Inverse Fourier Transform of the Hdamard's finite part - 2 dimensions
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I'm trying to compute the Fourier transform of the Hadamard's finite part distribution in two dimensions, and it's giving me a little trouble.
In two dimensions, we defined the finite part $textPFleft(1/k^2right)$, where $kequiv|veck|=sqrtk_x^2+k_y^2$, as the distribution such that for each test function $f$
beginequation
leftlangle textPFleft(frac1k^2right),frightrangle =textPFiint!d^2veckfracf(veck)k^2 =iint_k<1!!!d^2veckfracf(veck)-f(0)k^2+iint_k>1!!!d^2veckfracf(veck)k^2
endequation
Quoting my professor:
Sloppily then we can compute the inverse Fourier transform as
beginequation
frac1(2pi)^2leftlangle textPFleft(frac1k^2right),e^iveckcdotvecxrightrangle = frac1(2pi)^2textPFiint!d^2veckfrace^iveckcdotvecxk^2 = frac1(2pi)^2textPFint_0^infty!dk frackk^2int_0^2pidvartheta,e^ikrcosvartheta
endequation
Where we used spherical coordinates in two dimensions, and we called $requiv |vecx|=sqrtx^2+y^2$. Using Bessel $J_n$ functions, one eventually get
beginequation
frac1(2pi)^2leftlangle textPFleft(frac1k^2right),e^iveckcdotvecxrightrangle = frac1(2pi)^2textPFint_0^infty!dk, (2pi)fracJ_0(kr)k tag1
endequation
Using now the definition of finite part, and changing variable $uequiv rk$, it can be showed that
beginequation
frac1(2pi)^2leftlangle textPFleft(frac1k^2right),e^iveckcdotvecxrightrangle = frac12pileft-log(r)+int_0^1!dufracJ_0(u)-J_0(0)u+int_1^infty!dufracJ_0(u)uright tag2
endequation
First of all, I can't understand very well how do we get to equation (1), in the sense that it's seems that going in spherical coordinates allows us to "delete" one dimension, in the sense that equation (1) seems the Hadamard's finite part of the one dimensional variable $k$. I really don't understand the meaning of this equation.
Then, if we accept that, I cannot understand how to get to equation (2). Indeed if we rename $u=kr$ we get
beginequation
frac1(2pi)^2leftlangle textPFleft(frac1k^2right),e^iveckcdotvecxrightrangle = frac12piint_0^1/r!du,fracJ_0(u)-J_0(0)u +frac12piint_1/r^infty!du,fracJ_0(u)u
endequation
And i really can't understand how to go on, and get the $log(r)$ term.
distribution-theory fourier-transform
add a comment |Â
up vote
1
down vote
favorite
I'm trying to compute the Fourier transform of the Hadamard's finite part distribution in two dimensions, and it's giving me a little trouble.
In two dimensions, we defined the finite part $textPFleft(1/k^2right)$, where $kequiv|veck|=sqrtk_x^2+k_y^2$, as the distribution such that for each test function $f$
beginequation
leftlangle textPFleft(frac1k^2right),frightrangle =textPFiint!d^2veckfracf(veck)k^2 =iint_k<1!!!d^2veckfracf(veck)-f(0)k^2+iint_k>1!!!d^2veckfracf(veck)k^2
endequation
Quoting my professor:
Sloppily then we can compute the inverse Fourier transform as
beginequation
frac1(2pi)^2leftlangle textPFleft(frac1k^2right),e^iveckcdotvecxrightrangle = frac1(2pi)^2textPFiint!d^2veckfrace^iveckcdotvecxk^2 = frac1(2pi)^2textPFint_0^infty!dk frackk^2int_0^2pidvartheta,e^ikrcosvartheta
endequation
Where we used spherical coordinates in two dimensions, and we called $requiv |vecx|=sqrtx^2+y^2$. Using Bessel $J_n$ functions, one eventually get
beginequation
frac1(2pi)^2leftlangle textPFleft(frac1k^2right),e^iveckcdotvecxrightrangle = frac1(2pi)^2textPFint_0^infty!dk, (2pi)fracJ_0(kr)k tag1
endequation
Using now the definition of finite part, and changing variable $uequiv rk$, it can be showed that
beginequation
frac1(2pi)^2leftlangle textPFleft(frac1k^2right),e^iveckcdotvecxrightrangle = frac12pileft-log(r)+int_0^1!dufracJ_0(u)-J_0(0)u+int_1^infty!dufracJ_0(u)uright tag2
endequation
First of all, I can't understand very well how do we get to equation (1), in the sense that it's seems that going in spherical coordinates allows us to "delete" one dimension, in the sense that equation (1) seems the Hadamard's finite part of the one dimensional variable $k$. I really don't understand the meaning of this equation.
