On the uniqueness of the extension of continuous p-seminorms

Original title (in Romanian)

Asupra unicității prelungirii p-seminormelor continue

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Costica Mustata
“Tiberiu Popoviciu” Institute of Numerical Analysis, Romanian Academy, Romania

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C. Mustăţa, On the uniqueness of the extension of continuous p-seminorms, Rev. Anal. Numer. Teoria Approximatiei 2 (1973) no. 2, 173-177 (MR 53 # 8759) (in Romanian).

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Rev. Anal. Numer. Theoria Approximatiei

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Romanian Academy

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MR 53 # 8759

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[1] Czipser, J. si Geher, L., Extension of functions satisfying a Lipschitz condition, Acta Math. Acad. Sci. Hungar 6, 213-220, 1955
[2] Kolumban I., Ob edinstvenosti prodoljenia linein]h functionalov, Mathematica, Cluj, 4 (27), 267-270, 1962
[3] Phelps, R.P., Uniquencess of Hahn-Banach extension and unique best approximation, Trans. Amer. Math. Soc. 95, 238-255, 1960.

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1973-Mustata-RANTA-On-the-uniqueness-of-the-extension-of-continuous-p-seminorms

ASUPRA UNICITATTII PRELUNGIRII p p ppp-SEMINORMELOR CONTINUE

de COSTICA MUSTATA(Cluj)

