Lp ( ) (1
w SILU 223.6 E QWLQETSQ BANAHOWYM PROSTRANSTWOM. |
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p R I M E R Y. 2. w PROSTRANSTWE NEPRERYWNYH FUNKCIJ C[0; 1] S sup- |
NORMOJ OTOBRAVENIE '(f) = |
f (0) (f |
2 |
C[0; 1]) QWLQETSQ OGRANI^ENNYM |
LINEJNYM FUNKCIONALOM, PRI^EM• |
k'k |
= 1. |TOT LINEJNYJ FUNKCIONAL, |
ODNAKO |
, |
NE QWLQETSQ OGRANI^ENNYM |
, |
ESLI |
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C[0; 1] |
SNABVENO NORMOJ |
kfk1 = |
1 |
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Z0 |
jf (t)j dt. |
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3. pUSTX E | KONE^NOMERNOE NORMIROWANNOE PROSTRANSTWO, fe1; : : : ; eng |
| BAZIS W E , TO ESTX KAVDYJ WEKTOR f |
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2 |
E ODNOZNA^NO PREDSTAWIM W WI- |
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DE f |
= |
k=1 fkek |
(fk 2 ). pUSTX f 1; : : : ; ng | FIKSIROWANNYJ NABOR |
SKALQROWPTAK ^TO |
= ( 1 |
; : : : ; n) 2 |
n |
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tOGDA FORMULA |
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' (f ) = |
k=1 |
fk k (f = (f1; : : : ; fn ) 2 E) |
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OPREDELQET ' |
2 |
E . oBRATNO, ESLI ' |
2 |
E , TO, POLAGAQ k = |
'(ek ) (1 |
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k |
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n), POLU^IM '(f ) = '( |
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fk'(ek ) = |
' (f) |
(f |
2 |
E ). |
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k=1 |
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k=1 |
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iTAK |
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P |
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P |
BIEKTIWNOE |
) ! ' |
MEVDU |
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MY POLU^ILI SOOTWETSTWIE |
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O^EWIDNO |
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PROSTRANSTWAMI I E . |TO SOOTWETSTWIE LINEJNO, TAK ^TO E ALGEBRA-
I^ESKI IZOMORFNO WEKTORNOMU PROSTRANSTWU n . eSLI W n WWESTI NORMU
k k k' k( 2 n), TO E ' n .
w PRIMERE 3 MY RAZOBRALI ^ASTNYJ SLU^AJ KLASSI^ESKOJ ZADA^I NA- HOVDENIQ SOPRQVENNOGO• PROSTRANSTWA K DANNOMU NORMIROWANNOMU PRO- STRANSTWU E. |TA ZADA^A IZWESTNA KAK ZADA^A NAHOVDENIQ OB]EGO WIDA LINEJNOGO FUNKCIONALA; ONA SOSTOIT W OTYSKANII KONKRETNOGO BANAHO- WA PROSTRANSTWA IZOMETRI^ESKI IZOMORFNOGO PROSTRANSTWU E . nIVE MY PROILL@STRIRUEM RE[ENIE \TOJ ZADA^I DLQ PROSTRANSTW Lp( ).
u P R A V N E N I E. 4. dLQ KONE^NOMERNOGO PROSTRANSTWA C n , SNABV•EN-
NOGO NORMOJ k kp (1 p 1) (SM. 220.7), WY^ISLITE NORMU W PROSTRAN- STWE (C n ) .
x226. Lp( ) (1 p < 1)
rASSMOTRIM PROSTRANSTWO (E; A; ) S POLNOJ KONE^NOJ MEROJ . pUSTXp 1) | [KALA BANAHOWYH PROSTRANSTW NAD POLEM C W
USLOWIQH I OBOZNA^ENIQH 221.1.
