How to write $f(z)=sqrt{z}$ as a complex series around the origin
$begingroup$
I'm confused about the way to approach this exercises, I've been introduced complex analysis a few weeks ago.
If I have, for example, $f(z)=sqrt z$ what I'm tempted to do is to find some derivarives $f,f',f'',f''',...$ to see if I can guess the n-th term and write it's taylor polynomial around the origin.
Another way is to find some way to write $f(z)$ with terms whose series expansion are used frequently and well known.
My question is: what is necessary to be able to expand $f(z)$ as a power series around $z_0$? Is there a way am I missing? how can we do this kind of exercises?
Thanks for your time.
calculus complex-analysis taylor-expansion
$endgroup$
add a comment |
$begingroup$
I'm confused about the way to approach this exercises, I've been introduced complex analysis a few weeks ago.
If I have, for example, $f(z)=sqrt z$ what I'm tempted to do is to find some derivarives $f,f',f'',f''',...$ to see if I can guess the n-th term and write it's taylor polynomial around the origin.
Another way is to find some way to write $f(z)$ with terms whose series expansion are used frequently and well known.
My question is: what is necessary to be able to expand $f(z)$ as a power series around $z_0$? Is there a way am I missing? how can we do this kind of exercises?
Thanks for your time.
calculus complex-analysis taylor-expansion
$endgroup$
4
$begingroup$
Are you aware that there is no single-valued and continuous $sqrt{z}$ around the origin?
$endgroup$
– Adayah
Jun 20 '18 at 15:33
$begingroup$
Beginning complex analysis, you perhaps learned the definition of "analytic function". Check that $sqrt{z}$ is not analytic at $z=0$.
$endgroup$
– GEdgar
Jun 20 '18 at 15:35
$begingroup$
If you write down a definition of $sqrt z,$ the impossibility of doing this will become evident.
$endgroup$
– zhw.
Jun 20 '18 at 15:59
add a comment |
$begingroup$
I'm confused about the way to approach this exercises, I've been introduced complex analysis a few weeks ago.
If I have, for example, $f(z)=sqrt z$ what I'm tempted to do is to find some derivarives $f,f',f'',f''',...$ to see if I can guess the n-th term and write it's taylor polynomial around the origin.
Another way is to find some way to write $f(z)$ with terms whose series expansion are used frequently and well known.
My question is: what is necessary to be able to expand $f(z)$ as a power series around $z_0$? Is there a way am I missing? how can we do this kind of exercises?
Thanks for your time.
calculus complex-analysis taylor-expansion
$endgroup$
I'm confused about the way to approach this exercises, I've been introduced complex analysis a few weeks ago.
If I have, for example, $f(z)=sqrt z$ what I'm tempted to do is to find some derivarives $f,f',f'',f''',...$ to see if I can guess the n-th term and write it's taylor polynomial around the origin.
Another way is to find some way to write $f(z)$ with terms whose series expansion are used frequently and well known.
My question is: what is necessary to be able to expand $f(z)$ as a power series around $z_0$? Is there a way am I missing? how can we do this kind of exercises?
Thanks for your time.
calculus complex-analysis taylor-expansion
calculus complex-analysis taylor-expansion
edited Dec 23 '18 at 3:34
Eric Wofsey
185k13213339
185k13213339
asked Jun 20 '18 at 15:30
RelureRelure
2,1671035
2,1671035
4
$begingroup$
Are you aware that there is no single-valued and continuous $sqrt{z}$ around the origin?
$endgroup$
– Adayah
Jun 20 '18 at 15:33
$begingroup$
Beginning complex analysis, you perhaps learned the definition of "analytic function". Check that $sqrt{z}$ is not analytic at $z=0$.
$endgroup$
– GEdgar
Jun 20 '18 at 15:35
$begingroup$
If you write down a definition of $sqrt z,$ the impossibility of doing this will become evident.
$endgroup$
– zhw.
Jun 20 '18 at 15:59
add a comment |
4
$begingroup$
Are you aware that there is no single-valued and continuous $sqrt{z}$ around the origin?
$endgroup$
– Adayah
Jun 20 '18 at 15:33
$begingroup$
Beginning complex analysis, you perhaps learned the definition of "analytic function". Check that $sqrt{z}$ is not analytic at $z=0$.
$endgroup$
– GEdgar
Jun 20 '18 at 15:35
$begingroup$
If you write down a definition of $sqrt z,$ the impossibility of doing this will become evident.