Then, if we accept that, I cannot understand how to get to equation (2). Indeed if we rename $u=kr$ we get
beginequation
frac1(2pi)^2leftlangle textPFleft(frac1k^2right),e^iveckcdotvecxrightrangle = frac12piint_0^1/r!du,fracJ_0(u)-J_0(0)u +frac12piint_1/r^infty!du,fracJ_0(u)u
endequation
And i really can't understand how to go on, and get the $log(r)$ term.
distribution-theory fourier-transform
add a comment |Â
up vote
1
down vote
favorite
up vote
1
down vote
favorite
I'm trying to compute the Fourier transform of the Hadamard's finite part distribution in two dimensions, and it's giving me a little trouble.
In two dimensions, we defined the finite part $textPFleft(1/k^2right)$, where $kequiv|veck|=sqrtk_x^2+k_y^2$, as the distribution such that for each test function $f$
beginequation
leftlangle textPFleft(frac1k^2right),frightrangle =textPFiint!d^2veckfracf(veck)k^2 =iint_k<1!!!d^2veckfracf(veck)-f(0)k^2+iint_k>1!!!d^2veckfracf(veck)k^2
endequation
Quoting my professor:
Sloppily then we can compute the inverse Fourier transform as
beginequation
frac1(2pi)^2leftlangle textPFleft(frac1k^2right),e^iveckcdotvecxrightrangle = frac1(2pi)^2textPFiint!d^2veckfrace^iveckcdotvecxk^2 = frac1(2pi)^2textPFint_0^infty!dk frackk^2int_0^2pidvartheta,e^ikrcosvartheta
endequation
Where we used spherical coordinates in two dimensions, and we called $requiv |vecx|=sqrtx^2+y^2$. Using Bessel $J_n$ functions, one eventually get
beginequation
frac1(2pi)^2leftlangle textPFleft(frac1k^2right),e^iveckcdotvecxrightrangle = frac1(2pi)^2textPFint_0^infty!dk, (2pi)fracJ_0(kr)k tag1
endequation
Using now the definition of finite part, and changing variable $uequiv rk$, it can be showed that
beginequation
frac1(2pi)^2leftlangle textPFleft(frac1k^2right),e^iveckcdotvecxrightrangle = frac12pileft-log(r)+int_0^1!dufracJ_0(u)-J_0(0)u+int_1^infty!dufracJ_0(u)uright tag2
endequation
First of all, I can't understand very well how do we get to equation (1), in the sense that it's seems that going in spherical coordinates allows us to "delete" one dimension, in the sense that equation (1) seems the Hadamard's finite part of the one dimensional variable $k$. I really don't understand the meaning of this equation.
Then, if we accept that, I cannot understand how to get to equation (2). Indeed if we rename $u=kr$ we get
beginequation
frac1(2pi)^2leftlangle textPFleft(frac1k^2right),e^iveckcdotvecxrightrangle = frac12piint_0^1/r!du,fracJ_0(u)-J_0(0)u +frac12piint_1/r^infty!du,fracJ_0(u)u
endequation
And i really can't understand how to go on, and get the $log(r)$ term.
distribution-theory fourier-transform
I'm trying to compute the Fourier transform of the Hadamard's finite part distribution in two dimensions, and it's giving me a little trouble.