  1. Fie X X XXX un spaţiu liniar real și p ( 0 , 1 ] p ( 0 , 1 ] p in(0,1]p \in(0,1]p(0,1]. O funcţională : X R : X R ||||||:X rarr R\|\|\|: X \rightarrow R:XR se numeşte o p p ppp-normă pe X X XXX dacă ea verifică axiomele:
p 1 ) | x | 0 , x = 0 x = 0 , x X p 2 ) | x + y p | x | p + | y p , x , y X p 3 ) λ x | = | λ | x | , x X , λ R p 1 | x | 0 , x = 0 x = 0 ,      x X p 2 x + y p x p + y p ,      x , y X p 3 λ x | = | λ | x | ,      x X , λ R {:[{:p_(1))quad|||x||| >= 0","||x||=0<=>x=0",",x in X],[{:p_(2))quad|||x+y||^(p) <= |||x|||^(p)+|||y∣||^(p)",",x","y in X],[{:p_(3))quad||lambda x|||=|lambda|*||x|||",",x in X","lambda in R]:}\begin{array}{ll} \left.p_{1}\right) \quad\||x|\| \geqq 0,\|x\|=0 \Leftrightarrow x=0, & x \in X \\ \left.p_{2}\right) \quad\left\|\left|x+y\left\|^{p} \leqq\right\|\right| x\left|\left\|^{p}+\right\|\right| y \mid\right\|^{p}, & x, y \in X \\ \left.p_{3}\right) \quad\|\lambda x|\|=|\lambda| \cdot\| x|\|, & x \in X, \lambda \in R \end{array}p1)|x|0,x=0x=0,xXp2)|x+yp|x|p+|yp,x,yXp3)λx|=|λ|x|,xX,λR
Spatiul liniar real X X XXX, înzestrat cu p p ppp-norma ||| ||| îl numim spaţiu p p ppp-normat şi îl notăm ( X , | X , | X,|||||∣||X,\||\|\mid\|X,| ).
In lucrarea [4], w. RUESS defineşte conul convex al p p ppp-seminormelor continue definite pe spaţiul p p ppp-normat ( X , ) ( X , ) (X,||||||)(X,\| \| \|)(X,) :
(1) C X p τ = { h h : X R + , ) x , y X , λ R , h ( x + y ) h ( x ) + h ( y ) şi h ( λ x ) = | λ | p h ( x ) } (1) C X p τ = h h : X R + , x , y X , λ R , h ( x + y ) h ( x ) + h ( y )  şi  h ( λ x ) = | λ | p h ( x ) {:[(1)C_(X)^(p)tau={h∣h:X rarrR^(+),^(**))quad AA x","y in X","quad AA lambda in R","quad h(x+y) <= ],[{:h(x)+h(y)quad" şi "h(lambda x)=|lambda|^(p)h(x)}]:}\begin{gather*} C_{X}^{p} \tau=\left\{h \mid h: X \rightarrow R^{+},{ }^{*}\right) \quad \forall x, y \in X, \quad \forall \lambda \in R, \quad h(x+y) \leqq \tag{1}\\ \left.h(x)+h(y) \quad \text { şi } h(\lambda x)=|\lambda|^{p} h(x)\right\} \end{gather*}(1)CXpτ={hh:XR+,)x,yX,λR,h(x+y)h(x)+h(y) şi h(λx)=|λ|ph(x)}
şi spațiul generat de C X p C X p C_(X)^(p)C_{X}^{p}CXp, anume
(2) X τ p = C X p τ C X p τ . (2) X τ p = C X p τ C X p τ . {:(2)X_(tau)^(p)=C_(X)^(p)tau-C_(X)^(p)tau.:}\begin{equation*} X_{\tau}^{p}=C_{X}^{p} \tau-C_{X}^{p} \tau . \tag{2} \end{equation*}(2)Xτp=CXpτCXpτ.
Vom nota cu X # X # X^(#)X^{\#}X# spatiul liniar real al functionalelor lipschitziene definite pe spatiul p p ppp-normat ( X , | | | | | | X , | | | | | | X,||||||X,||||| |X,|||||| ) (vezi [2]):
