rộ tr ổ rt
H õ ợ ồ x H t tỷ PA (x) ổ tỗ t t
ự
ỹ tỗ t x A t t õ dA(x) = 0.
x A t t õ dA(x) = inf pA p x t
ữợ ú tỗ t ởt {yk } A s
0 lim y k x = dA (x) +.
k
{y } tỗ t ởt {ykj } ở tử tợ
p tý ỵ tở A A t õ õ t õ
k
p x = lim y kj x = lim y k x = dA (x).
j
k
p ừ x A
t sỷ p1 , p2 ừ x A õ
x p1 NA (p1 ), x p2 NA (p2 ) r p1 x, p2 p1 0
p2 x, p1 p2 0 ở t tự t õ p1 p2 0
tứ õ s r p1 = p2
A t ỗ õ rộ tr ổ rt
H
sỷ x, y H õ t õ
PA (x) PA (y) x y .
PA (x) PA (y)
2
x y, PA (x) PA (y) .
ự
ử ợ p = PA(x), y = PA(y) t õ
PA (x) x, PA (y) PA (x) 0.
ữỡ tỹ ợ p = PA(y), x = PA(x) t õ
PA (y) PA (x), y PA (y) 0.
ở t tự ử t tự
rt t s r
PA (x) PA (y) x y .
ử ợ x = y, p = PA(y), y = PA(x) t õ
PA (y) y, PA (x) PA (y) 0.
ữỡ tỹ ợ x = x, p = PA(x), y = PA(y) t õ
PA (x) x, PA (y) PA (x) 0.
ở t tự ử t tự
rt t s r
2
x y, PA (x) PA (y) PA (x) PA (y) .
ứ t s r ỵ s
ỵ t ỗ õ rộ A tr ổ rt H
ởt a A õ tỗ t ởt t x0 A s
a x0 , x x0 0, ợ ồ x A.
t x0 tr tr ữủ ồ ừ a
t A õ tỷ ừ A a t số a x0 ữủ ồ
tứ a A tỡ t = a x0 = 0 t
t, x x0 0, ợ ồ x A.
(2.2)
t, x x0 = a x0 , x x0 0
s r
sup t, x t, x0 < t, a .
xA
= 1/2
t, x0 + < t, a
t ữủ
sup t, x < < t, a .
xA
ự s H = x H| t, x = ợ = t, x0 t
t ỗ A ợ a x0 A s H = x H| t, x x0 =
0 q x0 A t
t, x x0 0, ợ ồ x A,
ữủ ồ ởt s tỹ ừ A t x0