How to make it formally correct?












2














Can someone help me formalizing this statement:



$$
z= x^0 +ix^1
$$

And therefore
$$
frac{partial}{partial z} = frac{partial}{partial (x^0 +ix^1)} = frac{partial}{partial x^0} + frac{1}{i} frac{partial}{partial x^1}
$$



My problem is with the last equality, I see it's right, but I'm not sure I'm allowed to do it in that way. Is there a way to procede more formally?










share|cite|improve this question


















  • 1




    It is customary to use $x$ and $y$ rather than $x^0$ and $x^1$ when talking about complex numbers. I've mostly seen $x^k$ notation when making arguments about unknown / arbitrary bases, not with concrete bases like this one. That being said, I don't really see anything wrong with it.
    – Arthur
    Nov 27 at 12:15












  • @Arthur Should I edit it using $x$ and $y$ to put it in a standard notation or it's fine?
    – Run like hell
    Nov 27 at 12:25












  • Nah. It's fine. I just found it a small curiousity. Some times unusual notation is bad. This time it's not.
    – Arthur
    Nov 27 at 12:29












  • @Runlikehell I have removed my comment, as I feel that the newly posted answer explains the situation more correctly. My apologies.
    – MisterRiemann
    Nov 27 at 12:39


















2














Can someone help me formalizing this statement:



$$
z= x^0 +ix^1
$$

And therefore
$$
frac{partial}{partial z} = frac{partial}{partial (x^0 +ix^1)} = frac{partial}{partial x^0} + frac{1}{i} frac{partial}{partial x^1}
$$



My problem is with the last equality, I see it's right, but I'm not sure I'm allowed to do it in that way. Is there a way to procede more formally?










share|cite|improve this question


















  • 1




    It is customary to use $x$ and $y$ rather than $x^0$ and $x^1$ when talking about complex numbers. I've mostly seen $x^k$ notation when making arguments about unknown / arbitrary bases, not with concrete bases like this one. That being said, I don't really see anything wrong with it.
    – Arthur
    Nov 27 at 12:15












  • @Arthur Should I edit it using $x$ and $y$ to put it in a standard notation or it's fine?
    – Run like hell
    Nov 27 at 12:25












  • Nah. It's fine. I just found it a small curiousity. Some times unusual notation is bad. This time it's not.
    – Arthur
    Nov 27 at 12:29












  • @Runlikehell I have removed my comment, as I feel that the newly posted answer explains the situation more correctly. My apologies.
    – MisterRiemann
    Nov 27 at 12:39
















2












2








2







Can someone help me formalizing this statement:



$$
z= x^0 +ix^1
$$

And therefore
$$
frac{partial}{partial z} = frac{partial}{partial (x^0 +ix^1)} = frac{partial}{partial x^0} + frac{1}{i} frac{partial}{partial x^1}
$$



My problem is with the last equality, I see it's right, but I'm not sure I'm allowed to do it in that way. Is there a way to procede more formally?










share|cite|improve this question













Can someone help me formalizing this statement:



$$
z= x^0 +ix^1
$$

And therefore
$$
frac{partial}{partial z} = frac{partial}{partial (x^0 +ix^1)} = frac{partial}{partial x^0} + frac{1}{i} frac{partial}{partial x^1}
$$



My problem is with the last equality, I see it's right, but I'm not sure I'm allowed to do it in that way. Is there a way to procede more formally?







calculus analysis multivariable-calculus partial-derivative formal-proofs






share|cite|improve this question













share|cite|improve this question











share|cite|improve this question




share|cite|improve this question










asked Nov 27 at 12:10









Run like hell

1405




1405








  • 1




    It is customary to use $x$ and $y$ rather than $x^0$ and $x^1$ when talking about complex numbers. I've mostly seen $x^k$ notation when making arguments about unknown / arbitrary bases, not with concrete bases like this one. That being said, I don't really see anything wrong with it.
    – Arthur
    Nov 27 at 12:15












  • @Arthur Should I edit it using $x$ and $y$ to put it in a standard notation or it's fine?
    – Run like hell
    Nov 27 at 12:25