1. pUSTX g 2 L1( ) I OTOBRAVENIE 'g : L1( ) ! C ZADANO FORMULOJ
'g (f) Z fg d (f 2 L1( )):
tOGDA |
'g 2 |
1 |
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PRI^EM |
k'gk |
= kgk1 . |
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L ( ) , |
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oTOBRAVENIE 'g KORREKTNO ZADANO, LINEJNO I OGRANI^ENO: |
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j'g(f )j = jZ fg d j kgk1 Z jfj d = kgk1kfk1 (f 2 L1 ( )): |
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iZ POLU^ENNOGO NERAWENSTWA SLEDUET TAKVE, ^TO k'gk kgk1 |
; DLQ DOKA- |
ZATELXSTWA OBRATNOGO NERAWENSTWA DOSTATO^NO S^ITATX, ^TO g = . pUSTX |
" > 0 PROIZWOLXNO, 0 < " < kgk1. oBOZNA^IM X" |
= |
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fx 2 |
E |
6: |
jg(x)j > |
kgk1 , "g. |
iZ OPREDELENIQ NORMY |
k k1 |
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SLEDUET |
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^TO |
X" > 0. |
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( . 221.1) |
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pOLOVIM f" = |
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g |
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X" (OTMETIM, ^TO kf"k1 = Z jf"j d = 1). tOGDA |
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jgj X" |
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'g |
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= |
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sup |
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'(f) |
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'g (f") |
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jZ |
f"g d = |
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Z j |
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1 j |
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X" |
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j j |
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k k |
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X" |
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kgk1 , ": |
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iZ PROIZWOLXNOSTI |
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" : k'gk kgk1: > |
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2. L1( ) ' L1( ). |
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g ! |
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w SILU P |
OPREDELENO IZOMETRI^ESKOE OTOBRAVENIE |
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PROSTRAN |
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. 1 |
1 |
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'g |
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- |
STWA L1( ) W L ( ) . pOKAVEM, ^TO \TO OTOBRAVENIE S@R_EKTIWNO. pUSTX |
' 2 L1( ) . pOLOVIM |
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(1) |
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' |
(X ) |
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'( X ) |
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(X |
2 |
A): |
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uBEDIMSQ, ^TO ' |
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1 |
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2 A). tOGDA POSLE- |
| ZARQD. pUSTX X = j=1 Xj (X; Xj |
DOWATELXNOSTX |
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n |
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j |
SHODITSQ PO NORMEP |
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1 K X , TAK KAK |
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j=1 |
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k k |
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n |
1 |
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Z j X , jX=1 |
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Xj j d = Zj=1n+1 Xj d = j=Xn+1 |
Xj ! 0 (n ! +1): |
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P |
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eSLI TEPERX f |
2 |
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L1( ) PROIZWOLXNA, TO SU]ESTWUET POSLEDOWATELXNOSTX |
fn |
2 K |
TAKAQ, ^TO |
kfn , fk1 |
! 0 |
