Poisson summation formula
Equation in Fourier analysis
In mathematics , the Poisson summation formula is an equation that relates the Fourier series coefficients of the periodic summation of a function to values of the function's continuous Fourier transform . Consequently, the periodic summation of a function is completely defined by discrete samples of the original function's Fourier transform. And conversely, the periodic summation of a function's Fourier transform is completely defined by discrete samples of the original function. The Poisson summation formula was discovered by Siméon Denis Poisson and is sometimes called Poisson resummation .
Forms of the equation
Consider an aperiodic function
with
Fourier transform
alternatively designated by
and
The basic Poisson summation formula is:
|
|
( Eq.1 ) |
Also consider periodic functions, where parameters
and
are in the same units as
:
Then Eq.1 is a special case (P=1, x=0) of this generalization: [1] [2]
|
|
( Eq.2 ) |
which is a
Fourier series
expansion with coefficients that are samples of function
Similarly:
|
|
( Eq.3 ) |
also known as the important Discrete-time Fourier transform .
A proof may be found in either Pinsky
[1]
or Zygmund.
[2]
Eq.2
, for instance, holds in the sense that if
, then the right-hand side is the (possibly divergent) Fourier series of the left-hand side. It follows from the
dominated convergence theorem
that
exists and is finite for almost every
. Furthermore it follows that
is integrable on any interval of length
So it is sufficient to show that the Fourier series coefficients of
are
Proceeding from the definition of the Fourier coefficients we have:
where the interchange of summation with integration is once again justified by dominated convergence. With a
change of variables
(
) this becomes:
Distributional formulation
These equations can be interpreted in the language of
distributions
[3]
[4]
:
§7.2
for a function
whose derivatives are all rapidly decreasing (see
Schwartz function
).
The Poisson summation formula arises as a particular case of the
Convolution Theorem on tempered distributions
,
using the
Dirac comb
distribution and its
Fourier series
:
In other words, the periodization of a
Dirac delta
resulting in a
Dirac comb
, corresponds to the discretization of its spectrum which is constantly one.
Hence, this again is a Dirac comb but with reciprocal increments.
For the case
Eq.1
readily follows:
Similarly:
Or: [5] : 143
The Poisson summation formula can also be proved quite conceptually using the compatibility of Pontryagin duality with short exact sequences such as [6]
Applicability
Eq.2
holds provided
is a continuous
integrable function
which satisfies
for some
and every
[7]
[8]
Note that such
is
uniformly continuous
, this together with the decay assumption on
, show that the series defining
converges uniformly to a continuous function.
Eq.2
holds in the strong sense that both sides converge uniformly and absolutely to the same limit.
[8]
Eq.2
holds in a
pointwise
sense under the strictly weaker assumption that
has bounded variation and
[2]
The Fourier series on the right-hand side of Eq.2 is then understood as a (conditionally convergent) limit of symmetric partial sums.
As shown above,
Eq.2
holds under the much less restrictive assumption that
is in
, but then it is necessary to interpret it in the sense that the right-hand side is the (possibly divergent) Fourier series of
[2]
In this case, one may extend the region where equality holds by considering summability methods such as
Cesàro summability
. When interpreting convergence in this way
Eq.2
, case
holds under the less restrictive conditions that
is integrable and 0 is a point of continuity of
. However
Eq.2
may fail to hold even when both
and
are integrable and continuous, and the sums converge absolutely.
[9]
Applications
Method of images
In
partial differential equations
, the Poisson summation formula provides a rigorous justification for the
fundamental solution
of the
heat equation
with absorbing rectangular boundary by the
method of images
. Here the
heat kernel
on
is known, and that of a rectangle is determined by taking the periodization. The Poisson summation formula similarly provides a connection between Fourier analysis on Euclidean spaces and on the tori of the corresponding dimensions.
[7]
In one dimension, the resulting solution is called a
theta function
.
In electrodynamics , the method is also used to accelerate the computation of periodic Green's functions . [10]
Sampling
In the statistical study of time-series, if
is a function of time, then looking only at its values at equally spaced points of time is called "sampling." In applications, typically the function
is
band-limited
, meaning that there is some cutoff frequency
such that
is zero for frequencies exceeding the cutoff:
for
For band-limited functions, choosing the sampling rate
guarantees that no information is lost: since
can be reconstructed from these sampled values. Then, by Fourier inversion, so can
This leads to the
Nyquist–Shannon sampling theorem
.