  • Nah. It's fine. I just found it a small curiousity. Some times unusual notation is bad. This time it's not.
    – Arthur
    Nov 27 at 12:29












  • @Runlikehell I have removed my comment, as I feel that the newly posted answer explains the situation more correctly. My apologies.
    – MisterRiemann
    Nov 27 at 12:39
















  • 1




    It is customary to use $x$ and $y$ rather than $x^0$ and $x^1$ when talking about complex numbers. I've mostly seen $x^k$ notation when making arguments about unknown / arbitrary bases, not with concrete bases like this one. That being said, I don't really see anything wrong with it.
    – Arthur
    Nov 27 at 12:15












  • @Arthur Should I edit it using $x$ and $y$ to put it in a standard notation or it's fine?
    – Run like hell
    Nov 27 at 12:25












  • Nah. It's fine. I just found it a small curiousity. Some times unusual notation is bad. This time it's not.
    – Arthur
    Nov 27 at 12:29












  • @Runlikehell I have removed my comment, as I feel that the newly posted answer explains the situation more correctly. My apologies.
    – MisterRiemann
    Nov 27 at 12:39










1




1




It is customary to use $x$ and $y$ rather than $x^0$ and $x^1$ when talking about complex numbers. I've mostly seen $x^k$ notation when making arguments about unknown / arbitrary bases, not with concrete bases like this one. That being said, I don't really see anything wrong with it.
– Arthur
Nov 27 at 12:15






It is customary to use $x$ and $y$ rather than $x^0$ and $x^1$ when talking about complex numbers. I've mostly seen $x^k$ notation when making arguments about unknown / arbitrary bases, not with concrete bases like this one. That being said, I don't really see anything wrong with it.
– Arthur
Nov 27 at 12:15














@Arthur Should I edit it using $x$ and $y$ to put it in a standard notation or it's fine?
– Run like hell
Nov 27 at 12:25






@Arthur Should I edit it using $x$ and $y$ to put it in a standard notation or it's fine?
– Run like hell
Nov 27 at 12:25














Nah. It's fine. I just found it a small curiousity. Some times unusual notation is bad. This time it's not.
– Arthur
Nov 27 at 12:29






Nah. It's fine. I just found it a small curiousity. Some times unusual notation is bad. This time it's not.
– Arthur
Nov 27 at 12:29














@Runlikehell I have removed my comment, as I feel that the newly posted answer explains the situation more correctly. My apologies.
– MisterRiemann
Nov 27 at 12:39






@Runlikehell I have removed my comment, as I feel that the newly posted answer explains the situation more correctly. My apologies.
– MisterRiemann
Nov 27 at 12:39












1 Answer
1






active

oldest

votes


















2














Not sure it's correct, as usually one defines:
$$
df(z)=frac{partial f}{partial z}dz+frac{partial f}{partial z^*}dz^*
$$

where $frac{partial}{partial z}$ and $frac{partial}{partial z^*}$ are called Wirtinger derivatives and are defined by:
$$
frac{partial}{partial z}:=frac{1}{2}left(frac{partial}{partial x^0}-ifrac{partial}{partial x^1}right)
$$

$$
frac{partial}{partial z^*}:=frac{1}{2}left(frac{partial}{partial x^0}+ifrac{partial}{partial x^1}right)
$$



Compared to your proposal, there is a 1/2 missing factor.




  • this is ok for holomorphic functions (thanks to Cauchy-Riemann)

  • but not in the general case


I can copy derivation from An Introduction to Complex Differentials and
Complex Differentiability p8-9, however this tutorial is really well written and I think it is better to directly read it.