In two dimensions, we defined the finite part $textPFleft(1/k^2right)$, where $kequiv|veck|=sqrtk_x^2+k_y^2$, as the distribution such that for each test function $f$
beginequation
leftlangle textPFleft(frac1k^2right),frightrangle =textPFiint!d^2veckfracf(veck)k^2 =iint_k<1!!!d^2veckfracf(veck)-f(0)k^2+iint_k>1!!!d^2veckfracf(veck)k^2
endequation
Quoting my professor:
Sloppily then we can compute the inverse Fourier transform as
beginequation
frac1(2pi)^2leftlangle textPFleft(frac1k^2right),e^iveckcdotvecxrightrangle = frac1(2pi)^2textPFiint!d^2veckfrace^iveckcdotvecxk^2 = frac1(2pi)^2textPFint_0^infty!dk frackk^2int_0^2pidvartheta,e^ikrcosvartheta
endequation
Where we used spherical coordinates in two dimensions, and we called $requiv |vecx|=sqrtx^2+y^2$. Using Bessel $J_n$ functions, one eventually get
beginequation
frac1(2pi)^2leftlangle textPFleft(frac1k^2right),e^iveckcdotvecxrightrangle = frac1(2pi)^2textPFint_0^infty!dk, (2pi)fracJ_0(kr)k tag1
endequation
Using now the definition of finite part, and changing variable $uequiv rk$, it can be showed that
beginequation
frac1(2pi)^2leftlangle textPFleft(frac1k^2right),e^iveckcdotvecxrightrangle = frac12pileft-log(r)+int_0^1!dufracJ_0(u)-J_0(0)u+int_1^infty!dufracJ_0(u)uright tag2
endequation
First of all, I can't understand very well how do we get to equation (1), in the sense that it's seems that going in spherical coordinates allows us to "delete" one dimension, in the sense that equation (1) seems the Hadamard's finite part of the one dimensional variable $k$. I really don't understand the meaning of this equation.
Then, if we accept that, I cannot understand how to get to equation (2). Indeed if we rename $u=kr$ we get
beginequation
frac1(2pi)^2leftlangle textPFleft(frac1k^2right),e^iveckcdotvecxrightrangle = frac12piint_0^1/r!du,fracJ_0(u)-J_0(0)u +frac12piint_1/r^infty!du,fracJ_0(u)u
endequation
And i really can't understand how to go on, and get the $log(r)$ term.
distribution-theory fourier-transform
edited Jul 14 at 14:51
asked Jul 14 at 14:41


M. M. R.
84
84
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1 Answer
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I'll change the notation slightly and use $k$ and $w$ for the vectors. Write the action of $1/|k|^2$ on $e^i k cdot w$ by definition as if $e^i k cdot w$ were a test function. In the resulting integral, make the change of variables $k_x = rho cos phi, k_y = rho sin phi$:
$$left< frac 1 , e^i k cdot w right> =
iint_ frac e^i k cdot w - 1 dk_x dk_y +
iint_ frac e^i k cdot w dk_x dk_y = \
int_0^1 int_0^2pi
frac exp(i rho (w_x cos phi + w_y sin phi)) - 1
rho dphi drho + \
int_1^infty int_0^2pi
frac exp(i rho (w_x cos phi + w_y sin phi)) rho dphi drho.$$
We know that
$$w_x cos phi + w_y sin phi =
r left( frac w_x r cos phi + frac w_y r sin phi right) =
r cos(phi + phi_0)$$
for some $phi_0$, where $r = |w|$; since we're integrating over a complete period, the result does not depend on $phi_0$, and
$$int_0^2pi e^i rho r cos phi dphi =
2 pi J_0(rho r) Rightarrow \
frac 1 2pi left< frac 1 , e^i k cdot w right> =
int_0^1 frac J_0(rho r) - 1 rho drho +
int_1^infty frac J_0(rho r) rho drho = \
int_0^r frac J_0(u) - 1 u du +
int_r^infty frac J_0(u) u du = \
left( int_0^1 frac J_0(u) - 1 u du +
int_1^r frac J_0(u) - 1 u du right) + \
left( int_1^infty frac J_0(u) u du +
int_r^1 frac J_0(u) u du right) = \
-ln r + int_0^1 frac J_0(u) - 1 u du +
int_1^infty frac J_0(u) u du = \
-ln r - gamma + ln 2.$$
The last step is for extra points.
