Preface by Hovik A. Matevossian
In this Editorial, we present an authentic translation of the Dissertation written by Mergelyan, S. N. called Best approximations in the complex domain. (Ph.D. thesis, Steklov Mathematical Institute of the USSR Academy of Sciences, Moscow, 1948, 56 pages (In Russian)). It presents phenomenal results from the outstanding Soviet and Armenian mathematician Sergey N. Mergelyan (1928–2008) in connection with his 95th birthday.
S. Mergelyan’s main scientific research included the theory of functions of complex variables, approximation theory, the theory of potentials, and harmonic functions.
S. Mergelyan carried out in-depth research and obtained valuable results in such areas as the best approximation by polynomials on an arbitrary continuum, weighted approximation by polynomials on the real axis, point approximation by polynomials on closed sets of the complex plane, uniform approximation by harmonic functions on compact sets, and entire functions on an unbounded continuum.
For the exceptional results obtained in the field of approximation theory, the Scientific Council of the Steklov Mathematical Institute of the USSR Academy of Sciences awarded the 20-year-old genius Sergey Mergelyan a degree of Doctor of Physical and Mathematical Sciences; he was the youngest Doctor of Sciences in the history of the USSR and the youngest Corresponding member of the Academy of Sciences of the Soviet Union (at 24 years old) (
https://en.wikipedia.org/wiki/Sergey_Mergelyan (accessed on 20 December 2023)).
In 1951, S. Mergelyan proved his famous theorem on approximation by polynomials (Mergelyan, S. N. Certain questions of the constructive theory of functions. Trudy Mat. Inst. Steklov 1951, 37, Acad. Sci. USSR, Moscow, 3–91). His theorem on the approximation of functions by polynomials has become classical among the theorems of Weierstrass and Runge.
The new terms “Mergelyan’s Theorem” and “Mergelyan Sets” have found their place in textbooks and monographs on approximation theory.
S. Mergelyan’s theorem answers the question about the possibility of polynomial approximation of the function of one complex variable: Every function continuous on a compact set and holomorphic in its interior can be represented in K by a uniformly converging sequence of polynomials if and only if the complement is connected (Mergelyan, S. N. Uniform approximations of functions of a complex variable. Uspekhi Mat. Nauk 1952, 7:2(48), 31–122).
S. Mergelyan’s theorem completes a large cycle of research on polynomial approximations, which began in 1885, and consists of classical results by Weierstrass, Runge, Walsh, M. Lavrentiev, M. Keldysh, and others. In these papers, a function that is continuous on a compact set and holomorphic in its interior is approximated by a function that is holomorphic on the entire compact set (that is, in a neighborhood of this set). Polynomial approximation is then obtained using the Runge theorem (1885) that every function that is holomorphic on a compact set whose complement is connected can be represented in this set by a uniformly converging sequence of complex polynomials.
S. Mergelyan’s further results were devoted to the study of the approximation of continuous functions that satisfy the smoothness properties for an arbitrary set (1962) and the solution of Bernstein’s approximation problem (1963).
I express my gratitude to the Leading Researcher of the Steklov Mathematical Institute of the Russian Academy of Sciences and Professor at Lomonosov Moscow State University, A.G. Sergeev, for supporting the idea of publishing Mergelyan’s Dissertation.
I express my gratitude to Professors Heinrich Begehr (FU Berlin) and Paul Gauthier (Université de Montréal) for their efforts in reading the translation of this Dissertation, for their valuable comments in clarifying and correctly using the terminology of the theory of approximation, and for their help in editing the manuscript.
I express my sincere gratitude to two staff members at the Library of Natural Sciences of the Russian Academy of Sciences, Tatiana and Irina, for providing the original manuscript of Mergelyan’s Dissertation, which is stored in the scientific collection of the library of the Steklov Mathematical Institute of the Russian Academy of Sciences.
I also express my gratitude to my graduate students M. Dorodnitsyn and A. Kovalev for typing this manuscript, and to my colleague V.N. Bobylev for their help in reading the manuscript and restoring the list of references.
S. N. Mergelyan “Best Approximations in the Complex Domain”
Introduction
Consider a finite closed domain
whose complement represents a connected set. Suppose that the function
is regular at interior points of
and continuous on
. The infimum of the values
with respect to all possible polynomials
of degree
we denote by
.
As is known [
1],
as
, and the rate of decrease of the number
is closely related to the properties of
on
, as well as to the properties of the domain
D.
In the case when the boundary
D is an analytic curve, it is established [
2] that if
has a continuous
derivative in
, satisfying there the Lipschitz condition of order
, then there is a constant
C for which
and, conversely, if
,
k is an integer and
, then the
derivative of
satisfies the Lipschitz condition of order
in
.