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(SM. 215.9). sLEDOWATELXNO, '(f ) = |
lim '(fn ) = lim |
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(fn) = |
(f): |
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> |
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3. |
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Lp( ) |
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Lq( ) ( |
1 |
+ |
1 |
= |
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1; |
1 < p; q |
< |
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+ ). w ^ASTNOSTI, |
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p |
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L2 ( ) |
' L2( ).' |
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1 |
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dLQ |
g 2 |
L |
q |
( ) |
ZADADIM LINEJNYJ FUNKCIONAL |
'g : L |
p |
( ) ! |
C |
ZDESX |
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Z |
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( |
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1 |
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+ |
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1) RAWENSTWOM 'g (f) |
= |
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fg d . w SILU |
221.7 fg |
2 |
L1( ), I |
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ZNA^IT, |
'g |
KORREKTNO ZADAN. pRI \TOM j'g(f )j Z |
jfgj d |
kgkqkfkp. |
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g g (q=p),1 |
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p |
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oTS@DA 'g |
OGRANI^EN I k'gk |
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kgkq . wZQW f0 = |
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kjjgjjq=p kp |
2 |
L |
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( ), |
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IMEEM |
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f0 |
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p = 1, TAK ^TO |
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'g |
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'g(f0) = |
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. iTAK, OTOBRAVENIE |
g p |
! 'g (g |
2 |
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L ( )) |
QWLQETSQ IZOMETRI^ESKIM OTOBRAVENIEM |
L ( ) |
W |
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L ( ) . oSTALOSX UBEDITXSQ, ^TO \TO OTOBRAVENIE QWLQETSQ S@R_EKCI- |
EJ. pUSTX ' 2 |
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Lp( ) . aNALOGI^NO DOKAZATELXSTWU P. 1 USTANAWLIWAEM, |
^TO (1) OPREDELQET ZARQD NA A ABSOL@TNO NEPRERYWNYJ OTNOSITELXNO |
I PO TEOREME rADONA-nIKODIMA POLU^AEM FORMULU (2) (!!). pO-PREVNEMU |
BUDEM S^ITATX, ^TO g |
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0. uBEDIMSQ, ^TO g |
2 |
Lq ( ) (TOGDA RAWENSTWO |
' = 'g |
SNOWA POLU^AETSQ PRIWEDENNYM WY[E SPOSOBOM |
(!!)). |
dLQ \TOGO |
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• |
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POLOVIM gn = g g,1[0;n] (n 2 N). tOGDA gnq |
! gq, I PO TEOREME fATU NAM |
NUVNO LI[X POKAZATX, ^TO INTEGRALY Z gnq |
d OGRANI^ENY W SOWOKUPNOS- |
TI. iMEEM: |
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Z |
gnq d = |
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gnq,1gn d = Z gnq,1g d = '(gnq,1) |
k'k kgnq,1kp |
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= k'k[Z gnp(q,1) d ]1=p = k'k[Z gnq d ]1=p: |
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oTS@DA |
Z |
gnq d k'kq |
(n 2 N): |
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4. u P R A V N E N I E. pOKAVITE, ^TO W OBOZNA^ENIQH 221.10 (`1 ) ' |
`1; (`p) ' `q ( |
1 |
+ |
1 |
= 1; 1 < p; q < +1). |
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p |
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x227. pRODOLVENIE OGRANI^ENNYH LINEJNYH OTOBRAVENIJ
PO NEPRERYWNOSTI
t E O R E M A. pUSTX E | NORMIROWANNOE PROSTRANSTWO I X | LINE- AL, PLOTNYJ W E. pUSTX F | BANAHOWO PROSTRANSTWO I A : X ! F |
OGRANI^ENNOE LINEJNOE OTOBRAVENIE. tOGDA SU]ESTWUET I OPREDELENO
ODNOZNA^NO OTOBRAVENIE A |
2 |
L(E; F ) SO SWOJSTWAMI: |
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A) Aj X = A, |