From $F(x,y)=U(x,y)+iV(x,y)$ the bivariate function associated to $f(z)$, the total differential is:
$$
dF=frac{partial}{partial x}F(x,y)dx+frac{partial}{partial y}F(x,y)dy
$$

which can be rewritten as follows (have a look at the cited reference):
$$
dF=frac{1}{2}left[frac{partial U}{partial x}+frac{partial V}{partial y}+ileft(frac{partial V}{partial x}-frac{partial U}{partial y}right)right]dz+frac{1}{2}left[frac{partial U}{partial x}-frac{partial V}{partial y}+ileft(frac{partial V}{partial x}+frac{partial U}{partial y}right)right]dz^*
$$

Assuming $f$ holomorphic and using Cauchy-Riemann conditions ($frac{partial U}{partial x}=frac{partial V}{partial y}$ and $frac{partial U}{partial y}=-frac{partial V}{partial x}$)
we get your suggestion:
$frac{partial}{partial z}=frac{partial}{partial x^0}-ifrac{partial}{partial x^1}$ and $frac{partial}{partial z^*}=0$





One of the big advantage of Wirtinger calculus is:




For the Wirtinger derivatives, the common rules for differentiation
known from real-valued analysis concerning the sum, product, and
composition of two functions hold as well...




in brief you can do differential calculus like in $mathbb{R}^n$. This is very handy for automatic differentiation, gradient computation... etc.






share|cite|improve this answer



















  • 1




    Thank you, so I guess my intuition was wrong since I didn't have the factor $1/2$ which, as the link provided shows, comes from taking the differential of $dx^0$ and $dx^1$ as a function of $dz$ and $dz^*$
    – Run like hell
    Nov 27 at 12:44













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1 Answer
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active

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2














Not sure it's correct, as usually one defines:
$$
df(z)=frac{partial f}{partial z}dz+frac{partial f}{partial z^*}dz^*
$$

where $frac{partial}{partial z}$ and $frac{partial}{partial z^*}$ are called Wirtinger derivatives and are defined by:
$$
frac{partial}{partial z}:=frac{1}{2}left(frac{partial}{partial x^0}-ifrac{partial}{partial x^1}right)
$$

$$
frac{partial}{partial z^*}:=frac{1}{2}left(frac{partial}{partial x^0}+ifrac{partial}{partial x^1}right)
$$



Compared to your proposal, there is a 1/2 missing factor.




  • this is ok for holomorphic functions (thanks to Cauchy-Riemann)

  • but not in the general case


I can copy derivation from An Introduction to Complex Differentials and
Complex Differentiability p8-9, however this tutorial is really well written and I think it is better to directly read it.





From $F(x,y)=U(x,y)+iV(x,y)$ the bivariate function associated to $f(z)$, the total differential is:
$$
dF=frac{partial}{partial x}F(x,y)dx+frac{partial}{partial y}F(x,y)dy
$$

which can be rewritten as follows (have a look at the cited reference):
$$
dF=frac{1}{2}left[frac{partial U}{partial x}+frac{partial V}{partial y}+ileft(frac{partial V}{partial x}-frac{partial U}{partial y}right)right]dz+frac{1}{2}left[frac{partial U}{partial x}-frac{partial V}{partial y}+ileft(frac{partial V}{partial x}+frac{partial U}{partial y}right)right]dz^*
$$

Assuming $f$ holomorphic and using Cauchy-Riemann conditions ($frac{partial U}{partial x}=frac{partial V}{partial y}$ and $frac{partial U}{partial y}=-frac{partial V}{partial x}$)
we get your suggestion:
$frac{partial}{partial z}=frac{partial}{partial x^0}-ifrac{partial}{partial x^1}$ and $frac{partial}{partial z^*}=0$





One of the big advantage of Wirtinger calculus is:




For the Wirtinger derivatives, the common rules for differentiation
known from real-valued analysis concerning the sum, product, and
composition of two functions hold as well...




in brief you can do differential calculus like in $mathbb{R}^n$. This is very handy for automatic differentiation, gradient computation... etc.






share|cite|improve this answer



















  • 1




    Thank you, so I guess my intuition was wrong since I didn't have the factor $1/2$ which, as the link provided shows, comes from taking the differential of $dx^0$ and $dx^1$ as a function of $dz$ and $dz^*$
    – Run like hell
    Nov 27 at 12:44


















2














Not sure it's correct, as usually one defines:
$$
df(z)=frac{partial f}{partial z}dz+frac{partial f}{partial z^*}dz^*
$$

where $frac{partial}{partial z}$ and $frac{partial}{partial z^*}$ are called Wirtinger derivatives and are defined by:
$$
frac{partial}{partial z}:=frac{1}{2}left(frac{partial}{partial x^0}-ifrac{partial}{partial x^1}right)
$$

$$
frac{partial}{partial z^*}:=frac{1}{2}left(frac{partial}{partial x^0}+ifrac{partial}{partial x^1}right)
$$



Compared to your proposal, there is a 1/2 missing factor.