[1]
Ewald summation
Computationally, the Poisson summation formula is useful since a slowly converging summation in real space is guaranteed to be converted into a quickly converging equivalent summation in Fourier space. [11] (A broad function in real space becomes a narrow function in Fourier space and vice versa.) This is the essential idea behind Ewald summation .
Approximations of integrals
The Poisson summation formula is also useful to bound the errors obtained when an integral is approximated by a (Riemann) sum. Consider an approximation of
as
, where
is the size of the bin. Then, according to
Eq.2
this approximation coincides with
. The error in the approximation can then be bounded as
. This is particularly useful when the Fourier transform of
is rapidly decaying if
.
Lattice points in a sphere
The Poisson summation formula may be used to derive Landau's asymptotic formula for the number of lattice points in a large Euclidean sphere. It can also be used to show that if an integrable function,
and
both have
compact support
then
[1]
Number theory
In number theory , Poisson summation can also be used to derive a variety of functional equations including the functional equation for the Riemann zeta function . [12]
One important such use of Poisson summation concerns
theta functions
: periodic summations of Gaussians . Put
, for
a complex number in the upper half plane, and define the theta function:
The relation between
and
turns out to be important for number theory, since this kind of relation is one of the defining properties of a
modular form
. By choosing
and using the fact that
one can conclude:
by putting
It follows from this that
has a simple transformation property under
and this can be used to prove Jacobi's formula for the number of different ways to express an integer as the sum of eight perfect squares.
Sphere packings
Cohn & Elkies [13] proved an upper bound on the density of sphere packings using the Poisson summation formula, which subsequently led to a proof of optimal sphere packings in dimension 8 and 24.
Other
-
Let
for
and
for
to get
- It can be used to prove the functional equation for the theta function.
- Poisson's summation formula appears in Ramanujan's notebooks and can be used to prove some of his formulas, in particular it can be used to prove one of the formulas in Ramanujan's first letter to Hardy. [ clarification needed ]
- It can be used to calculate the quadratic Gauss sum.
Generalizations
The Poisson summation formula holds in
Euclidean space
of arbitrary dimension. Let
be the
lattice
in
consisting of points with integer coordinates. For a function
in
, consider the series given by summing the translates of
by elements of
:
Theorem
For
in
, the above series converges pointwise almost everywhere, and thus defines a periodic function
on
lies in
with
Moreover, for all
in
(Fourier transform on
) equals
(Fourier transform on
).
When
is in addition continuous, and both
and
decay sufficiently fast at infinity, then one can "invert" the domain back to
and make a stronger statement. More precisely, if
for some C , δ > 0, then [8] : VII §2
where both series converge absolutely and uniformly on Λ. When d = 1 and x = 0, this gives Eq.1 above.
More generally, a version of the statement holds if Λ is replaced by a more general lattice in
. The
dual lattice
Λ′ can be defined as a subset of the dual vector space or alternatively by
Pontryagin duality
. Then the statement is that the sum of delta-functions at each point of Λ, and at each point of Λ′, are again Fourier transforms as distributions, subject to correct normalization.
This is applied in the theory of theta functions , and is a possible method in geometry of numbers . In fact in more recent work on counting lattice points in regions it is routinely used − summing the indicator function of a region D over lattice points is exactly the question, so that the LHS of the summation formula is what is sought and the RHS something that can be attacked by mathematical analysis .
Selberg trace formula
Further generalization to locally compact abelian groups is required in number theory . In non-commutative harmonic analysis , the idea is taken even further in the Selberg trace formula, but takes on a much deeper character.
A series of mathematicians applying harmonic analysis to number theory, most notably Martin Eichler,
Atle Selberg
,
Robert Langlands
, and James Arthur, have generalised the Poisson summation formula to the Fourier transform on non-commutative locally compact reductive algebraic groups
with a discrete subgroup
such that
has finite volume. For example,
can be the real points of
and
can be the integral points of
. In this setting,
plays the role of the real number line in the classical version of Poisson summation, and
plays the role of the integers
that appear in the sum. The generalised version of Poisson summation is called the Selberg Trace Formula, and has played a role in proving many cases of Artin's conjecture and in Wiles's proof of Fermat's Last Theorem. The left-hand side of
Eq.1
becomes a sum over irreducible unitary representations of
, and is called "the spectral side," while the right-hand side becomes a sum over conjugacy classes of
, and is called "the geometric side."
