In mathematics, the Abel transform, named for Niels Henrik Abel, is an integral transform often used in the analysis of spherically symmetric or axially symmetric functions. The Abel transform of a function f(r) is given by: Wikibooks Wikiversity has more about this subject: School of Mathematics Wikiquote has a collection of quotations related to: Mathematics Look up Mathematics in Wiktionary, the free dictionary Wikimedia Commons has media related to: Mathematics Interactive Mathematics Miscellany and Puzzles — A collection of articles on various math topics, with interactive Java... Niels Henrik Abel (August 5, 1802–April 6, 1829), Norwegian mathematician, was born in Finnøy. ... In mathematics, an integral transform is any transform T of the following form: The input of this transform is a function f, and the output is another function Tf. ...

Assuming f(r) drops to zero more quickly than 1/r, the inverse Abel transform is given by

In image analysis, the forward Abel transform is used to project an optically thin, axially symmetric emission function onto a plane, and the reverse Abel transform is used to calculate the emission function given a projection (i.e. a scan or a photograph) of that emission function. Image analysis is the extraction of useful information from images; mainly from digital images by means of digital image processing techniques. ...

Geometrical interpretation

A geometrical interpretation of the Abel transform in two dimensions. An observer (I) looks along a line perpendicular to the x-axis a distance y above the origin. What the observer sees is the projection (i.e. the integral) of the circularly symmetric function f(r) along the line of sight. The function f(r) is represented in gray in this figure. The observer is assumed to be located infinitely far from the origin so that the limits of integration are ±∞

In two dimensions, the Abel transform F(y) can be interpreted as the projection of a circularly symmetric function f(r) along a set of parallel lines of sight which are a distance y from the origin. Referring to the figure on the right, the observer (I) will see Image File history File links AbelTransform. ... Image File history File links AbelTransform. ...

where f(r) is the circularly symmetric function represented by the gray color in the figure. It is assumed that the observer is actually at x = ∞ so that the limits of integration are ±∞ and all lines of sight are parallel to the x-axis. Realizing that the radius r is related to x and y via r² = x² + y², it follows that In classical geometry, a radius of a circle or sphere is any line segment with one endpoint on the circle (i. ...

The path of integration in r does not pass through zero, and since both f(r) and the above expression for dx are even functions, we may write:

Substituting the expression for dx in terms of r and rewriting the integration limits accordingly yields the Abel transform.

The Abel transform may be extended to higher dimensions. Of particular interest is the extension to three dimensions. If we have an axially symmetric function f(ρ,z) where ρ² = x² + y² is the cylindrical radius, then we may want to know the projection of that function onto a plane parallel to the z axis. Without loss of generality, we can take that plane to be the yz-plane so that Without loss of generality or simply WLOG is a frequently used expression in mathematics. ...

which is just the Abel transform of f(ρ,z) in ρ and y.

A particular type of axial symmetry is spherical symmetry. In this case, we have a function f(r) where r² = x² + y² + z². The projection onto, say, the yz-plane will then be circularly symmetric and expressible as F(s) where s² = x² + y². Carrying out the integration, we have:

which is again, the Abel transform of f(r) in r and s.

Relationship to other integral transforms

Relationship to the Fourier and Hankel transforms

The Abel transform is one member of the FHA cycle of integral operators. For example, in two dimensions, if we define A as the Abel transform operator, F as the Fourier transform operator and H as the zeroth-order Hankel transform operator, then the special case of the Projection-slice theorem for circularly symmetric functions states that: In mathematics, the projection-slice theorem in two dimensions states that the Fourier transform of the projection of a two-dimensional function f(r) onto a line is equal to a slice through the origin of the two-dimensional Fourier transform of that function which is parallel to the projection... The Fourier transform, named after Joseph Fourier, is an integral transform that re-expresses a function in terms of sinusoidal basis functions, i. ... In mathematics, the Hankel transform of order ν of a function f(r) is given by: where Jν is the Bessel function of the first kind of order ν with ν ≥ −1/2. ... In mathematics, the projection-slice theorem in two dimensions states that the Fourier transform of the projection of a two-dimensional function f(r) onto a line is equal to a slice through the origin of the two-dimensional Fourier transform of that function which is parallel to the projection...

In other words, applying the Abel transform to a 1-dimensional function and then applying the Fourier transform to that result is the same as applying the Hankel transform to that function. This concept can be extended to higher dimensions.

Relationship to the Radon transform

The Abel transform is a projection of f(r) along a particular axis. The two-dimensional Radon transform gives the Abel transform as a function of not only the distance along the viewing axis, but of the angle of the viewing axis as well. In mathematics, the Radon transform in two dimensions is the integral transform The Radon transform integrates a function over lines in the plane, mapping a function of position to a function of the slope and the y-intercept. ... This article is about angles in geometry. ...

References

Bracewell, R. (1965). The Fourier Transform and its Applications, McGraw-Hill, New York. ISBN 0070070164.