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e |
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B) keAk |
= kAk. |
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fn |
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e |
f 2 |
E |
PROIZWOLEN I |
PROIZWOLXNAQ POSLEDOWATELXNOSTX |
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pUSTX |
! |
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n |
TAKAQ |
, |
^TO |
fn |
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oSTAETSQ PROWERITX |
, |
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KORREKTNO OPREDELENNYJ• |
\LEMENT IZ L(E; F ), UDOWLETWORQ@]IJ USLOWIQM |
A) I B). oTMETIM SNA^ALA, |
^TO lim Afn SU]ESTWUET, TAK KAK |
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(Afn) | |
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FUNDAMENTALXNAQ POSLEDOWATELXNOSTX W BANAHOWOM PROSTRANSTWE F :
kAfn , Afmk = kA(fn , fm)k kAk kfn , fmk ! 0 (n; m ! +1):
|TOT PREDEL NE ZAWISIT OT WYBORA POSLEDOWATELXNOSTI (fn), SHODQ]EJSQ K f (!!). iZ ARIFMETI^ESKIH SWOJSTW PREDELA SLEDUET, ^TO A | LINEJNOE
OTOBRAVENIE IZ E W F . uSLOWIE A) WYPOLNENO PO POSTROENI@. nAKONEC, |
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OTKUDA SLEDUET, ^TO A | OGRANI^ENNYJ LINEJNYJ OPERATOR I kAk kAk. |
oBRATNOE NERAWENSTWO |
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SLEDUET IZ TOGO, |
^TO A | PRODOLVENIE |
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k k k |
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x228. tEOREMA hANA-bANAHA
tEOREMA hANA-bANAHA USTANAWLIWAET WOZMOVNOSTX PRODOLVENIQ FUNK- CIONALA S PODPROSTRANSTWA NA WSE• PROSTRANSTWO S SOHRANENIEM OPRE- DEL•ENNYH SWOJSTW. oTMETIM, NAPRIMER, ^TO S POMO]X@ \TOJ TEOREMY MOVNO OTWETITX (POLOVITELXNO) NA SLEDU@]IJ WOPROS: SU]ESTWUET LI HOTQ BY ODIN NENULEWOJ OGRANI^ENNYJ LINEJNYJ FUNKCIONAL NA PROIZ- WOLXNOM NORMIROWANNOM PROSTRANSTWE E (6= f g)?
1. t E O R E M A. [g.hAN, s.bANAH]. pUSTX E | WEKTORNOE PRO-
STRANSTWO NAD POLEM (= C ILI R) I k k | POLUNORMA NA E (SM. |
148.5), X | LINEAL W E I ' : X ! | LINEJNYJ FUNKCIONAL TAKOJ, |
^TO j'(f )j kfk (f 2 X ). tOGDA SU]ESTWUET LINEJNYJ FUNKCIONAL |
: E |
! TAKOJ, ^TO |
A) |
j X = ', |
AKSIOMY PORQDKA NA SAMOM DELE WYPOLNENY (!!). pROWERIM, ^TO MNOVES-
TWO ('i) INDUKTIWNO. pUSTX ('j )j |
2 |
J | SOWER[ENNO UPORQDO^ENNOE POD- |
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MNOVESTWO MNOVESTWA ('i). oPREDELIM NA LINEALE X0 |
2 |
LINEJNYJ |
FUNKCIONAL |
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ESLI |
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fUNKCIONAL |
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'0 |
(f) 'j(f ), |
f |
2 Xj . |
'0 |
SOPREDELEN KOR |
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REKTNO: ESLI f |
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Xj1 |
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2 |
J ), TO W SILU SOWER[ENNOJ UPORQDO- |
^ENNOSTI SEMEJSTWA ('j )j2J : |
'j1 |
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'j2 , LIBO 'j2 |
'j1 . pUSTX, NAPRIMER, |
'j2 'j1 . tOGDA Xj2 |
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Xj1 I 'j1 (f ) = 'j2 (f) (f |
2 |
Xj2 ). pRI \TOM '0 |
| MAVORANTA SEMEJSTWA ('j )j |
2 |
J , I INDUKTIWNOSTX ('i) USTANOWLENA. pO |
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TEOREME cORNA SU]ESTWUET MAKSIMALXNOE PRODOLVENIE FUNKCIONALA ', |
TO ESTX LINEJNYJ FUNKCIONAL |
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: Y ! R, GDE Y | LINEAL W E, PRI^EM• |
j |
X = '; |
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(f) |
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2 |
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Y ). eSLI Y = E, TO SU]ESTWUET g |
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Y I, |
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PRIMENQQ K KONSTRUKCI@, IZLOVENNU@ W NA^ALE DOKAZATELXSTWA, PRO- |
DOLVIM |
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NA LINEAL f g + fj f 2 Y; 2 Rg W PROTIWORE^IE S MAKSIMALX- |
NOSTX@ |
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oTMETIM RQD SLEDSTWIJ TEOREMY hANA-bANAHA DLQ NORMIROWANNYH PROSTRANSTW.