The Poisson summation formula is the archetype for vast developments in harmonic analysis and number theory.
Convolution theorem
The Poisson summation formula is a particular case of the convolution theorem on tempered distributions . If one of the two factors is the Dirac comb , one obtains periodic summation on one side and sampling on the other side of the equation. Applied to the Dirac delta function and its Fourier transform , the function that is constantly 1, this yields the Dirac comb identity .
See also
- Fourier analysis § Summary
- Post's inversion formula
- Voronoi formula
- Discrete-time Fourier transform
- Explicit formulae for L-functions
References
- 1 2 3 4 Pinsky, M. (2002), Introduction to Fourier Analysis and Wavelets. , Brooks Cole, ISBN 978-0-534-37660-4
- 1 2 3 4 Zygmund, Antoni (1968), Trigonometric Series (2nd ed.), Cambridge University Press (published 1988), ISBN 978-0-521-35885-9
- ↑ Córdoba, A., "La formule sommatoire de Poisson", Comptes Rendus de l'Académie des Sciences, Série I , 306 : 373–376
- ↑ Hörmander, L. (1983), The analysis of linear partial differential operators I , Grundl. Math. Wissenschaft., vol. 256, Springer, doi : 10.1007/978-3-642-96750-4 , ISBN 3-540-12104-8 , MR 0717035
-
↑
Oppenheim, Alan V.
;
Schafer, Ronald W.
; Buck, John R. (1999).
Discrete-time signal processing
(2nd
ed.). Upper Saddle River, N.J.: Prentice Hall.
ISBN
0-13-754920-2
.
samples of the Fourier transform of an aperiodic sequence x[n] can be thought of as DFS coefficients of a periodic sequence obtained through summing periodic replicas of x[n].
- ↑ Deitmar, Anton; Echterhoff, Siegfried (2014), Principles of Harmonic Analysis , Universitext (2 ed.), doi : 10.1007/978-3-319-05792-7 , ISBN 978-3-319-05791-0
- 1 2 Grafakos, Loukas (2004), Classical and Modern Fourier Analysis , Pearson Education, Inc., pp. 253–257, ISBN 0-13-035399-X
- 1 2 3 Stein, Elias; Weiss, Guido (1971), Introduction to Fourier Analysis on Euclidean Spaces , Princeton, N.J.: Princeton University Press, ISBN 978-0-691-08078-9
- ↑ Katznelson, Yitzhak (1976), An introduction to harmonic analysis (Second corrected ed.), New York: Dover Publications, Inc, ISBN 0-486-63331-4
- ↑ Kinayman, Noyan; Aksun, M. I. (1995). "Comparative study of acceleration techniques for integrals and series in electromagnetic problems". Radio Science . 30 (6): 1713–1722. doi : 10.1029/95RS02060 . hdl : 11693/48408 .
- ↑ Woodward, Philipp M. (1953). Probability and Information Theory, with Applications to Radar . Academic Press, p. 36.
- ↑ H. M. Edwards (1974). Riemann's Zeta Function . Academic Press, pp. 209–11. ISBN 0-486-41740-9 .
- ↑ Cohn, Henry; Elkies, Noam (2003), "New upper bounds on sphere packings I", Ann. of Math. , 2, 157 (2): 689–714, arXiv : math/0110009 , doi : 10.4007/annals.2003.157.689 , MR 1973059
Further reading
- Benedetto, J.J.; Zimmermann, G. (1997), "Sampling multipliers and the Poisson summation formula" , J. Fourier Ana. App. , 3 (5), archived from the original on 2011-05-24 , retrieved 2008-06-19
- Gasquet, Claude; Witomski, Patrick (1999), Fourier Analysis and Applications , Springer, pp. 344–352, ISBN 0-387-98485-2
- Higgins, J.R. (1985), "Five short stories about the cardinal series" , Bull. Amer. Math. Soc. , 12 (1): 45–89, doi : 10.1090/S0273-0979-1985-15293-0