(3) f X # , M 0 , x , y X , | f ( x ) f ( y ) | M | x y | f X # , M 0 , x , y X , | f ( x ) f ( y ) | M | x y | AA f inX^(#),EE M >= 0,AA x,y in X,|f(x)-f(y)| <= M*|||x-y|||\forall f \in X^{\#}, \exists M \geqq 0, \forall x, y \in X,|f(x)-f(y)| \leqq M \cdot|\|x-y\||fX#,M0,x,yX,|f(x)f(y)|M|xy|
Cu Φ Φ Phi\PhiΦ vom nota funcţionala nulă pe ( X , | | | | | | X , | | | | | | X,||||||X,||||| |X,|||||| ).
C X p τ C X p τ C_(X)^(p_(tau))C_{X}^{p_{\tau}}CXpτ este un con convex din spaţiul liniar X # X # X^(#)X^{\#}X#. Intr-adevăr dacă h C γ τ p p τ h C γ τ p p τ h inC_(gamma_(tau)^(p))^(p_(tau))h \in C_{\gamma_{\tau}^{p}}^{p_{\tau}}hCγτppτ atunci pentru orice x 1 , x 2 X x 1 , x 2 X x_(1),x_(2)in Xx_{1}, x_{2} \in Xx1,x2X avem | h ( x 1 ) h ( x 2 ) | h ( x 1 x 2 ) h x 1 h x 2 h x 1 x 2 |h(x_(1))-h(x_(2))| <= h(x_(1)-x_(2))\left|h\left(x_{1}\right)-h\left(x_{2}\right)\right| \leqq h\left(x_{1}-x_{2}\right)|h(x1)h(x2)|h(x1x2) de unde, pentru x 1 x 2 x 1 x 2 x_(1)!=x_(2)x_{1} \neq x_{2}x1x2
(4) | h ( x 1 ) h ( x 2 ) | x 1 x 2 h ( x 1 x 2 ) x 1 x 2 sup x y x , y X h ( x y ) x y (4) h x 1 h x 2 x 1 x 2 h x 1 x 2 x 1 x 2 sup x y x , y X h ( x y ) x y {:(4)(|h(x_(1))-h(x_(2))|)/(||x_(1)-x_(2)||∣) <= (h(x_(1)-x_(2)))/(||∣x_(1)-x_(2)||||) <= s u p_({:[x!=y],[x","y in X]:})(h(x-y))/(||x-y||||):}\begin{equation*} \frac{\left|h\left(x_{1}\right)-h\left(x_{2}\right)\right|}{\left\|x_{1}-x_{2}\right\| \mid} \leqq \frac{h\left(x_{1}-x_{2}\right)}{\left\|\mid x_{1}-x_{2}\right\| \|} \leqq \sup _{\substack{x \neq y \\ x, y \in X}} \frac{h(x-y)}{\|x-y\| \|} \tag{4} \end{equation*}(4)|h(x1)h(x2)|x1x2h(x1x2)x1x2supxyx,yXh(xy)xy
Pe de altă parte h C X p h C X p h inC_(X)^(p)h \in C_{X}^{p}hCXp dacă şi numai dacă sup x X { θ } h ( x ) x < sup x X { θ } h ( x ) x < s u p_(x in X-{theta})(h(x))/(||x||) < oo\sup _{x \in X-\{\theta\}} \frac{h(x)}{\|x\|}<\inftysupxX{θ}h(x)x< (vezi [4] pag. 16, Obs. 2.6) și avînd în vedere (4) rezultă că h X # h X # h inX^(#)h \in X^{\#}hX#.
Evident X τ p X τ p X_(tau)^(p)X_{\tau}^{p}Xτp este un subspaţiu liniar al lui X # X # X^(#)X^{\#}X#.
Pe| X τ p X τ p X_(tau)^(p)X_{\tau}^{p}Xτp se definesc următoarele norme (vezi Definitia 2.7 şi Lema 2.8 din [4]) :
(5) X 1 : X τ p R + (5) X 1 : X τ p R + {:(5)||||_(X)^(1):X_(tau)^(p)rarrR^(+):}:}\begin{equation*} \left\|\|_{X}^{1}: X_{\tau}^{p} \rightarrow R^{+}\right. \tag{5} \end{equation*}(5)X1:XτpR+
f X τ p , f X 1 = sup x 1 | f ( x ) | , f X τ p , f X 1 = sup x 1 | f ( x ) | , AA f inX_(tau)^(p),||f||_(X)^(1)=s u p_(||x|||| <= 1)|f(x)|,\forall f \in X_{\tau}^{p},\|f\|_{X}^{1}=\sup _{\|x\| \| \leqslant 1}|f(x)|,fXτp,fX1=supx1|f(x)|,
(6)
