Fubini's Theorem, but $dxdy = -dydx$
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As differential $2$-forms, clearly $dxdy=-dydx$ by alternation. Yet just as clearly, $int_0^1 int_0^1 dxdy neq -int_0^1 int_0^1 dydx$. Where's the abuse of notation here?
integration differential-forms
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add a comment |
$begingroup$
As differential $2$-forms, clearly $dxdy=-dydx$ by alternation. Yet just as clearly, $int_0^1 int_0^1 dxdy neq -int_0^1 int_0^1 dydx$. Where's the abuse of notation here?
integration differential-forms
$endgroup$
$begingroup$
You've written the same on both sides of the second equation. And you haven't explained where and why you see a problem.
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– Henrik
Nov 6 '18 at 6:43
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Sorry, fat fingers.
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– Feryll
Nov 6 '18 at 6:44
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You need to match up the orientation when integrating differential forms by definition, to avoid exactly this problem.
$endgroup$
– user10354138
Nov 6 '18 at 7:08
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Check carefully how the integral of a $2$-form over an oriented $2$-dimensional manifold is defined. In particular, how is the integral of $f,dxwedge dy$ over a region in $Bbb R^2$ (with the usual orientation) defined?
$endgroup$
– Ted Shifrin
Nov 7 '18 at 23:14
add a comment |
$begingroup$
As differential $2$-forms, clearly $dxdy=-dydx$ by alternation. Yet just as clearly, $int_0^1 int_0^1 dxdy neq -int_0^1 int_0^1 dydx$. Where's the abuse of notation here?
integration differential-forms
$endgroup$
As differential $2$-forms, clearly $dxdy=-dydx$ by alternation. Yet just as clearly, $int_0^1 int_0^1 dxdy neq -int_0^1 int_0^1 dydx$. Where's the abuse of notation here?
integration differential-forms
integration differential-forms
edited Nov 6 '18 at 6:44
Feryll
asked Nov 6 '18 at 6:41
FeryllFeryll
544212
544212
$begingroup$
You've written the same on both sides of the second equation. And you haven't explained where and why you see a problem.
$endgroup$
– Henrik
Nov 6 '18 at 6:43
$begingroup$
Sorry, fat fingers.
$endgroup$
– Feryll
Nov 6 '18 at 6:44
$begingroup$
You need to match up the orientation when integrating differential forms by definition, to avoid exactly this problem.
$endgroup$
– user10354138
Nov 6 '18 at 7:08
$begingroup$
Check carefully how the integral of a $2$-form over an oriented $2$-dimensional manifold is defined. In particular, how is the integral of $f,dxwedge dy$ over a region in $Bbb R^2$ (with the usual orientation) defined?
$endgroup$
– Ted Shifrin
Nov 7 '18 at 23:14
add a comment |
$begingroup$
You've written the same on both sides of the second equation. And you haven't explained where and why you see a problem.
$endgroup$
– Henrik
Nov 6 '18 at 6:43
$begingroup$
Sorry, fat fingers.
$endgroup$
– Feryll
Nov 6 '18 at 6:44
$begingroup$
You need to match up the orientation when integrating differential forms by definition, to avoid exactly this problem.
$endgroup$
– user10354138
Nov 6 '18 at 7:08
$begingroup$
Check carefully how the integral of a $2$-form over an oriented $2$-dimensional manifold is defined. In particular, how is the integral of $f,dxwedge dy$ over a region in $Bbb R^2$ (with the usual orientation) defined?
$endgroup$
– Ted Shifrin
Nov 7 '18 at 23:14
$begingroup$
You've written the same on both sides of the second equation. And you haven't explained where and why you see a problem.
$endgroup$
– Henrik
Nov 6 '18 at 6:43
$begingroup$
You've written the same on both sides of the second equation. And you haven't explained where and why you see a problem.
$endgroup$
– Henrik
Nov 6 '18 at 6:43
$begingroup$
Sorry, fat fingers.
$endgroup$
– Feryll
Nov 6 '18 at 6:44
$begingroup$
Sorry, fat fingers.
$endgroup$
– Feryll
Nov 6 '18 at 6:44
$begingroup$
You need to match up the orientation when integrating differential forms by definition, to avoid exactly this problem.
