Task concerning Stone Weierstrass












0












$begingroup$


Let $D$ be the open unit circle with center $0$ in $mathbb{C}$.



Let $overline D$ be the closed unit circle with center $0$ in $mathbb{C}$.



Let $mathbb{C}[z]_{big|overline D}$ be the set of all functions $f:overline Dtomathbb{C}$ of polynomials of the type $f(z)=sum_{j=0}^{N}a_jz^j$.



The task has something to do with the complex version of Stone-Weierstrass.




I need to show which of the conditions for the sentence are not fulfilled:




$mathbb{C}[z]_{big|overline D}$ is a algebra in $C_b(D,mathbb{C})$ where $C_b(D,mathbb{C})$ describes the set of all continuous and bounded functions from $D$ to $mathbb{C}$. I think there should be no problem because $mathbb{C}[z]_{big|overline D}$ is a linear subspace and for $f,g in mathbb{C}[z]_{big|overline D}$ $Rightarrow fg in mathbb{C}[z]_{big|overline D}$.



Then the next one: $z_1 neq z_2 Rightarrow f(z_1) neq f(z_2)$ should be no problem either by choosing $f(z)=z$ And for every $z$ there exists a $f$ such that $f(z)neq 0$ by simply choosing $f=1$ constant.



So If I didn't make any mistakes so far the only thing that can go wrong is $f in mathbb{C}[z]_{big|overline D} Rightarrow overline f in mathbb{C}[z]_{big|overline D}$.



$overline f(z)=sum_{j=0}^{N}overline a_j z^{-j}$. Is this not in $mathbb{C}[z]_{big|overline D}$ because $z$ can have negative exponents?






real-analysis complex-analysis







share|cite|improve this question











$endgroup$












  • $begingroup$
    It is true that $sum_{j=0}^N overline a_j z^{-j}$ is not in $left.Bbb C[z]rightrvert_{overline D}$, but it's not true that $overline f(z)=sum_{j=0}^N overline a_j z^{-j}$ (unless $N=0$). In fact, $$overline f(z)=sum_{j=0}^N overline a_j overline z^{j}.$$
    $endgroup$
    – Saucy O'Path
    Dec 15 '18 at 16:38










  • $begingroup$
    So for $f in mathbb{C}[z]_{big|overline D} Rightarrow overline f in mathbb{C}[z]_{big|overline D}$ is fulfilled?
    $endgroup$
    – conrad
    Dec 15 '18 at 16:51










  • $begingroup$
    I'm wondering, is $overline f$ a polynomial of the type $f(z)=sum_{j=0}^{N}a_jz^j$?
    $endgroup$
    – conrad
    Dec 15 '18 at 17:00












  • $begingroup$
    No, it isn't${}$.
    $endgroup$
    – Saucy O'Path
    Dec 15 '18 at 21:20
















0












$begingroup$


Let $D$ be the open unit circle with center $0$ in $mathbb{C}$.



Let $overline D$ be the closed unit circle with center $0$ in $mathbb{C}$.



Let $mathbb{C}[z]_{big|overline D}$ be the set of all functions $f:overline Dtomathbb{C}$ of polynomials of the type $f(z)=sum_{j=0}^{N}a_jz^j$.



The task has something to do with the complex version of Stone-Weierstrass.




I need to show which of the conditions for the sentence are not fulfilled:




$mathbb{C}[z]_{big|overline D}$ is a algebra in $C_b(D,mathbb{C})$ where $C_b(D,mathbb{C})$ describes the set of all continuous and bounded functions from $D$ to $mathbb{C}$. I think there should be no problem because $mathbb{C}[z]_{big|overline D}$ is a linear subspace and for $f,g in mathbb{C}[z]_{big|overline D}$ $Rightarrow fg in mathbb{C}[z]_{big|overline D}$.