2. pUSTX X | LINEAL W NORMIROWANNOM PROSTRANSTWE E I ' |
OGRANI^ENNYJ LINEJNYJ FUNKCIONAL NA X. tOGDA SU]ESTWUET |
2 E |
TAKOJ, ^TO |
j X = '; |
k k = k'k. |
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oPREDELIM NA WEKTORNOM PROSTRANSTWE E WYPUKLU@ FUNKCI@ kfk |
k'k kfk, GDE kfk | NORMA WEKTORA f W NORMIROWANNOM PROSTRANSTWE E, |
A k'k | NORMA FUNKCIONALA ' 2 X . pRIMENIM P. 1 K (E; k k ) I |
FUNKCIONALU ': SU]ESTWUET LINEJNYJ FUNKCIONAL NA E TAKOJ, ^TO |
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j X = '; j (f)j kfk = k'k kfk (f 2 E): |
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oTS@DA SLEDUET, ^TO |
2 E I k k k'k. nERAWENSTWO |
k k k'k |
SLEDUET IZ TOGO, ^TO |
| PRODOLVENIE ': |
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3. pUSTX E | NORMIROWANNOE PROSTRANSTWO, = f |
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E. tOGDA |
SU]ESTWUET |
2 E TAKOJ, ^TO k k = 1; (f) = kfk.6 |
2 |
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pOLOVIM X = f f j 2 g; '( f ) kfk ( f 2 X). tOGDA
'2 X ; k'k = 1. oSTAETSQ• PRIMENITX P. 2. >
4.eSLI NORMIROWANNOE PROSTRANSTWO E NETRIWIALXNO, TO NETRI- WIALXNO I PROSTRANSTWO E .
|TO SLEDSTWIE P. 3. >
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5. pUSTX X | LINEAL W NORMIROWANNOM PROSTRANSTWE E; g 2 EnX I |
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f2X k |
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E TAKOJ, ^TO |
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inf |
g + f |
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> 0. tOGDA SU]ESTWUET FUNKCIONAL |
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(g) = I (f) = 0 (f 2 X). |
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rASSMOTRIM LINEAL Y = f g + f j f 2 X; 2 g I OPREDELIM NA NEM• |
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6 |
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FUNKCIONAL '( g + f) |
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SLEDUET, ^TO ' 2 Y . w KA^ESTWE |
WOZXMEM PRODOLVENIE ' PO P. 2. |
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6. u P R A V N E N I E. zAWER[ITE DOKAZATELXSTWO TEOREMY hANAbANAHA W SLU^AE SEPARABELXNOGO NORMIROWANNOGO PROSTRANSTWA E, NE ISPOLXZUQ TEOREMU cORNA fUKAZANIE: ISPOLXZOWATX TEOREMU x227g.
x229. wTOROE SOPRQV•ENNOE PROSTRANSTWO
1. pUSTX E | NORMIROWANNOE PROSTRANSTWO I E | SOPRQVENNOE•
K NEMU PROSTRANSTWO, QWLQ@]EESQ BANAHOWYM (SM. 223.6). mOVNO RASSMOTRETX SOPRQVENNOE• K PROSTRANSTWU E ; ONO NAZYWAETSQ WTORYM SO-
PRQV•ENNYM K PROSTRANSTWU E. tAKIM OBRAZOM, PO OPREDELENI@ E
(E ) .(mOVNO, RAZUMEETSQ,PRODOLVITX PROCESS I RASSMOTRETX E ; E
I T. D.)