X : X τ p R + X : X τ p R + ||||_(X):X_(tau)^(p)rarrR^(+):}\left\|\|_{X}: X_{\tau}^{p} \rightarrow R^{+}\right.X:XτpR+
f X τ p , f X = q ( B 1 ( 0 , 1 ) X p τ B 1 ( 0 , 1 ) c X p τ ) ( f ) f X τ p , f X = q B 1 ( 0 , 1 ) X p τ B 1 ( 0 , 1 ) c X p τ ( f ) AA f inX_(tau)^(p),quad||f||_(X)=q_((B_(1)(0,1)nn nnint_(X)^(p)tau-B_(1)(0,1)nnc_(X)^(p)tau))(f)\forall f \in X_{\tau}^{p}, \quad\|f\|_{X}=q_{\left(B_{1}(0,1) \cap \cap \int_{X}^{p} \tau-B_{1}(0,1) \cap c_{X}^{p} \tau\right)}(f)fXτp,fX=q(B1(0,1)XpτB1(0,1)cXpτ)(f)
unde q q qqq este funcţionala lui Minkowski ataşată mulțimii B 1 ( 0 , 1 ) C X p τ B 1 ( 0 , 1 ) C X p τ B_(1)(0,1)nnC_(X)^(p)tauB_{1}(0,1) \cap C_{X}^{p} \tauB1(0,1)CXpτ - B 1 ( 0 , 1 ) C X p τ B 1 ( 0 , 1 ) C X p τ -B_(1)(0,1)nnC_(X)^(p_(tau))-B_{1}(0,1) \cap C_{X}^{p_{\tau}}B1(0,1)CXpτ iar B 1 ( 0 , 1 ) = { f X τ p , f X 1 1 } B 1 ( 0 , 1 ) = f X τ p , f X 1 1 B_(1)(0,1)={f inX_(tau)^(p),||f||_(X)^(1) <= 1}B_{1}(0,1)=\left\{f \in X_{\tau}^{p},\|f\|_{X}^{1} \leqq 1\right\}B1(0,1)={fXτp,fX11}.
Conform Lemei 2.8 din [4], ( X τ p , X X τ p , X X_(tau)^(p),||||_(X)X_{\tau}^{p},\| \|_{X}Xτp,X ) este un spațiu Banach şi pentru orice f X τ p , f X 1 f X f X τ p , f X 1 f X f inX_(tau)^(p),||f||_(X)^(1) <= ||f||_(X)f \in X_{\tau}^{p},\|f\|_{X}^{1} \leqq\|f\|_{X}fXτp,fX1fX. Dacă h h hhh este chiar din C X p τ C X p τ C_(X)^(p)tauC_{X}^{p} \tauCXpτ atunci h X = h X 1 h X = h X 1 ||h||_(X)=||h||_(X)^(1)\|h\|_{X}=\|h\|_{X}^{1}hX=hX1.
‥ Fie Y Y YYY un subspatiu liniar al lui ( X , | | | | | | ) X , | | | ) (X,||||||):}\left(X,\left|\left|\left||| |)\right.\right.\right.\right.(X,||||||). Pentru h C X p τ h C X p τ h inC_(X)^(p_(tau))h \in C_{X}^{p_{\tau}}hCXpτ vom nota cu h | Y h Y h|_(Y)\left.h\right|_{Y}h|Y restrictia lui h h hhh pe subspatiul Y Y YYY.
teorema 1. (Teorema 2.9 din [4]). Fie Y Y YYY un subspatiu liniar al lui ( X X XXX, III III) si h C X p τ h C X p τ h inC_(X)^(p_(tau))h \in C_{X}^{p_{\tau}}hCXpτ. Atunci functionala
(7)
H : X R + H : X R + H:X rarrR^(+)H: X \rightarrow R^{+}H:XR+
x X H ( x ) = inf y Y { h | Y ( y ) + h | Y Y x y } x X H ( x ) = inf y Y h Y ( y ) + h Y Y x y AA x in X quad H(x)=i n f_(y in Y){h|_(Y)(y)+||h|_(Y)||_(Y)*||∣x-y||||}\forall x \in X \quad H(x)=\inf _{y \in Y}\left\{\left.h\right|_{Y}(y)+\left\|\left.h\right|_{Y}\right\|_{Y} \cdot\|\mid x-y\| \|\right\}xXH(x)=infyY{h|Y(y)+h|YYxy}
verifică proprietățile:
(8) H C X p τ ; H | Y = h | Y ; H X = h | Y Y . (8) H C X p τ ; H Y = h Y ; H X = h Y Y . {:(8)H inC_(X)^(p)tau;H|_(Y)=h|_(Y);||H||_(X)=||h|_(Y)||_(Y).:}\begin{equation*} H \in C_{X}^{p} \tau ;\left.H\right|_{Y}=\left.h\right|_{Y} ;\|H\|_{X}=\left\|\left.h\right|_{Y}\right\|_{Y} . \tag{8} \end{equation*}(8)HCXpτ;H|Y=h|Y;HX=h|YY.