$endgroup$
– zhw.
Jun 20 '18 at 15:59
4
4
$begingroup$
Are you aware that there is no single-valued and continuous $sqrt{z}$ around the origin?
$endgroup$
– Adayah
Jun 20 '18 at 15:33
$begingroup$
Are you aware that there is no single-valued and continuous $sqrt{z}$ around the origin?
$endgroup$
– Adayah
Jun 20 '18 at 15:33
$begingroup$
Beginning complex analysis, you perhaps learned the definition of "analytic function". Check that $sqrt{z}$ is not analytic at $z=0$.
$endgroup$
– GEdgar
Jun 20 '18 at 15:35
$begingroup$
Beginning complex analysis, you perhaps learned the definition of "analytic function". Check that $sqrt{z}$ is not analytic at $z=0$.
$endgroup$
– GEdgar
Jun 20 '18 at 15:35
$begingroup$
If you write down a definition of $sqrt z,$ the impossibility of doing this will become evident.
$endgroup$
– zhw.
Jun 20 '18 at 15:59
$begingroup$
If you write down a definition of $sqrt z,$ the impossibility of doing this will become evident.
$endgroup$
– zhw.
Jun 20 '18 at 15:59
add a comment |
1 Answer
1
active
oldest
votes
$begingroup$
A Taylor series expansion is possible for a function $fcolonmathbb Ctomathbb C$ is possible (converges) at $z_0$ if $f$ is complex analytic in a neighborhood of the point $z_0$.
The square root function has two issues.
First, at the origin it is not even analytic: it does not satisfy the Cauchy–Riemann equations because it is not even differentiable.
In addition, outside the origin it is not single-valued.
If you choose a branch (see below) of the square root, then it is analytic in a neighborhood of any $z_0neq0$.
There is also a possibility to have a series expansion around singular points.
This is known as a Laurent series, but at this stage you should focus on analytic functions and Taylor series.
But even the Laurent series does not exist for all functions.
It requires that the singularity is of a suitable nice kind, and the one of the square root at the origin does not qualify.
Every non-zero complex number has two square roots, just like a number $x>0$ has two square roots which differ by sign.
Taking a branch means choosing one of the two in a consistent way so that the square root function becomes continuous and single-valued.
This concept should appear at some later point in your studies in more detail, but this is the idea.
Some other functions have more than two options to choose from.
For example, the number $1+i$ has three cubic roots and infinitely many logarithms.
$endgroup$
$begingroup$
Hi @Joonas, what do you mean by a branch?
$endgroup$
– Relure
Jun 20 '18 at 15:45
1
$begingroup$
@Relure Every non-zero complex number has two square roots, just like a number $x>0$ has two square roots which differ by sign. Taking a branch means choosing one of the two in a consistent way so that the square root function becomes continuous and single-valued. This concept should appear at some later point in your studies in more detail, but this is the idea. Some other functions have more than two options to choose from.
$endgroup$
– Joonas Ilmavirta
Jun 20 '18 at 15:50
2
$begingroup$
I like most of this answer (+1) but I worry that the third paragraph might give the OP or other readers the idea that there will be a Laurent series for $sqrt z$ around $0$. The best one can actually get is (as far as I know) a Puiseux series (which in this case would be trivial: $z^{1/2}$).
$endgroup$
– Andreas Blass
Jun 20 '18 at 16:55
$begingroup$
@AndreasBlass Fair enough, that was indeed misleading. I edited the paragraph a bit.
$endgroup$
– Joonas Ilmavirta
Jun 20 '18 at 18:59
add a comment |
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
});
}
});
Sign up or log in
StackExchange.ready(function () {
StackExchange.helpers.onClickDraftSave('#login-link');
});
Sign up using Google
Sign up using Facebook
Sign up using Email and Password
Post as a guest
Required, but never shown
StackExchange.ready(
function () {
StackExchange.openid.initPostLogin('.new-post-login', 'https%3a%2f%2fmath.stackexchange.com%2fquestions%2f2826230%2fhow-to-write-fz-sqrtz-as-a-complex-series-around-the-origin%23new-answer', 'question_page');
}
);
Post as a guest
Required, but never shown
1 Answer
1
active
oldest
votes
1 Answer
1
active
oldest
votes
active
oldest
votes
active
oldest
votes
$begingroup$
A Taylor series expansion is possible for a function $fcolonmathbb Ctomathbb C$ is possible (converges) at $z_0$ if $f$ is complex analytic in a neighborhood of the point $z_0$.