2. mEVDU ISHODNYM PROSTRANSTWOM E I EGO WTORYM SOPRQVENNYM• IMEETSQ TESNAQ SWQZX. ~TOBY PROANALIZIROWATX EE•, WWEDEM• OTOBRAVENIE
b |
) : E |
! E (ONO ^ASTO NAZYWAETSQ KANONI^ESKIM), SOPOSTAWLQ@]EE |
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2 |
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E , DEJSTWU@]IJ PO FORMULE |
KAVDOMU \LEMENTU f |
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\LEMENT f |
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(1) |
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f (') '(f ) (' 2 E ): |
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b |
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w SAMOM DELE, f | OGRANI^ENNYJ LINEJNYJ FUNKCIONAL NA BANAHOWOM |
PROSTRANSTWE Eb, TAK KAK |
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jfb(')j = j'(f )j kfk k'k (' 2 E ): |
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3. kANONI^ESKOE OTOBRAVENIE |
b |
) : E ! E | IZOMETRIQ. |
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w SOOTWETSTWII S OPREDELENIEM 223.7 NUVNO UBEDITXSQ, ^TO OTOBRAVE- |
NIE |
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SOHRANQET NORMU. iZ (2) SLEDUET, ^TO kfk kfk. oBRATNO, PUSTX |
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f = I '0 |
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TAKOJ, ^TO |
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= 1 I '0(f) = |
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('0 SU]ESTWUET |
kfk kfk: > |
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W SILU |
228.3). |
tOGDA IZ RAWENSTWA |
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f ('0) = '0(f ) |
= |
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SLEDUET |
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^TO |
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b 4. |
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bANAHOWO |
PROSTRANSTWO |
E |
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NAZYWAETSQ |
REFLEKSIWNYM, |
ESLI |
b |
) : E |
! E | IZOMETRI^ESKIJ IZOMORFIZM E NA E . |
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pRIMERY REFLEKSIWNYH BANAHOWYH PROSTRANSTW: EWKLIDOWY PROSTRAN- |
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STWA R |
; C |
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, PROSTRANSTWA L ( ) (1 < p < 1). |
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u P R A V N E N I Q. 5. pOKAVITE, ^TO KAVDOE KONE^NOMERNOE NORMIROWANNOE PROSTRANSTWO REFLEKSIWNO.
6. uBEDITESX, ^TO PROSTRANSTWO c0 WSEH KOMPLEKSNYH POSLEDOWATELX-
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n |
k |
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n j |
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NOSTEJ f = (f1; f2 |
; : : :) SO SWOJSTWOM lim fn = 0 I S NORMOJ |
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sup |
fn |
, |
| NE REFLEKSIWNOE BANAHOWO PROSTRANSTWO.
7. pUSTX E | BANAHOWO PROSTRANSTWO, PRI^EM• E REFLEKSIWNO. pOKAVITE, ^TO E TAKVE REFLEKSIWNO.
x230. tEOREMA bANAHA-{TEJNGAUZA
1. [pRINCIP RAWNOMERNOJ OGRANI^ENNOSTI]. pUSTX E | BANAHOWO PRO-
STRANSTWO, F | NORMIROWANNOE PROSTRANSTWO, |
F L(E; F ), PRI^•EM |
T 2F k |
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T 2F k |
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1 |
sup |
T f |
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< + PRI KAVDOM f |
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E. tOGDA |
sup |
T |
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< + . |
pOLOVIM An = T |
2F |
ff j kT fk ng. tOGDA |
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(i) |
E = |
1 An,T |
(ii) KAVDOE An ZAMKNUTO. |
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S |
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n=1 |
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pO TEOREME b\RA 217.4 KAKOE-LIBO An IMEET NEPUSTU@ WNUTRENNOSTX:
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n 6 ; |
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N1 |
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A = |
. pUSTX B" |
(a) |
2 |
A . sU]ESTWUET N1 |
TAKOE, ^TO B"( ) |
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f |
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pUSTX sup |
T a |
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< k ( N); POLAGAQ N1 |
= n + k, IMEEM (TAK KAK f + a |
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B"(a)): |
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kfk < " ) kT fk kT(f + a)k + kT ak n + k:g |
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tEPERX |
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kfk 1 k |
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2 F ) k |
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" |
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T f |
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= 1 |
sup |
T ("f ) |
N1=": |
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rASSMOTRIM RQD WAVNYH SLEDSTWIJ.