Then the next one: $z_1 neq z_2 Rightarrow f(z_1) neq f(z_2)$ should be no problem either by choosing $f(z)=z$ And for every $z$ there exists a $f$ such that $f(z)neq 0$ by simply choosing $f=1$ constant.



So If I didn't make any mistakes so far the only thing that can go wrong is $f in mathbb{C}[z]_{big|overline D} Rightarrow overline f in mathbb{C}[z]_{big|overline D}$.



$overline f(z)=sum_{j=0}^{N}overline a_j z^{-j}$. Is this not in $mathbb{C}[z]_{big|overline D}$ because $z$ can have negative exponents?






real-analysis complex-analysis







share|cite|improve this question











$endgroup$












  • $begingroup$
    It is true that $sum_{j=0}^N overline a_j z^{-j}$ is not in $left.Bbb C[z]rightrvert_{overline D}$, but it's not true that $overline f(z)=sum_{j=0}^N overline a_j z^{-j}$ (unless $N=0$). In fact, $$overline f(z)=sum_{j=0}^N overline a_j overline z^{j}.$$
    $endgroup$
    – Saucy O'Path
    Dec 15 '18 at 16:38










  • $begingroup$
    So for $f in mathbb{C}[z]_{big|overline D} Rightarrow overline f in mathbb{C}[z]_{big|overline D}$ is fulfilled?
    $endgroup$
    – conrad
    Dec 15 '18 at 16:51










  • $begingroup$
    I'm wondering, is $overline f$ a polynomial of the type $f(z)=sum_{j=0}^{N}a_jz^j$?
    $endgroup$
    – conrad
    Dec 15 '18 at 17:00












  • $begingroup$
    No, it isn't${}$.
    $endgroup$
    – Saucy O'Path
    Dec 15 '18 at 21:20














0












0








0





$begingroup$


Let $D$ be the open unit circle with center $0$ in $mathbb{C}$.



Let $overline D$ be the closed unit circle with center $0$ in $mathbb{C}$.



Let $mathbb{C}[z]_{big|overline D}$ be the set of all functions $f:overline Dtomathbb{C}$ of polynomials of the type $f(z)=sum_{j=0}^{N}a_jz^j$.



The task has something to do with the complex version of Stone-Weierstrass.




I need to show which of the conditions for the sentence are not fulfilled:




$mathbb{C}[z]_{big|overline D}$ is a algebra in $C_b(D,mathbb{C})$ where $C_b(D,mathbb{C})$ describes the set of all continuous and bounded functions from $D$ to $mathbb{C}$. I think there should be no problem because $mathbb{C}[z]_{big|overline D}$ is a linear subspace and for $f,g in mathbb{C}[z]_{big|overline D}$ $Rightarrow fg in mathbb{C}[z]_{big|overline D}$.



Then the next one: $z_1 neq z_2 Rightarrow f(z_1) neq f(z_2)$ should be no problem either by choosing $f(z)=z$ And for every $z$ there exists a $f$ such that $f(z)neq 0$ by simply choosing $f=1$ constant.



So If I didn't make any mistakes so far the only thing that can go wrong is $f in mathbb{C}[z]_{big|overline D} Rightarrow overline f in mathbb{C}[z]_{big|overline D}$.



$overline f(z)=sum_{j=0}^{N}overline a_j z^{-j}$. Is this not in $mathbb{C}[z]_{big|overline D}$ because $z$ can have negative exponents?






real-analysis complex-analysis







share|cite|improve this question











$endgroup$




Let $D$ be the open unit circle with center $0$ in $mathbb{C}$.



Let $overline D$ be the closed unit circle with center $0$ in $mathbb{C}$.



Let $mathbb{C}[z]_{big|overline D}$ be the set of all functions $f:overline Dtomathbb{C}$ of polynomials of the type $f(z)=sum_{j=0}^{N}a_jz^j$.



The task has something to do with the complex version of Stone-Weierstrass.