The square root function has two issues.
First, at the origin it is not even analytic: it does not satisfy the Cauchy–Riemann equations because it is not even differentiable.
In addition, outside the origin it is not single-valued.
If you choose a branch (see below) of the square root, then it is analytic in a neighborhood of any $z_0neq0$.
There is also a possibility to have a series expansion around singular points.
This is known as a Laurent series, but at this stage you should focus on analytic functions and Taylor series.
But even the Laurent series does not exist for all functions.
It requires that the singularity is of a suitable nice kind, and the one of the square root at the origin does not qualify.
Every non-zero complex number has two square roots, just like a number $x>0$ has two square roots which differ by sign.
Taking a branch means choosing one of the two in a consistent way so that the square root function becomes continuous and single-valued.
This concept should appear at some later point in your studies in more detail, but this is the idea.
Some other functions have more than two options to choose from.
For example, the number $1+i$ has three cubic roots and infinitely many logarithms.
$endgroup$
$begingroup$
Hi @Joonas, what do you mean by a branch?
$endgroup$
– Relure
Jun 20 '18 at 15:45
1
$begingroup$
@Relure Every non-zero complex number has two square roots, just like a number $x>0$ has two square roots which differ by sign. Taking a branch means choosing one of the two in a consistent way so that the square root function becomes continuous and single-valued. This concept should appear at some later point in your studies in more detail, but this is the idea. Some other functions have more than two options to choose from.
$endgroup$
– Joonas Ilmavirta
Jun 20 '18 at 15:50
2
$begingroup$
I like most of this answer (+1) but I worry that the third paragraph might give the OP or other readers the idea that there will be a Laurent series for $sqrt z$ around $0$. The best one can actually get is (as far as I know) a Puiseux series (which in this case would be trivial: $z^{1/2}$).
$endgroup$
– Andreas Blass
Jun 20 '18 at 16:55
$begingroup$
@AndreasBlass Fair enough, that was indeed misleading. I edited the paragraph a bit.
$endgroup$
– Joonas Ilmavirta
Jun 20 '18 at 18:59
add a comment |
$begingroup$
A Taylor series expansion is possible for a function $fcolonmathbb Ctomathbb C$ is possible (converges) at $z_0$ if $f$ is complex analytic in a neighborhood of the point $z_0$.
The square root function has two issues.
First, at the origin it is not even analytic: it does not satisfy the Cauchy–Riemann equations because it is not even differentiable.
In addition, outside the origin it is not single-valued.
If you choose a branch (see below) of the square root, then it is analytic in a neighborhood of any $z_0neq0$.
There is also a possibility to have a series expansion around singular points.
This is known as a Laurent series, but at this stage you should focus on analytic functions and Taylor series.
But even the Laurent series does not exist for all functions.
It requires that the singularity is of a suitable nice kind, and the one of the square root at the origin does not qualify.
Every non-zero complex number has two square roots, just like a number $x>0$ has two square roots which differ by sign.
Taking a branch means choosing one of the two in a consistent way so that the square root function becomes continuous and single-valued.
This concept should appear at some later point in your studies in more detail, but this is the idea.
Some other functions have more than two options to choose from.
For example, the number $1+i$ has three cubic roots and infinitely many logarithms.
$endgroup$
$begingroup$
Hi @Joonas, what do you mean by a branch?
$endgroup$
– Relure
Jun 20 '18 at 15:45
1
$begingroup$
@Relure Every non-zero complex number has two square roots, just like a number $x>0$ has two square roots which differ by sign. Taking a branch means choosing one of the two in a consistent way so that the square root function becomes continuous and single-valued. This concept should appear at some later point in your studies in more detail, but this is the idea. Some other functions have more than two options to choose from.
$endgroup$
– Joonas Ilmavirta
Jun 20 '18 at 15:50
2
$begingroup$
I like most of this answer (+1) but I worry that the third paragraph might give the OP or other readers the idea that there will be a Laurent series for $sqrt z$ around $0$. The best one can actually get is (as far as I know) a Puiseux series (which in this case would be trivial: $z^{1/2}$).
$endgroup$
– Andreas Blass
Jun 20 '18 at 16:55
$begingroup$
@AndreasBlass Fair enough, that was indeed misleading. I edited the paragraph a bit.
