Moment problem explained
to the sequence of
moments
More generally, one may consider
for an arbitrary sequence of functions
.
Introduction
In the classical setting,
is a measure on the
real line, and
is the sequence
. In this form the question appears in
probability theory, asking whether there is a
probability measure having specified
mean,
variance and so on, and whether it is unique.
There are three named classical moment problems: the Hamburger moment problem in which the support of
is allowed to be the whole real line; the
Stieltjes moment problem, for
; and the
Hausdorff moment problem for a bounded interval, which
without loss of generality may be taken as
.
The moment problem also extends to complex analysis as the trigonometric moment problem in which the Hankel matrices are replaced by Toeplitz matrices and the support of is the complex unit circle instead of the real line.
Existence
A sequence of numbers
is the sequence of moments of a measure
if and only if a certain positivity condition is fulfilled; namely, the
Hankel matrices
,
should be positive semi-definite. This is because a positive-semidefinite Hankel matrix corresponds to a linear functional
such that
and
(non-negative for sum of squares of polynomials). Assume
can be extended to
. In the univariate case, a non-negative polynomial can always be written as a sum of squares. So the linear functional
is positive for all the non-negative polynomials in the univariate case. By Haviland's theorem, the linear functional has a measure form, that is
. A condition of similar form is necessary and sufficient for the existence of a measure
supported on a given interval
.
One way to prove these results is to consider the linear functional
that sends a polynomial
to
If
are the moments of some measure
supported on
, then evidently
Vice versa, if holds, one can apply the M. Riesz extension theorem and extend
to a functional on the space of continuous functions with compact support
), so that
By the Riesz representation theorem, holds iff there exists a measure
supported on
, such that
for every
.
Thus the existence of the measure
is equivalent to . Using a representation theorem for positive polynomials on
, one can reformulate as a condition on Hankel matrices.
Uniqueness (or determinacy)
See also: Carleman's condition and Krein's condition. The uniqueness of
in the Hausdorff moment problem follows from the
Weierstrass approximation theorem, which states that
polynomials are
dense under the
uniform norm in the space of
continuous functions on
. For the problem on an infinite interval, uniqueness is a more delicate question. There are distributions, such as
log-normal distributions, which have finite moments for all the positive integers but where other distributions have the same moments.
Formal solution
When the solution exists, it can be formally written using derivatives of the Dirac delta function as
d\mu(x)=\rho(x)dx, \rho(x)=
\delta(n)(x)mn
.The expression can be derived by taking the inverse Fourier transform of its
characteristic function.
Variations
See also: Chebyshev–Markov–Stieltjes inequalities. An important variation is the truncated moment problem, which studies the properties of measures with fixed first moments (for a finite). Results on the truncated moment problem have numerous applications to extremal problems, optimisation and limit theorems in probability theory.
Probability
The moment problem has applications to probability theory. The following is commonly used:[1]
By checking Carleman's condition, we know that the standard normal distribution is a determinate measure, thus we have the following form of the central limit theorem:
See also
References
- Book: Shohat . James Alexander . Jacob D. . Tamarkin . Jacob Tamarkin . The Problem of Moments . American mathematical society . New York . 1943 . 978-1-4704-1228-9.
- Book: Akhiezer . Naum I. . Naum Akhiezer . The classical moment problem and some related questions in analysis . registration . Hafner Publishing Co. . New York . 1965 . (translated from the Russian by N. Kemmer)
- Book: Kreĭn . M. G. . Nudel′man . A. A. . Translations of Mathematical Monographs . The Markov Moment Problem and Extremal Problems . American Mathematical Society . Providence, Rhode Island . 1977 . 978-0-8218-4500-4 . 0065-9282 . 10.1090/mmono/050.
- Book: Schmüdgen, Konrad . Graduate Texts in Mathematics. The Moment Problem . Springer International Publishing . Cham . 2017 . 277 . 978-3-319-64545-2 . 0072-5285 . 10.1007/978-3-319-64546-9.
Notes and References
- Web site: Sodin . Sasha . March 5, 2019 . The classical moment problem . live . https://web.archive.org/web/20220701072907/https://webspace.maths.qmul.ac.uk/a.sodin/teaching/moment/clmp.pdf . 1 Jul 2022.