I need to show which of the conditions for the sentence are not fulfilled:




$mathbb{C}[z]_{big|overline D}$ is a algebra in $C_b(D,mathbb{C})$ where $C_b(D,mathbb{C})$ describes the set of all continuous and bounded functions from $D$ to $mathbb{C}$. I think there should be no problem because $mathbb{C}[z]_{big|overline D}$ is a linear subspace and for $f,g in mathbb{C}[z]_{big|overline D}$ $Rightarrow fg in mathbb{C}[z]_{big|overline D}$.



Then the next one: $z_1 neq z_2 Rightarrow f(z_1) neq f(z_2)$ should be no problem either by choosing $f(z)=z$ And for every $z$ there exists a $f$ such that $f(z)neq 0$ by simply choosing $f=1$ constant.



So If I didn't make any mistakes so far the only thing that can go wrong is $f in mathbb{C}[z]_{big|overline D} Rightarrow overline f in mathbb{C}[z]_{big|overline D}$.



$overline f(z)=sum_{j=0}^{N}overline a_j z^{-j}$. Is this not in $mathbb{C}[z]_{big|overline D}$ because $z$ can have negative exponents?






real-analysis complex-analysis




real-analysis complex-analysis






share|cite|improve this question















share|cite|improve this question













share|cite|improve this question




share|cite|improve this question








edited Dec 15 '18 at 17:00







conrad

















asked Dec 15 '18 at 16:34









conradconrad

757




757












  • $begingroup$
    It is true that $sum_{j=0}^N overline a_j z^{-j}$ is not in $left.Bbb C[z]rightrvert_{overline D}$, but it's not true that $overline f(z)=sum_{j=0}^N overline a_j z^{-j}$ (unless $N=0$). In fact, $$overline f(z)=sum_{j=0}^N overline a_j overline z^{j}.$$
    $endgroup$
    – Saucy O'Path
    Dec 15 '18 at 16:38










  • $begingroup$
    So for $f in mathbb{C}[z]_{big|overline D} Rightarrow overline f in mathbb{C}[z]_{big|overline D}$ is fulfilled?
    $endgroup$
    – conrad
    Dec 15 '18 at 16:51










  • $begingroup$
    I'm wondering, is $overline f$ a polynomial of the type $f(z)=sum_{j=0}^{N}a_jz^j$?
    $endgroup$
    – conrad
    Dec 15 '18 at 17:00












  • $begingroup$
    No, it isn't${}$.
    $endgroup$
    – Saucy O'Path
    Dec 15 '18 at 21:20


















  • $begingroup$
    It is true that $sum_{j=0}^N overline a_j z^{-j}$ is not in $left.Bbb C[z]rightrvert_{overline D}$, but it's not true that $overline f(z)=sum_{j=0}^N overline a_j z^{-j}$ (unless $N=0$). In fact, $$overline f(z)=sum_{j=0}^N overline a_j overline z^{j}.$$
    $endgroup$
    – Saucy O'Path
    Dec 15 '18 at 16:38










  • $begingroup$
    So for $f in mathbb{C}[z]_{big|overline D} Rightarrow overline f in mathbb{C}[z]_{big|overline D}$ is fulfilled?
    $endgroup$
    – conrad
    Dec 15 '18 at 16:51










  • $begingroup$
    I'm wondering, is $overline f$ a polynomial of the type $f(z)=sum_{j=0}^{N}a_jz^j$?
    $endgroup$
    – conrad
    Dec 15 '18 at 17:00












  • $begingroup$
    No, it isn't${}$.
    $endgroup$
    – Saucy O'Path
    Dec 15 '18 at 21:20
















$begingroup$
It is true that $sum_{j=0}^N overline a_j z^{-j}$ is not in $left.Bbb C[z]rightrvert_{overline D}$, but it's not true that $overline f(z)=sum_{j=0}^N overline a_j z^{-j}$ (unless $N=0$). In fact, $$overline f(z)=sum_{j=0}^N overline a_j overline z^{j}.$$
$endgroup$
– Saucy O'Path
Dec 15 '18 at 16:38