Thus, in the case of an analytic domain, the dependence of on is similar to the dependence of the rate of best approximation on the properties of a function in the real domain.
From the results related to the investigation of the rate of best approximation in the case of non-analytic domains, we note the following two ideas.
Let
denote the distance of the image of the circle
under the conformal mapping
onto the complement of
to the boundary of
D. If for some
then from inequality
it follows that the
derivative of
satisfies the Lipschitz condition of order
in
[
2].
If
denotes the angle made by the tangent to the boundary of
D at the point
with the axis
, and
represents a point on
distant from some fixed point
at arc distance
s, and
satisfies a Lipschitz condition of positive order, then from the fact that
is regular in
D and satisfies the inequality
it follows, as A. I. Markouchevitch showed [
3], that for any positive
The present paper is devoted to studying the rate of best approximation in the general case when D represents an arbitrary domain of the Carathéodory class. Some issues are also considered that are somehow related to the theory of best approximation.
In
Section 1 we establish upper estimates for the quantities
for domains with various features, for example for domains with a corner point, convex domains, etc.
In
Section 2 given the rate of approximation and the domain
D, the necessary properties of the function are investigated, and the theorems are local in nature, since the same rate of approximation imposes different restrictions on the function at different boundary points, depending on the behavior of the domain
D near these points. Thus, it is possible to verify the accuracy of the estimates
Section 1.
Direct and inverse theorems on the rate of best approximation in domains with a smooth boundary are highlighted in
Section 3.
Here we show that the above-mentioned analogy between and the rate of best approximation in the real domain , which holds for analytic domains, already disappears in the case of some of the domains with a smooth boundary.
Inequality
0.1 extends to arbitrary domains with a smooth boundary. Estimates for
are also given depending on the degree of smoothness of the boundary
D.
It is known [
4] that if two domains
and
have only one common boundary point, and
and
are regular in
,
, respectively, and are continuous at the closures and take equal value at the common boundary point, then, as soon as in each of the domains
,
the corresponding function can be uniformly approximated by polynomials, then there exists a sequence of polynomials uniformly converging in
to
and, at the same time, in
to
. In
Section 4 the results concerning the study of the rate of simultaneous approximation in two touching domains are presented. (The Introduction specifies
Section 4, but this
Section 4 is missing from the manuscript. It is worth noting that the material stated as
Section 4 is discussed in
Section 3 (
editor’s comment)) In this case, the rate of approximation depends on a third factor—the relative position of the domains
and
. It is proved that if a certain relationship is satisfied between the rate of simultaneous approximation and the order of contact of the boundaries
and
, then the convergence of the sequence of polynomials
to zero in
also automatically implies convergence to zero in
.
In
Section 5 some quasi-analytic classes of functions are introduced and criteria for belonging to them are given in terms of the best approximation. Related here is the question of the distribution of zeros of the analytic function
located on the boundary of the domain of regularity of
under the assumption that
is continuous in a closed domain.
In
Section 6 the best approximation on various discontinuous sets is considered, and in some cases a dependency is established between the rate of approximation, the behavior of the function and the properties of the sets on which the approximation occurs.
I take this opportunity to express my deep gratitude to Academician M.V. Keldysh, whose advice and instructions provided me with great assistance in carrying out this work.
1. Direct Theorems for Domains with Different Types of Singularities
Let us present the formulation of one of Warschawski’s results, which we will use in the future.
Let
D be a Jordan domain bounded by a curve
passing through
. Suppose that in the neighborhood
of the point
the boundary
of the domain
D consists of two arcs
and
, the equations of which in polar coordinates are
respectively. Let there also be limits
Let
denote the function that conformally maps the domain
D onto the circle
so that
goes to
;
.
Theorem 1.1 (S. E. Warschawski [
5])
. Ifthen in the neighborhood of the point we havewhere C does not depend on z. This theorem makes it possible in a number of cases to investigate the behavior of conformal mapping functions in a closed domain, as well as to estimate the distance of the level lines of the Green’s function to a boundary point, depending on the behavior of the boundary near this point.
Let D be a bounded domain with a simply connected complement; henceforth, by we mean the level line of the Green’s function of the complement to .
is a bounded domain bounded by
. If
is regular in
and does not exceed unity there in absolute value, then for any integer
there is a polynomial
of degree
n, for which
where
is an absolute constant (the degree of difference
can be reduced, however the question of determining it as accurately as possible does not interest us now).
The proof is easy to derive by composing a Fejér interpolation polynomial with uniformly distributed nodes, estimating the remainder term represented by the Cauchy integral, and considering that, firstly, if the diameter
D is less than one, then
and secondly, the distance of any point
to
exceeds
, where
is an absolute constant.