Functionala H H HHH din teorema 1 se numeşte o prelungire a restrictiei lui h C X p τ h C X p τ h inC_(X)^(p_(tau))h \in C_{X}^{p_{\tau}}hCXpτ pe Y Y YYY, de pe Y Y YYY pe X X XXX cu păstrarea normei de pe Y Y YYY.
2. În general, prelungirea H H HHH, cu proprietățile (8) nu este unică. În cele ce urmează vom găsi o condiţie necesată şi suficientă pentru unicitatea unei astfel de prelungiri. Pentru alte tipuri de functionale, condiţii pentru unicitatea prelungirii se pot găsi în lucrările [1], [2], [3].
Definifia 1. Fie ( X , X , X,||||X,\| \|X, ) un spatiu liniar normat, V V VVV o submultime a sa nevidă şi Y Y YYY un subspațiu liniar al lui X X XXX. Vom zice că subspatiul Y Y YYY este V V VVV-cebîsevian dacă dîndu-se v V v V v in Vv \in VvV există un singur element y 0 Y y 0 Y y_(0)in Yy_{0} \in Yy0Y astfel ca
(9)
v y 0 = inf y Y v y = d ( v , Y ) . v y 0 = inf y Y v y = d ( v , Y ) . ||v-y_(0)||=i n f_(y in Y)||v-y||=d(v,Y).\left\|v-y_{0}\right\|=\inf _{y \in Y}\|v-y\|=d(v, Y) .vy0=infyYvy=d(v,Y).
Fie Y Y YYY un subspatiu liniar al lui ( X , X , X,||||||X,\| \| \|X, ). Vom nota
(10) Y X τ p = { f X τ p , f ( y ) = 0 pentru toti y Y } . (10) Y X τ p = f X τ p , f ( y ) = 0  pentru toti  y Y . {:(10)Y_(X_(tau)^(p))^(_|_)={f inX_(tau)^(p),f(y)=0" pentru toti "y in Y}.:}\begin{equation*} Y_{X_{\tau}^{p}}^{\perp}=\left\{f \in X_{\tau}^{p}, f(y)=0 \text { pentru toti } y \in Y\right\} . \tag{10} \end{equation*}(10)YXτp={fXτp,f(y)=0 pentru toti yY}.
Evident Y X τ p Y X τ p Y_(X_(tau)^(p))^(_|_)Y_{X_{\tau}^{p}}^{\perp}YXτp este un subspatiu al lui X τ p X τ p X_(tau)^(p)X_{\tau}^{p}Xτp.
Lema 1. Fie Y Y YYY un subspatiu liniar al lui ( X , ) ( X , ) (X,||||||)(X,\| \| \|)(X,) şi h C X p τ h C X p τ h inC_(X)^(p)tauh \in C_{X}^{p} \tauhCXpτ. Atunci are loc următoavea egalitate:
(11)
h | Y Y = d ( h , Y X p ) . h Y Y = d h , Y X p . ||h|_(Y)||_(Y)=d(h,Y_(X^(p))^(_|_)).\left\|\left.h\right|_{Y}\right\|_{Y}=d\left(h, Y_{X^{p}}^{\perp}\right) .h|YY=d(h,YXp).
Demonstratie. Conform teoremer 1 , dacă h C X p h C X p h inC_(X)^(p)h \in C_{X}^{p}hCXp, pentru h | Y h Y h|_(Y)\left.h\right|_{Y}h|Y există H C X p τ H C X p τ H inC_(X)^(p)tauH \in C_{X}^{p} \tauHCXpτ cu proprietățile (8). Conform Lemei 2.8 (d) din [4] avem:
h | Y Y = h | Y Y 1 = sup y 1 < 1 y Y h ( y ) = sup y 1 1 < 1 y Y | h ( y ) | = = sup | ( h g ) ( y ) | sup | ( h g ) ( x ) | = h Y Y = h Y Y 1 = sup y 1 < 1 y Y h ( y ) = sup y 1 1 < 1 y Y | h ( y ) | = = sup | ( h g ) ( y ) | sup | ( h g ) ( x ) | = {:[||h|_(Y)||_(Y)=||h|_(Y)||_(Y)^(1)=s u p_({:[||y||||_(1) < 1],[y in Y]:})h(y)=s u p_({:[||y_(1)||||_(1) < 1],[y in Y]:})|h(y)|=],[=s u p|(h-g)(y)| <= s u p|(h-g)(x)|=]:}\begin{aligned} & \left\|\left.h\right|_{Y}\right\|_{Y}=\left\|\left.h\right|_{Y}\right\|_{Y}^{1}=\sup _{\substack{\|y\| \|_{1}<1 \\ y \in Y}} h(y)=\sup _{\substack{\left\|y_{1}\right\| \|_{1}<1 \\ y \in Y}}|h(y)|= \\ & =\sup |(h-g)(y)| \leqq \sup |(h-g)(x)|= \end{aligned}h|YY=h|YY1=supy1<1yYh(y)=supy11<1yY|h(y)|==sup|(hg)(y)|sup|(hg)(x)|=
= h g X 1 h g X . = h g X 1 h g X . {:=||h-g||_(X)^(1) <= ||h-g||_(X).:}\begin{aligned} & =\|h-g\|_{X}^{1} \leqq\|h-g\|_{X} . \end{aligned}=hgX1hgX.
De aici rezultă că
h | Y Y inf ε Y h g X p = d ( h , Y X τ p ) h Y Y inf ε Y h g X p = d h , Y X τ p ||h|_(Y)||_(Y) <= i n f_(epsi inY^(_|_))||h-g||_(X)^(p)=d(h,Y_(X_(tau)^(p))^(_|_))\left\|\left.h\right|_{Y}\right\|_{Y} \leqq \inf _{\varepsilon \in Y^{\perp}}\|h-g\|_{X}^{p}=d\left(h, Y_{X_{\tau}^{p}}^{\perp}\right)h|YYinfεYhgXp=d(h,YXτp)
Invers, conform teoremei 1 avem:
h | Y Y = H X = h ( h H ) X d ( h , Y X τ p ) . h Y Y = H X = h ( h H ) X d h , Y X τ p . ||h|_(Y)||_(Y)=||H||_(X)=||h-(h-H)||_(X) >= d(h,Y_(X_(tau)^(p))^(_|_)).\left\|\left.h\right|_{Y}\right\|_{Y}=\|H\|_{X}=\|h-(h-H)\|_{X} \geqq d\left(h, Y_{X_{\tau}^{p}}^{\perp}\right) .h|YY=HX=h(hH)Xd(h,YXτp).
Deci
h | Y Y = d ( h , Y X τ p ) . h Y Y = d h , Y X τ p . ||h|_(Y)||_(Y)=d(h,Y_(X_(tau)^(p))^(_|_)).\left\|\left.h\right|_{Y}\right\|_{Y}=d\left(h, Y_{X_{\tau}^{p}}^{\perp}\right) .h|YY=d(h,YXτp).
teorema 2. Fie Y Y YYY un subspatiu liniar al lui ( X , X , X,||||X,\| \|X, |||) şi h C X p h C X p h inC_(X)^(p)h \in C_{X}^{p}hCXp. Următoavele două afirmații sînt echivalente:
a) Oricare ar fi h C X p τ , h | Y h C X p τ , h Y h inC_(X)^(p)tau,h|_(Y)h \in C_{X}^{p} \tau,\left.h\right|_{Y}hCXpτ,h|Y are o prelungire H H HHH, cave verifică proprietătile (8) unică.
b) Y X τ p Y X τ p Y_(X_(tau)^(p))^(_|_)Y_{X_{\tau}^{p}}^{\perp}YXτp este C X p τ C X p τ C_(X)^(p)tau-C_{X}^{p} \tau-CXpτ cebîşevian.
Demonstratie. a) =>\Rightarrow b). Mai întîi observăm că pentru orice h C X p τ h C X p τ h inC_(X)^(p)tauh \in C_{X}^{p} \tauhCXpτ există un element g 0 Y X τ p g 0 Y X τ p g_(0)inY_(X_(tau)^(p))^(_|_)g_{0} \in Y_{X_{\tau}^{p}}^{\perp}g0YXτp astfel ca h g 0 X = d ( h , Y X τ p ) h g 0 X = d h , Y X τ p ||h-g_(0)||_(X)=d(h,Y_(X_(tau)^(p))^(_|_))\left\|h-g_{0}\right\|_{X}=d\left(h, Y_{X_{\tau}^{p}}^{\perp}\right)hg0X=d(h,YXτp). Intr-adevăr, conform TEOREMEI 1 și LEMEI 1 , h | Y 1 , h Y 1,h|_(Y)1,\left.h\right|_{Y}1,h|Y are o prelungire H C X p τ H C X p τ H inC_(X)^(p)tauH \in C_{X}^{p} \tauHCXpτ astfel ca