add a comment |Â
1 Answer
1
active
oldest
votes
1 Answer
1
active
oldest
votes
active
oldest
votes
active
oldest
votes
up vote
2
down vote
accepted
I'll change the notation slightly and use $k$ and $w$ for the vectors. Write the action of $1/|k|^2$ on $e^i k cdot w$ by definition as if $e^i k cdot w$ were a test function. In the resulting integral, make the change of variables $k_x = rho cos phi, k_y = rho sin phi$:
$$left< frac 1 , e^i k cdot w right> =
iint_ frac e^i k cdot w - 1 dk_x dk_y +
iint_ frac e^i k cdot w dk_x dk_y = \
int_0^1 int_0^2pi
frac exp(i rho (w_x cos phi + w_y sin phi)) - 1
rho dphi drho + \
int_1^infty int_0^2pi
frac exp(i rho (w_x cos phi + w_y sin phi)) rho dphi drho.$$
We know that
$$w_x cos phi + w_y sin phi =
r left( frac w_x r cos phi + frac w_y r sin phi right) =
r cos(phi + phi_0)$$
for some $phi_0$, where $r = |w|$; since we're integrating over a complete period, the result does not depend on $phi_0$, and
$$int_0^2pi e^i rho r cos phi dphi =
2 pi J_0(rho r) Rightarrow \
frac 1 2pi left< frac 1 , e^i k cdot w right> =
int_0^1 frac J_0(rho r) - 1 rho drho +
int_1^infty frac J_0(rho r) rho drho = \
int_0^r frac J_0(u) - 1 u du +
int_r^infty frac J_0(u) u du = \
left( int_0^1 frac J_0(u) - 1 u du +
int_1^r frac J_0(u) - 1 u du right) + \
left( int_1^infty frac J_0(u) u du +
int_r^1 frac J_0(u) u du right) = \
-ln r + int_0^1 frac J_0(u) - 1 u du +
int_1^infty frac J_0(u) u du = \
-ln r - gamma + ln 2.$$
The last step is for extra points.
add a comment |Â
up vote
2
down vote
accepted
I'll change the notation slightly and use $k$ and $w$ for the vectors. Write the action of $1/|k|^2$ on $e^i k cdot w$ by definition as if $e^i k cdot w$ were a test function. In the resulting integral, make the change of variables $k_x = rho cos phi, k_y = rho sin phi$:
$$left< frac 1 , e^i k cdot w right> =
iint_ frac e^i k cdot w - 1 dk_x dk_y +
iint_ frac e^i k cdot w dk_x dk_y = \
int_0^1 int_0^2pi
frac exp(i rho (w_x cos phi + w_y sin phi)) - 1
rho dphi drho + \
int_1^infty int_0^2pi
frac exp(i rho (w_x cos phi + w_y sin phi)) rho dphi drho.$$
We know that
$$w_x cos phi + w_y sin phi =
r left( frac w_x r cos phi + frac w_y r sin phi right) =
r cos(phi + phi_0)$$
for some $phi_0$, where $r = |w|$; since we're integrating over a complete period, the result does not depend on $phi_0$, and
$$int_0^2pi e^i rho r cos phi dphi =
2 pi J_0(rho r) Rightarrow \
frac 1 2pi left< frac 1 , e^i k cdot w right> =
int_0^1 frac J_0(rho r) - 1 rho drho +
int_1^infty frac J_0(rho r) rho drho = \
int_0^r frac J_0(u) - 1 u du +
int_r^infty frac J_0(u) u du = \
left( int_0^1 frac J_0(u) - 1 u du +
int_1^r frac J_0(u) - 1 u du right) + \
left( int_1^infty frac J_0(u) u du +
int_r^1 frac J_0(u) u du right) = \
-ln r + int_0^1 frac J_0(u) - 1 u du +
int_1^infty frac J_0(u) u du = \
-ln r - gamma + ln 2.$$
The last step is for extra points.