$endgroup$
– Joonas Ilmavirta
Jun 20 '18 at 18:59
add a comment |
$begingroup$
A Taylor series expansion is possible for a function $fcolonmathbb Ctomathbb C$ is possible (converges) at $z_0$ if $f$ is complex analytic in a neighborhood of the point $z_0$.
The square root function has two issues.
First, at the origin it is not even analytic: it does not satisfy the Cauchy–Riemann equations because it is not even differentiable.
In addition, outside the origin it is not single-valued.
If you choose a branch (see below) of the square root, then it is analytic in a neighborhood of any $z_0neq0$.
There is also a possibility to have a series expansion around singular points.
This is known as a Laurent series, but at this stage you should focus on analytic functions and Taylor series.
But even the Laurent series does not exist for all functions.
It requires that the singularity is of a suitable nice kind, and the one of the square root at the origin does not qualify.
Every non-zero complex number has two square roots, just like a number $x>0$ has two square roots which differ by sign.
Taking a branch means choosing one of the two in a consistent way so that the square root function becomes continuous and single-valued.
This concept should appear at some later point in your studies in more detail, but this is the idea.
Some other functions have more than two options to choose from.
For example, the number $1+i$ has three cubic roots and infinitely many logarithms.
$endgroup$
A Taylor series expansion is possible for a function $fcolonmathbb Ctomathbb C$ is possible (converges) at $z_0$ if $f$ is complex analytic in a neighborhood of the point $z_0$.
The square root function has two issues.
First, at the origin it is not even analytic: it does not satisfy the Cauchy–Riemann equations because it is not even differentiable.
In addition, outside the origin it is not single-valued.
If you choose a branch (see below) of the square root, then it is analytic in a neighborhood of any $z_0neq0$.
There is also a possibility to have a series expansion around singular points.
This is known as a Laurent series, but at this stage you should focus on analytic functions and Taylor series.
But even the Laurent series does not exist for all functions.
It requires that the singularity is of a suitable nice kind, and the one of the square root at the origin does not qualify.
Every non-zero complex number has two square roots, just like a number $x>0$ has two square roots which differ by sign.
Taking a branch means choosing one of the two in a consistent way so that the square root function becomes continuous and single-valued.
This concept should appear at some later point in your studies in more detail, but this is the idea.
Some other functions have more than two options to choose from.
For example, the number $1+i$ has three cubic roots and infinitely many logarithms.
edited Jun 20 '18 at 18:59
answered Jun 20 '18 at 15:41
Joonas IlmavirtaJoonas Ilmavirta
20.6k94282
20.6k94282
$begingroup$
Hi @Joonas, what do you mean by a branch?
$endgroup$
– Relure
Jun 20 '18 at 15:45
1
$begingroup$
@Relure Every non-zero complex number has two square roots, just like a number $x>0$ has two square roots which differ by sign. Taking a branch means choosing one of the two in a consistent way so that the square root function becomes continuous and single-valued. This concept should appear at some later point in your studies in more detail, but this is the idea. Some other functions have more than two options to choose from.
$endgroup$
– Joonas Ilmavirta
Jun 20 '18 at 15:50
2
$begingroup$
I like most of this answer (+1) but I worry that the third paragraph might give the OP or other readers the idea that there will be a Laurent series for $sqrt z$ around $0$. The best one can actually get is (as far as I know) a Puiseux series (which in this case would be trivial: $z^{1/2}$).
$endgroup$
– Andreas Blass
Jun 20 '18 at 16:55
$begingroup$
@AndreasBlass Fair enough, that was indeed misleading. I edited the paragraph a bit.
$endgroup$
– Joonas Ilmavirta
Jun 20 '18 at 18:59
add a comment |
$begingroup$
Hi @Joonas, what do you mean by a branch?
$endgroup$
– Relure
Jun 20 '18 at 15:45
1
$begingroup$
@Relure Every non-zero complex number has two square roots, just like a number $x>0$ has two square roots which differ by sign. Taking a branch means choosing one of the two in a consistent way so that the square root function becomes continuous and single-valued. This concept should appear at some later point in your studies in more detail, but this is the idea. Some other functions have more than two options to choose from.
$endgroup$
– Joonas Ilmavirta
Jun 20 '18 at 15:50
2
$begingroup$
I like most of this answer (+1) but I worry that the third paragraph might give the OP or other readers the idea that there will be a Laurent series for $sqrt z$ around $0$. The best one can actually get is (as far as I know) a Puiseux series (which in this case would be trivial: $z^{1/2}$).