$begingroup$
It is true that $sum_{j=0}^N overline a_j z^{-j}$ is not in $left.Bbb C[z]rightrvert_{overline D}$, but it's not true that $overline f(z)=sum_{j=0}^N overline a_j z^{-j}$ (unless $N=0$). In fact, $$overline f(z)=sum_{j=0}^N overline a_j overline z^{j}.$$
$endgroup$
– Saucy O'Path
Dec 15 '18 at 16:38












$begingroup$
So for $f in mathbb{C}[z]_{big|overline D} Rightarrow overline f in mathbb{C}[z]_{big|overline D}$ is fulfilled?
$endgroup$
– conrad
Dec 15 '18 at 16:51




$begingroup$
So for $f in mathbb{C}[z]_{big|overline D} Rightarrow overline f in mathbb{C}[z]_{big|overline D}$ is fulfilled?
$endgroup$
– conrad
Dec 15 '18 at 16:51












$begingroup$
I'm wondering, is $overline f$ a polynomial of the type $f(z)=sum_{j=0}^{N}a_jz^j$?
$endgroup$
– conrad
Dec 15 '18 at 17:00






$begingroup$
I'm wondering, is $overline f$ a polynomial of the type $f(z)=sum_{j=0}^{N}a_jz^j$?
$endgroup$
– conrad
Dec 15 '18 at 17:00














$begingroup$
No, it isn't${}$.
$endgroup$
– Saucy O'Path
Dec 15 '18 at 21:20




$begingroup$
No, it isn't${}$.
$endgroup$
– Saucy O'Path
Dec 15 '18 at 21:20










0






active

oldest

votes











Your Answer





StackExchange.ifUsing("editor", function () {
return StackExchange.using("mathjaxEditing", function () {
StackExchange.MarkdownEditor.creationCallbacks.add(function (editor, postfix) {
StackExchange.mathjaxEditing.prepareWmdForMathJax(editor, postfix, [["$", "$"], ["\\(","\\)"]]);
});
});
}, "mathjax-editing");

StackExchange.ready(function() {
var channelOptions = {
tags: "".split(" "),
id: "69"
};
initTagRenderer("".split(" "), "".split(" "), channelOptions);

StackExchange.using("externalEditor", function() {
// Have to fire editor after snippets, if snippets enabled
if (StackExchange.settings.snippets.snippetsEnabled) {
StackExchange.using("snippets", function() {
createEditor();
});
}
else {
createEditor();
}
});

function createEditor() {
StackExchange.prepareEditor({
heartbeatType: 'answer',
autoActivateHeartbeat: false,
convertImagesToLinks: true,
noModals: true,
showLowRepImageUploadWarning: true,
reputationToPostImages: 10,
bindNavPrevention: true,
postfix: "",
imageUploader: {
brandingHtml: "Powered by u003ca class="icon-imgur-white" href="https://imgur.com/"u003eu003c/au003e",
contentPolicyHtml: "User contributions licensed under u003ca href="https://creativecommons.org/licenses/by-sa/3.0/"u003ecc by-sa 3.0 with attribution requiredu003c/au003e u003ca href="https://stackoverflow.com/legal/content-policy"u003e(content policy)u003c/au003e",
allowUrls: true
},
noCode: true, onDemand: true,
discardSelector: ".discard-answer"
,immediatelyShowMarkdownHelp:true
});


}
});














draft saved

draft discarded


















StackExchange.ready(
function () {
StackExchange.openid.initPostLogin('.new-post-login', 'https%3a%2f%2fmath.stackexchange.com%2fquestions%2f3041684%2ftask-concerning-stone-weierstrass%23new-answer', 'question_page');
}
);

Post as a guest















Required, but never shown

























0






active

oldest

votes








0






active

oldest

votes









active

oldest

votes






active

oldest

votes
















draft saved

draft discarded




















































Thanks for contributing an answer to Mathematics Stack Exchange!