$endgroup$
– user10354138
Nov 6 '18 at 7:08
$begingroup$
You need to match up the orientation when integrating differential forms by definition, to avoid exactly this problem.
$endgroup$
– user10354138
Nov 6 '18 at 7:08
$begingroup$
Check carefully how the integral of a $2$-form over an oriented $2$-dimensional manifold is defined. In particular, how is the integral of $f,dxwedge dy$ over a region in $Bbb R^2$ (with the usual orientation) defined?
$endgroup$
– Ted Shifrin
Nov 7 '18 at 23:14
$begingroup$
Check carefully how the integral of a $2$-form over an oriented $2$-dimensional manifold is defined. In particular, how is the integral of $f,dxwedge dy$ over a region in $Bbb R^2$ (with the usual orientation) defined?
$endgroup$
– Ted Shifrin
Nov 7 '18 at 23:14
add a comment |
2 Answers
2
active
oldest
votes
$begingroup$
$(x,y)mapsto(y,x)$ is an orientation-reversing map of the plane to itself.
For orientation-reversing maps one has to introduce a minus sign factor in the
change of variables:
$$int_0^1int_0^1,dxwedge,dy=-int_0^1int_0^1-,dywedge,dx
=int_0^1int_0^1,dywedge,dx.$$
$endgroup$
add a comment |
$begingroup$
Expanding upon Lord Shark's answer: remember that when changing coordinate systems, you need to introduce the determinant of the Jacobian of the coordinate transformation. In your case, the Jacobian is
$$
J(x,y) = begin{bmatrix}
frac{partial y}{partial x} & frac{partial x}{partial x} \
frac{partial y}{partial y} & frac{partial x}{partial y}
end{bmatrix}
=begin{bmatrix} 0 & 1 \ 1 & 0 end{bmatrix} , ,
$$
whose determinant is $-1$. Therefore,
$$
int_0^1 int_0^1 dx wedge dy = int_0^1 int_0^1 det(J(x,y)) , (-dy wedge dx) = int_0^1 int_0^1 dy wedge dx
$$
As Lord Shark pointed out, any orientation-reversing transformation will have a Jacobian whose determinant is $-1$.
$endgroup$
add a comment |
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2 Answers
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active
oldest
votes
2 Answers
2
active
oldest
votes
active
oldest
votes
active
oldest
votes
$begingroup$
$(x,y)mapsto(y,x)$ is an orientation-reversing map of the plane to itself.
For orientation-reversing maps one has to introduce a minus sign factor in the
change of variables:
$$int_0^1int_0^1,dxwedge,dy=-int_0^1int_0^1-,dywedge,dx
=int_0^1int_0^1,dywedge,dx.$$
$endgroup$
add a comment |
$begingroup$
$(x,y)mapsto(y,x)$ is an orientation-reversing map of the plane to itself.
For orientation-reversing maps one has to introduce a minus sign factor in the
change of variables:
$$int_0^1int_0^1,dxwedge,dy=-int_0^1int_0^1-,dywedge,dx
=int_0^1int_0^1,dywedge,dx.$$
$endgroup$
add a comment |
$begingroup$
$(x,y)mapsto(y,x)$ is an orientation-reversing map of the plane to itself.
For orientation-reversing maps one has to introduce a minus sign factor in the
change of variables:
$$int_0^1int_0^1,dxwedge,dy=-int_0^1int_0^1-,dywedge,dx
=int_0^1int_0^1,dywedge,dx.$$
$endgroup$
$(x,y)mapsto(y,x)$ is an orientation-reversing map of the plane to itself.