$endgroup$
– Andreas Blass
Jun 20 '18 at 16:55
$begingroup$
@AndreasBlass Fair enough, that was indeed misleading. I edited the paragraph a bit.
$endgroup$
– Joonas Ilmavirta
Jun 20 '18 at 18:59
$begingroup$
Hi @Joonas, what do you mean by a branch?
$endgroup$
– Relure
Jun 20 '18 at 15:45
$begingroup$
Hi @Joonas, what do you mean by a branch?
$endgroup$
– Relure
Jun 20 '18 at 15:45
1
1
$begingroup$
@Relure Every non-zero complex number has two square roots, just like a number $x>0$ has two square roots which differ by sign. Taking a branch means choosing one of the two in a consistent way so that the square root function becomes continuous and single-valued. This concept should appear at some later point in your studies in more detail, but this is the idea. Some other functions have more than two options to choose from.
$endgroup$
– Joonas Ilmavirta
Jun 20 '18 at 15:50
$begingroup$
@Relure Every non-zero complex number has two square roots, just like a number $x>0$ has two square roots which differ by sign. Taking a branch means choosing one of the two in a consistent way so that the square root function becomes continuous and single-valued. This concept should appear at some later point in your studies in more detail, but this is the idea. Some other functions have more than two options to choose from.
$endgroup$
– Joonas Ilmavirta
Jun 20 '18 at 15:50
2
2
$begingroup$
I like most of this answer (+1) but I worry that the third paragraph might give the OP or other readers the idea that there will be a Laurent series for $sqrt z$ around $0$. The best one can actually get is (as far as I know) a Puiseux series (which in this case would be trivial: $z^{1/2}$).
$endgroup$
– Andreas Blass
Jun 20 '18 at 16:55
$begingroup$
I like most of this answer (+1) but I worry that the third paragraph might give the OP or other readers the idea that there will be a Laurent series for $sqrt z$ around $0$. The best one can actually get is (as far as I know) a Puiseux series (which in this case would be trivial: $z^{1/2}$).
$endgroup$
– Andreas Blass
Jun 20 '18 at 16:55
$begingroup$
@AndreasBlass Fair enough, that was indeed misleading. I edited the paragraph a bit.
$endgroup$
– Joonas Ilmavirta
Jun 20 '18 at 18:59
$begingroup$
@AndreasBlass Fair enough, that was indeed misleading. I edited the paragraph a bit.
$endgroup$
– Joonas Ilmavirta
Jun 20 '18 at 18:59
add a comment |
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.
Sign up or log in
StackExchange.ready(function () {
StackExchange.helpers.onClickDraftSave('#login-link');
});
Sign up using Google
Sign up using Facebook
Sign up using Email and Password
Post as a guest
Required, but never shown
StackExchange.ready(
function () {
StackExchange.openid.initPostLogin('.new-post-login', 'https%3a%2f%2fmath.stackexchange.com%2fquestions%2f2826230%2fhow-to-write-fz-sqrtz-as-a-complex-series-around-the-origin%23new-answer', 'question_page');
}
);
Post as a guest
Required, but never shown
Sign up or log in
StackExchange.ready(function () {
StackExchange.helpers.onClickDraftSave('#login-link');
});
Sign up using Google
Sign up using Facebook
Sign up using Email and Password
Post as a guest
Required, but never shown
Sign up or log in
StackExchange.ready(function () {
StackExchange.helpers.onClickDraftSave('#login-link');
});
Sign up using Google
Sign up using Facebook
Sign up using Email and Password
Post as a guest
Required, but never shown
Sign up or log in
StackExchange.ready(function () {
StackExchange.helpers.onClickDraftSave('#login-link');
});
Sign up using Google
Sign up using Facebook
Sign up using Email and Password
Sign up using Google
Sign up using Facebook
Sign up using Email and Password
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
4
$begingroup$
Are you aware that there is no single-valued and continuous $sqrt{z}$ around the origin?
$endgroup$
– Adayah
Jun 20 '18 at 15:33
$begingroup$
Beginning complex analysis, you perhaps learned the definition of "analytic function". Check that $sqrt{z}$ is not analytic at $z=0$.
$endgroup$
– GEdgar
Jun 20 '18 at 15:35
$begingroup$
If you write down a definition of $sqrt z,$ the impossibility of doing this will become evident.
$endgroup$
– zhw.
Jun 20 '18 at 15:59