  • Please be sure to answer the question. Provide details and share your research!

But avoid …



  • Asking for help, clarification, or responding to other answers.

  • Making statements based on opinion; back them up with references or personal experience.


Use MathJax to format equations. MathJax reference.


To learn more, see our tips on writing great answers.




draft saved


draft discarded














StackExchange.ready(
function () {
StackExchange.openid.initPostLogin('.new-post-login', 'https%3a%2f%2fmath.stackexchange.com%2fquestions%2f3041684%2ftask-concerning-stone-weierstrass%23new-answer', 'question_page');
}
);

Post as a guest















Required, but never shown





















































Required, but never shown














Required, but never shown












Required, but never shown







Required, but never shown

































Required, but never shown














Required, but never shown












Required, but never shown







Required, but never shown







-

Popular posts from this blog

Plaza Victoria

Image
Plaza Victoria Plaza Victoria en 2017 Otros nombres Plaza de La Victoria Localización El Almendral, Valparaíso, Chile Vías adyacentes Chacabuco, Molina, Plaza Victoria y Edwards Creación 1841 Metro Bellavista Ubicación 33°02′46″S 71°37′11″O  /  -33.04624167, -71.61980833 Coordenadas: 33°02′46″S 71°37′11″O  /  -33.04624167, -71.61980833 [editar datos en Wikidata] La Plaza Victoria , también llamada Plaza de la Victoria , [ 1 ] ​ es una plaza ubicada en el sector El Almendral del plan de la ciudad de Valparaíso, Chile, de importancia patrimonial, que incluye valiosas estatuas y especies vegetales. [ 2 ] ​ La plaza se creó como tal en la primera mitad del siglo XIX, tomando previamente los nombres de Plaza Nueva y Plaza de Orrego . Si bien desde sus inicios tuvo relevancia histórica, no fue sino a partir de fines de los años 1860 cuando comenzó a cobrar mayor importancia y se convirtió en uno de los principales centros sociales de l
Read more

In PowerPoint, is there a keyboard shortcut for bulleted / numbered list?

Image
.everyoneloves__top-leaderboard:empty,.everyoneloves__mid-leaderboard:empty,.everyoneloves__bot-mid-leaderboard:empty{ height:90px;width:728px;box-sizing:border-box; } 2 OneNote has two useful keyboard shortcuts for bulleted and numbered lists, namely Ctrl + . and Ctrl + - . Is there something similar for PowerPoint where the work with lists is also very common? Haven't found anything here: Use keyboard shortcuts in PowerPoint. keyboard-shortcuts microsoft-powerpoint share | improve this question edited Aug 14 '14 at 14:49 Linger 2,826 10 28 40
Read more

How to put 3 figures in Latex with 2 figures side by side and 1 below these side by side images but in...

Image
1 0 I want to put 3 images in LateX such as 2 figures are side by side horizotally and 3rd figure below these 2 side by side figures but in middle. I have following code: begin{figure}[H] centering begin{minipage}{.5textwidth} centering includegraphics[width=.50linewidth]{Text/Images/Genelec_8010_AP.jpg} captionof{figure}{Genelec 8010 AP} label{fig:Genelec 8010} end{minipage}% begin{minipage}{.5textwidth} centering includegraphics[width=.69linewidth]{Text/Images/Genelec_8020_CPM.jpg} captionof{figure}{Genelec 8020 CPM} label{fig:Genelec 8020} end{minipage}% end{figure} %%Here is my 3rd picture code begin{figure} centering includegraphics[width=5cm]{Text/Images/Genelec_8030_BPM.jpg} caption{Genelec 8030 BPM} label{fig:Genelec 8030} end{figure} I want to ask ho
Read more