For orientation-reversing maps one has to introduce a minus sign factor in the
change of variables:
$$int_0^1int_0^1,dxwedge,dy=-int_0^1int_0^1-,dywedge,dx
=int_0^1int_0^1,dywedge,dx.$$
answered Nov 6 '18 at 7:16
Lord Shark the UnknownLord Shark the Unknown
102k1159132
102k1159132
add a comment |
add a comment |
$begingroup$
Expanding upon Lord Shark's answer: remember that when changing coordinate systems, you need to introduce the determinant of the Jacobian of the coordinate transformation. In your case, the Jacobian is
$$
J(x,y) = begin{bmatrix}
frac{partial y}{partial x} & frac{partial x}{partial x} \
frac{partial y}{partial y} & frac{partial x}{partial y}
end{bmatrix}
=begin{bmatrix} 0 & 1 \ 1 & 0 end{bmatrix} , ,
$$
whose determinant is $-1$. Therefore,
$$
int_0^1 int_0^1 dx wedge dy = int_0^1 int_0^1 det(J(x,y)) , (-dy wedge dx) = int_0^1 int_0^1 dy wedge dx
$$
As Lord Shark pointed out, any orientation-reversing transformation will have a Jacobian whose determinant is $-1$.
$endgroup$
add a comment |
$begingroup$
Expanding upon Lord Shark's answer: remember that when changing coordinate systems, you need to introduce the determinant of the Jacobian of the coordinate transformation. In your case, the Jacobian is
$$
J(x,y) = begin{bmatrix}
frac{partial y}{partial x} & frac{partial x}{partial x} \
frac{partial y}{partial y} & frac{partial x}{partial y}
end{bmatrix}
=begin{bmatrix} 0 & 1 \ 1 & 0 end{bmatrix} , ,
$$
whose determinant is $-1$. Therefore,
$$
int_0^1 int_0^1 dx wedge dy = int_0^1 int_0^1 det(J(x,y)) , (-dy wedge dx) = int_0^1 int_0^1 dy wedge dx
$$
As Lord Shark pointed out, any orientation-reversing transformation will have a Jacobian whose determinant is $-1$.
$endgroup$
add a comment |
$begingroup$
Expanding upon Lord Shark's answer: remember that when changing coordinate systems, you need to introduce the determinant of the Jacobian of the coordinate transformation. In your case, the Jacobian is
$$
J(x,y) = begin{bmatrix}
frac{partial y}{partial x} & frac{partial x}{partial x} \
frac{partial y}{partial y} & frac{partial x}{partial y}
end{bmatrix}
=begin{bmatrix} 0 & 1 \ 1 & 0 end{bmatrix} , ,
$$
whose determinant is $-1$. Therefore,
$$
int_0^1 int_0^1 dx wedge dy = int_0^1 int_0^1 det(J(x,y)) , (-dy wedge dx) = int_0^1 int_0^1 dy wedge dx
$$
As Lord Shark pointed out, any orientation-reversing transformation will have a Jacobian whose determinant is $-1$.
$endgroup$
Expanding upon Lord Shark's answer: remember that when changing coordinate systems, you need to introduce the determinant of the Jacobian of the coordinate transformation. In your case, the Jacobian is
$$
J(x,y) = begin{bmatrix}
frac{partial y}{partial x} & frac{partial x}{partial x} \
frac{partial y}{partial y} & frac{partial x}{partial y}
end{bmatrix}
=begin{bmatrix} 0 & 1 \ 1 & 0 end{bmatrix} , ,
$$
whose determinant is $-1$. Therefore,
$$
int_0^1 int_0^1 dx wedge dy = int_0^1 int_0^1 det(J(x,y)) , (-dy wedge dx) = int_0^1 int_0^1 dy wedge dx
$$
As Lord Shark pointed out, any orientation-reversing transformation will have a Jacobian whose determinant is $-1$.
answered Nov 30 '18 at 22:45
mcatmcat
1087
1087
add a comment |
add a comment |
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$begingroup$
You've written the same on both sides of the second equation. And you haven't explained where and why you see a problem.
$endgroup$
– Henrik
Nov 6 '18 at 6:43
$begingroup$
Sorry, fat fingers.
$endgroup$
– Feryll
Nov 6 '18 at 6:44
$begingroup$
You need to match up the orientation when integrating differential forms by definition, to avoid exactly this problem.
$endgroup$
– user10354138
Nov 6 '18 at 7:08
$begingroup$
Check carefully how the integral of a $2$-form over an oriented $2$-dimensional manifold is defined. In particular, how is the integral of $f,dxwedge dy$ over a region in $Bbb R^2$ (with the usual orientation) defined?
$endgroup$
– Ted Shifrin
Nov 7 '18 at 23:14