h | Y Y = d ( h , Y X τ p ) = h ( h H ) X . h Y Y = d h , Y X τ p = h ( h H ) X . ||h|_(Y)||_(Y)=d(h,Y_(X_(tau)^(p))^(_|_))=||h-(h-H)||_(X).\left\|\left.h\right|_{Y}\right\|_{Y}=d\left(h, Y_{X_{\tau}^{p}}^{\perp}\right)=\|h-(h-H)\|_{X} .h|YY=d(h,YXτp)=h(hH)X.
Deci g 0 = h H g 0 = h H g_(0)=h-Hg_{0}=h-Hg0=hH.
Să presupunem acum că Y X Y X Y_(X)^(_|_)Y_{X}^{\perp}YX nu este C X p C X p C_(X)^(p)C_{X}^{p}CXp - cevişevian; atunci există h C X p τ h C X p τ h inC_(X)^(p)tauh \in C_{X}^{p} \tauhCXpτ şi există g 1 , g 2 din Y X τ p τ , g 1 g 2 g 1 , g 2 din Y X τ p τ , g 1 g 2 g_(1),g_(2)dinY_(X_(tau)^(p))^(tau),g_(1)!=g_(2)g_{1}, g_{2} \operatorname{din} Y_{X_{\tau}^{p}}^{\tau}, g_{1} \neq g_{2}g1,g2dinYXτpτ,g1g2 astfel ca
(12) h g 1 X = h g 2 X = d ( h , Y X τ p ) = h | Y Y . (12) h g 1 X = h g 2 X = d h , Y X τ p = h Y Y . {:(12)||h-g_(1)||_(X)=||h-g_(2)||_(X)=d(h,Y_(X_(tau)^(p))^(_|_))=||h|_(Y)||_(Y).:}\begin{equation*} \left\|h-g_{1}\right\|_{X}=\left\|h-g_{2}\right\|_{X}=d\left(h, Y_{X_{\tau}^{p}}^{\perp}\right)=\left\|\left.h\right|_{Y}\right\|_{Y} . \tag{12} \end{equation*}(12)hg1X=hg2X=d(h,YXτp)=h|YY.
Dar atunci, avînd în vedere şi egalitățile (12) rezultă că h g 1 h g 1 h-g_(1)h-g_{1}hg1 şi h g 2 h g 2 h-g_(2)h-g_{2}hg2 sînt două prelungiri diferite ale lui h | Y h Y h|_(Y)\left.h\right|_{Y}h|Y.
b) =>\Rightarrow a). Să presupunem că există h C X p h C X p h inC_(X)^(p)h \in C_{X}^{p}hCXp astfel ca h | Y h Y h|_(Y)\left.h\right|_{Y}h|Y să aibă prelungirile H 1 , H 2 C X p τ , H 1 H 2 cu H 1 , H 2 C X p τ , H 1 H 2 cu H_(1),H_(2)inC_(X)^(p)tau,H_(1)!=H_(2)cuH_{1}, H_{2} \in C_{X}^{p} \tau, H_{1} \neq H_{2} \mathrm{cu}H1,H2CXpτ,H1H2cu proprietățile (8). Atunci din mema 1 rezultă că:
H 1 X = H 1 ( H 1 H 2 ) X = h | Y Y = H 1 | Y Y = d ( H 1 , Y X τ p ) . H 1 X = H 1 H 1 H 2 X = h Y Y = H 1 Y Y = d H 1 , Y X τ p . ||H_(1)||_(X)=||H_(1)-(H_(1)-H_(2))||_(X)=||h|_(Y)||_(Y)=||H_(1)|_(Y)||_(Y)=d(H_(1),Y_(X_(tau)^(p))^(_|_)).\left\|H_{1}\right\|_{X}=\left\|H_{1}-\left(H_{1}-H_{2}\right)\right\|_{X}=\left\|\left.h\right|_{Y}\right\|_{Y}=\left\|\left.H_{1}\right|_{Y}\right\|_{Y}=d\left(H_{1}, Y_{X_{\tau}^{p}}^{\perp}\right) .H1X=H1(H1H2)X=h|YY=H1|YY=d(H1,YXτp).
Dar aceasta înseamnă că pentru H 1 H 1 H_(1)H_{1}H1 există două elemente din Y x τ p Y x τ p Y_(x_(tau)^(p))^(_|_)Y_{x_{\tau}^{p}}^{\perp}Yxτp pentru care are loc (9) şi anume Φ Φ Phi\PhiΦ şi H 1 H 2 Φ H 1 H 2 Φ H_(1)-H_(2)!=PhiH_{1}-H_{2} \neq \PhiH1H2Φ, deci Y X τ p Y X τ p Y_(X)^(_|_)_(tau)^(p)Y_{X}^{\perp}{ }_{\tau}^{p}YXτp nu este C X p C X p C_(X)^(p)C_{X}^{p}CXp r-cebîşevian.