add a comment |Â
up vote
2
down vote
accepted
up vote
2
down vote
accepted
I'll change the notation slightly and use $k$ and $w$ for the vectors. Write the action of $1/|k|^2$ on $e^i k cdot w$ by definition as if $e^i k cdot w$ were a test function. In the resulting integral, make the change of variables $k_x = rho cos phi, k_y = rho sin phi$:
$$left< frac 1 , e^i k cdot w right> =
iint_ frac e^i k cdot w - 1 dk_x dk_y +
iint_ frac e^i k cdot w dk_x dk_y = \
int_0^1 int_0^2pi
frac exp(i rho (w_x cos phi + w_y sin phi)) - 1
rho dphi drho + \
int_1^infty int_0^2pi
frac exp(i rho (w_x cos phi + w_y sin phi)) rho dphi drho.$$
We know that
$$w_x cos phi + w_y sin phi =
r left( frac w_x r cos phi + frac w_y r sin phi right) =
r cos(phi + phi_0)$$
for some $phi_0$, where $r = |w|$; since we're integrating over a complete period, the result does not depend on $phi_0$, and
$$int_0^2pi e^i rho r cos phi dphi =
2 pi J_0(rho r) Rightarrow \
frac 1 2pi left< frac 1 , e^i k cdot w right> =
int_0^1 frac J_0(rho r) - 1 rho drho +
int_1^infty frac J_0(rho r) rho drho = \
int_0^r frac J_0(u) - 1 u du +
int_r^infty frac J_0(u) u du = \
left( int_0^1 frac J_0(u) - 1 u du +
int_1^r frac J_0(u) - 1 u du right) + \
left( int_1^infty frac J_0(u) u du +
int_r^1 frac J_0(u) u du right) = \
-ln r + int_0^1 frac J_0(u) - 1 u du +
int_1^infty frac J_0(u) u du = \
-ln r - gamma + ln 2.$$
The last step is for extra points.
I'll change the notation slightly and use $k$ and $w$ for the vectors. Write the action of $1/|k|^2$ on $e^i k cdot w$ by definition as if $e^i k cdot w$ were a test function. In the resulting integral, make the change of variables $k_x = rho cos phi, k_y = rho sin phi$:
$$left< frac 1 , e^i k cdot w right> =
iint_ frac e^i k cdot w - 1 dk_x dk_y +
iint_ frac e^i k cdot w dk_x dk_y = \
int_0^1 int_0^2pi
frac exp(i rho (w_x cos phi + w_y sin phi)) - 1
rho dphi drho + \
int_1^infty int_0^2pi
frac exp(i rho (w_x cos phi + w_y sin phi)) rho dphi drho.$$
We know that
$$w_x cos phi + w_y sin phi =
r left( frac w_x r cos phi + frac w_y r sin phi right) =
r cos(phi + phi_0)$$
for some $phi_0$, where $r = |w|$; since we're integrating over a complete period, the result does not depend on $phi_0$, and
$$int_0^2pi e^i rho r cos phi dphi =
2 pi J_0(rho r) Rightarrow \
frac 1 2pi left< frac 1 , e^i k cdot w right> =
int_0^1 frac J_0(rho r) - 1 rho drho +
int_1^infty frac J_0(rho r) rho drho = \
int_0^r frac J_0(u) - 1 u du +
int_r^infty frac J_0(u) u du = \
left( int_0^1 frac J_0(u) - 1 u du +
int_1^r frac J_0(u) - 1 u du right) + \
left( int_1^infty frac J_0(u) u du +
int_r^1 frac J_0(u) u du right) = \
-ln r + int_0^1 frac J_0(u) - 1 u du +
int_1^infty frac J_0(u) u du = \
-ln r - gamma + ln 2.$$
The last step is for extra points.
answered Jul 15 at 2:06
Maxim
2,150113
2,150113
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