  • this is ok for holomorphic functions (thanks to Cauchy-Riemann)

  • but not in the general case


I can copy derivation from An Introduction to Complex Differentials and
Complex Differentiability p8-9, however this tutorial is really well written and I think it is better to directly read it.





From $F(x,y)=U(x,y)+iV(x,y)$ the bivariate function associated to $f(z)$, the total differential is:
$$
dF=frac{partial}{partial x}F(x,y)dx+frac{partial}{partial y}F(x,y)dy
$$

which can be rewritten as follows (have a look at the cited reference):
$$
dF=frac{1}{2}left[frac{partial U}{partial x}+frac{partial V}{partial y}+ileft(frac{partial V}{partial x}-frac{partial U}{partial y}right)right]dz+frac{1}{2}left[frac{partial U}{partial x}-frac{partial V}{partial y}+ileft(frac{partial V}{partial x}+frac{partial U}{partial y}right)right]dz^*
$$

Assuming $f$ holomorphic and using Cauchy-Riemann conditions ($frac{partial U}{partial x}=frac{partial V}{partial y}$ and $frac{partial U}{partial y}=-frac{partial V}{partial x}$)
we get your suggestion:
$frac{partial}{partial z}=frac{partial}{partial x^0}-ifrac{partial}{partial x^1}$ and $frac{partial}{partial z^*}=0$





One of the big advantage of Wirtinger calculus is:




For the Wirtinger derivatives, the common rules for differentiation
known from real-valued analysis concerning the sum, product, and
composition of two functions hold as well...




in brief you can do differential calculus like in $mathbb{R}^n$. This is very handy for automatic differentiation, gradient computation... etc.






share|cite|improve this answer



















  • 1




    Thank you, so I guess my intuition was wrong since I didn't have the factor $1/2$ which, as the link provided shows, comes from taking the differential of $dx^0$ and $dx^1$ as a function of $dz$ and $dz^*$
    – Run like hell
    Nov 27 at 12:44
















2












2








2






Not sure it's correct, as usually one defines:
$$
df(z)=frac{partial f}{partial z}dz+frac{partial f}{partial z^*}dz^*
$$

where $frac{partial}{partial z}$ and $frac{partial}{partial z^*}$ are called Wirtinger derivatives and are defined by:
$$
frac{partial}{partial z}:=frac{1}{2}left(frac{partial}{partial x^0}-ifrac{partial}{partial x^1}right)
$$

$$
frac{partial}{partial z^*}:=frac{1}{2}left(frac{partial}{partial x^0}+ifrac{partial}{partial x^1}right)
$$



Compared to your proposal, there is a 1/2 missing factor.




  • this is ok for holomorphic functions (thanks to Cauchy-Riemann)

  • but not in the general case


I can copy derivation from An Introduction to Complex Differentials and
Complex Differentiability p8-9, however this tutorial is really well written and I think it is better to directly read it.





From $F(x,y)=U(x,y)+iV(x,y)$ the bivariate function associated to $f(z)$, the total differential is:
$$
dF=frac{partial}{partial x}F(x,y)dx+frac{partial}{partial y}F(x,y)dy
$$

which can be rewritten as follows (have a look at the cited reference):
$$
dF=frac{1}{2}left[frac{partial U}{partial x}+frac{partial V}{partial y}+ileft(frac{partial V}{partial x}-frac{partial U}{partial y}right)right]dz+frac{1}{2}left[frac{partial U}{partial x}-frac{partial V}{partial y}+ileft(frac{partial V}{partial x}+frac{partial U}{partial y}right)right]dz^*
$$

Assuming $f$ holomorphic and using Cauchy-Riemann conditions ($frac{partial U}{partial x}=frac{partial V}{partial y}$ and $frac{partial U}{partial y}=-frac{partial V}{partial x}$)
we get your suggestion:
$frac{partial}{partial z}=frac{partial}{partial x^0}-ifrac{partial}{partial x^1}$ and $frac{partial}{partial z^*}=0$