SUR L'UNICITÉ DU PROLONGEMENT DES p p ppp-SÉMINORMES CONTINUES

RÉSUME

Soit ( X , X , X,||||||X,\| \| \|X, ) un espace p p ppp-normé réel ( p ( 0 , 1 ] p ( 0 , 1 ] p in(0,1]p \in(0,1]p(0,1] ), Y Y YYY un sousespace de ( X , ) ( X , ) (X,||∣||)(X,\|\mid\|)(X,) et soit C X p C X p C_(X)^(p)C_{X}^{p}CXp r le cône des p p ppp-séminormes continues sur ( X , ) ( X , ) (X,||||||)(X,\| \| \|)(X,). Soit h C X p τ h C X p τ h inC_(X)^(p)tauh \in C_{X}^{p} \tauhCXpτ et H H HHH un prolongement de h | Y h Y h|_(Y)\left.h\right|_{Y}h|Y de Y Y YYY sur X X XXX qui conserve 1a norme de Y Y YYY. On montre que H H HHH est un prolongement unique si et seulement si Y C X p τ C X p τ Y C X p τ C X p τ ∣Y_(C)^(_|_)_(X)^(p)tau-C_(X)^(p)tau\mid Y_{C}^{\perp}{ }_{X}^{p} \tau-C_{X}^{p} \tauYCXpτCXpτ lest un sousespace de Tchébycheff pour les éléments de C X p τ C X p τ C_(X)^(p)tauC_{X}^{p} \tauCXpτ.

BIBLIOGRAFIE

[1] Kolumban I., Ob edinstvenosti prodoljenia lineinîh functionalov, Mathematica, vol. 4
[2] (24), 2, 1962), 267-270. schitziene, "Revista de analiză numerică şi teoria aproximației", vol. 2, fasc. 1, (1973), "81-87.
[3] Phelps R. R., Uniqueness of Hahn-Banach extension and unique best approximction, Trans. Amer. Math. Soc., 95, (1960), 238-255.
[4] Ruess W., Ein Dualkegel für p-konvexe topologische lineare Räume, Gesellschaft für Mathematik und Datenverarbeltung, Bonn, Nr. 60, (1973).
Institutul de calcul din Clui
al Academiei Republicii Socialiste
Románia
Primit la 28. V. 1973.

    • R + R + R+\mathrm{R}+R+ reprezintă mulțimea numerelor reale nenegative.
1973

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