Let
D now contain
and the equation of its boundary in polar coordinate be
where
is a single-valued continuous function.
Theorem 1.2. If all derivative numbers of the function are uniformly bounded from above by the number k and the function is regular in D, has a continuous derivative in , the modulus of continuity of which is , then Proof. Let be an arbitrary boundary point of D, and let map the complement of to the circle so that goes to .
It is easy to see that from the condition
that it follows, in the notation of Warschawski’s theorem
so putting
we have
that is, if
belongs to the circumference
, then the distance
z to
does not exceed
.
Let us denote by the domain to which D goes under the transformation . From it follows that the existence of a constant for which the distance of the boundary to the boundary D exceeds .
Thus, for some
the domain
bounded by the level line
is contained in the domain
. It follows that the function
is analytic in
, uniformly bounded there in
R; therefore, according to the remark made above, there is a polynomial
of degree
n such that
where
But
Let us put
; we have
From the fact that
is the modulus of continuity of some function other than a constant, it follows that, firstly,
, and secondly,
, so we have
Next, we apply a well-known technique. The function
satisfies a first-order Lipschitz condition with a Lipschitz constant equal to
so, according to what has been proved, we can find a polynomial
of degree
for which
Proceeding similarly with
, and finally with
we come to the proof of the theorem.
Thus, Theorem 1.2 gives an estimate of the rate of approximation for domains with corner points. □
Let D be bounded by a finite number of smooth curves that make angles with each other, the internal openings of which do not exceed , and be regular in D and satisfy the Lipschitz condition of order in .
Assuming the domain
D is star-shaped with respect to one of its points, it is easy to show by the reasoning used in the proof of Theorem 1.2 that
for any
.
We can free ourselves from the artificial restriction of star-shapedness by using an additional reasoning based on the “averaging” method of academician M.V. Keldysh [
6], which we set out in §3 when proving Theorem 1.3.
If no additional restrictions are imposed on the smoothness of the boundary
D, then, as will be seen, in §3, the numbers
can increase arbitrarily quickly: for any positive function
there exists
such that
satisfies a Lipschitz condition of order
in
; however,
Now, let the domain D have an incoming point, and still means the equation of the boundary in polar coordinates; assume that exists everywhere outside , and also that at the point two arcs of the boundary of D touch the axis , and decreases monotonically as , ; by we denote the distance of the point to the level line .
Theorem 1.3. If is regular in D, its derivative has modulus of continuity in , then Proof. The proof of this theorem is similar to the above proof of Theorem 1.2.
If, in particular, at the point
we have the algebraic order of tangency of two arcs of the boundary
then, using Warschawski’s theorem, we have
Hence, in this case
In
Section 2 it will be shown that this estimate is exact, in the sense that for some functions satisfying the conditions of Theorem 1.3 and two constants
and
From the noted special cases we can conclude that in order to obtain a certain rate of approach in there is no need to require that the function behaves well enough at all boundary points; the necessary and sufficient properties of the function for a given rate of approximation and domain depend at each point only on the behavior of the boundary of the domain near this point. □
This circumstance constitutes a distinguishing feature of the best approximations in the complex domain from the best approximations in the real domain.
2. Inverse Theorems
Let D denote a domain bounded by the Jordan curve . By we denote the distance of the image of the circle under a conformal mapping of the exterior of the unit circle to the complement of to a boundary point .
Let denote an arbitrary subdomain of D having only the property that the ratio of the distance of any point of to to the distance of the same point to is bounded from above uniformly with respect to all points of . The class of functions that are regular in some domain G and whose derivative satisfies a Lipschitz condition of order in is denoted by .
It should be noted that by the modulus of continuity
of the function
in
we mean the supremum of the quantities
by all possible pairs
belonging to
and such that
can be connected to
by a rectifiable curve lying entirely in
and by length not exceeding
; for some domains this definition obviously does not coincide with the definition of the modulus of continuity that is given in the real domain; accordingly, a different meaning, generally speaking, is attached to the satisfaction of the Lipschitz condition in a closed domain
.
It is easy to see that the quantity
is closely related to the properties of the function
, while
represents an artificial formation in relation to
.
Indeed, for any function decreasing monotonically to zero, one can construct a domain such that the fact that implies infinite differentiability of at individual points on the boundary of the domain. As a similar example, we can take a domain with an incoming point and a sufficiently large order of contact of two boundary arcs at .
The following proposition also applies to this question, the proof of which we will not dwell on.