One of the big advantage of Wirtinger calculus is:




For the Wirtinger derivatives, the common rules for differentiation
known from real-valued analysis concerning the sum, product, and
composition of two functions hold as well...




in brief you can do differential calculus like in $mathbb{R}^n$. This is very handy for automatic differentiation, gradient computation... etc.






share|cite|improve this answer














Not sure it's correct, as usually one defines:
$$
df(z)=frac{partial f}{partial z}dz+frac{partial f}{partial z^*}dz^*
$$

where $frac{partial}{partial z}$ and $frac{partial}{partial z^*}$ are called Wirtinger derivatives and are defined by:
$$
frac{partial}{partial z}:=frac{1}{2}left(frac{partial}{partial x^0}-ifrac{partial}{partial x^1}right)
$$

$$
frac{partial}{partial z^*}:=frac{1}{2}left(frac{partial}{partial x^0}+ifrac{partial}{partial x^1}right)
$$



Compared to your proposal, there is a 1/2 missing factor.




  • this is ok for holomorphic functions (thanks to Cauchy-Riemann)

  • but not in the general case


I can copy derivation from An Introduction to Complex Differentials and
Complex Differentiability p8-9, however this tutorial is really well written and I think it is better to directly read it.





From $F(x,y)=U(x,y)+iV(x,y)$ the bivariate function associated to $f(z)$, the total differential is:
$$
dF=frac{partial}{partial x}F(x,y)dx+frac{partial}{partial y}F(x,y)dy
$$

which can be rewritten as follows (have a look at the cited reference):
$$
dF=frac{1}{2}left[frac{partial U}{partial x}+frac{partial V}{partial y}+ileft(frac{partial V}{partial x}-frac{partial U}{partial y}right)right]dz+frac{1}{2}left[frac{partial U}{partial x}-frac{partial V}{partial y}+ileft(frac{partial V}{partial x}+frac{partial U}{partial y}right)right]dz^*
$$

Assuming $f$ holomorphic and using Cauchy-Riemann conditions ($frac{partial U}{partial x}=frac{partial V}{partial y}$ and $frac{partial U}{partial y}=-frac{partial V}{partial x}$)
we get your suggestion:
$frac{partial}{partial z}=frac{partial}{partial x^0}-ifrac{partial}{partial x^1}$ and $frac{partial}{partial z^*}=0$





One of the big advantage of Wirtinger calculus is:




For the Wirtinger derivatives, the common rules for differentiation
known from real-valued analysis concerning the sum, product, and
composition of two functions hold as well...




in brief you can do differential calculus like in $mathbb{R}^n$. This is very handy for automatic differentiation, gradient computation... etc.







share|cite|improve this answer














share|cite|improve this answer



share|cite|improve this answer








edited Nov 27 at 13:01

























answered Nov 27 at 12:35









Picaud Vincent

1,20838




1,20838








  • 1




    Thank you, so I guess my intuition was wrong since I didn't have the factor $1/2$ which, as the link provided shows, comes from taking the differential of $dx^0$ and $dx^1$ as a function of $dz$ and $dz^*$
    – Run like hell
    Nov 27 at 12:44
















  • 1




    Thank you, so I guess my intuition was wrong since I didn't have the factor $1/2$ which, as the link provided shows, comes from taking the differential of $dx^0$ and $dx^1$ as a function of $dz$ and $dz^*$
    – Run like hell
    Nov 27 at 12:44










1




1




Thank you, so I guess my intuition was wrong since I didn't have the factor $1/2$ which, as the link provided shows, comes from taking the differential of $dx^0$ and $dx^1$ as a function of $dz$ and $dz^*$
– Run like hell
Nov 27 at 12:44






Thank you, so I guess my intuition was wrong since I didn't have the factor $1/2$ which, as the link provided shows, comes from taking the differential of $dx^0$ and $dx^1$ as a function of $dz$ and $dz^*$
– Run like hell
Nov 27 at 12:44




















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