Proposition 2.1. If two domains and have one common boundary point , and the function in coincides with the function that is regular in and continuous in , and , then for any function decreasing towards zero one can specify such a large order of contact of the boundaries of and at that from the inequalityit follows that if one of the functions is identically equal to zero in the corresponding domain, then the same can be stated regarding the other function, i.e., pairs of functions satisfying (2.1) constitute a quasi-analytic class. This Proposition can be deduced from a theorem to be proved later.
Theorem 2.1. Ifthen for any , ; if , is infinitely differentiable in . Proof. For any integer
we define the integer
from the condition
The numbers
obviously constitute an increasing sequence. Let
z denote an arbitrary point of
,
,
p be some integer, and
be a polynomial of degree
m that least deviates from
in
.
But from the definition of it follows that there exists such that the disc is contained entirely in the domain , bounded by the outer line of the level .
It is known that if
, then
,
. Keeping in mind that
and setting
, we apply this to the estimate of the difference under the integral in
(the last inequality follows from (
2.3)).
Let
denote, as usual, the integer part of
a and
. Let us put
; since
and
is small enough, then
. From the conditions of the theorem it follows that
Consequently, the series
representing
in
D uniformly converges in
, i.e.,
is continuous in
.
Let us estimate its modulus of continuity in the closed domain
. Let
be points of
. We have
But the common term of the sum just written does not exceed
where the integration path lies entirely in
.
The first multiplier, similar to the above, can be easily estimated using the Cauchy integral, and with respect to the second we assume that
As a result, we obtain
But
We estimate the general term of the last sum on the right side of (
2.6) based on (
2.5). Thus, we have
Now assuming
and taking any pair of points
from
with the condition
, we have
where
is the modulus of continuity of
in
.
Consequently, the inclusion is proven for any . □
Corollary 2.1. From the above reasoning it can be seen that for is infinitely differentiable in .
Thus, an arbitrarily slow approximation rate ensures infinite differentiability of the approximated function at some boundary points, if only the domain is located appropriately near these points.
Applying Warschawski’s result on conformal mapping stated above, in many cases it is possible, by estimating , to formulate the previous theorem directly in terms of the boundary of the domain.
Let the domain
D contain, for some
, the segment
and its boundary near
be determined by the equation
that is, we have a domain with an incoming point of algebraic tangent order.
In this case, according to Theorem 1.1,
therefore, it can be proved that
Corollary 2.2. If for some then for any ; if the proof is carried out carefully, then it can be shown that even , and this result is quite accurate, in the sense that, as it follows from Theorem 2.1, it is invertible: if , then . Now, let
. We denote
The rate of increase of the numbers
depends on the rate of decrease of the numbers
and will be closer to the rate of increase of
, the closer
to any geometric progression. Namely, we will show that for any
,
In the case of
this shows that
(
does not depend on n), i.e.,
is analytic in
.
Indeed, for any
, using the Cauchy integral, one can obtain the estimate
putting
and taking into account (
2.9) we obtain, summing over
k, the required inequality. In particular, let
In this case, it is easy to calculate the right-hand side of (
2.8):
For
close to one, the estimate (
2.10) does not differ much from the exact one. Indeed, consider the function
where
, and
and
are functions inverse to the corresponding ones, and
z belongs to the domain
D, symmetrically located relative to the
axis so that the distance of its boundary point
to the level line
is equal to
and
does not intersect with
anywhere other than
.
Since the function
is regular in the domain
bounded by
, then, as is known, there exists a polynomial
so that
But
Setting
we obtain
Let us estimate
from below. We have
Estimating the integral and assuming the existence of
,
, we obtain
In our case
and
; substituting in (
2.12) we have
that is, for small
the estimate (
2.10) does not differ much from the exact one.
So, for an arbitrarily slow rate of approximation, the corresponding structure of the domain can guarantee sufficiently “good” properties of the function at some boundary points. Therefore, the question arises: is it possible, by means of an appropriate construction of the domain, to ensure that the rate of approximation, different from the progression, would guarantee the analyticity of the function at some points on the boundary?
Regarding this question, the following can be stated:
Theorem 2.2. For any function satisfying, for any , the conditionand any bounded domain D of the Carathéodory class, there exists a function regular in D, continuous in , the rate of approximation to which satisfies the inequalityhowever for which the boundary of D is a cut. Proof. Let us denote, for brevity,
From (
2.13) it follows that
on the outer line of the level
; we mark a point
so that for the set
, each boundary point of
D would be a limit point. This is obviously possible, since
implies that
.
Let
denote the distance of the point
to
. Let us create the function
We have, obviously,
In addition, since the function
is regular in
and uniformly bounded in
n, then there exists a polynomial
for which
Let us show that the boundary of D is a cut-off for , if only decreases sufficiently quickly.
Let be analytically extendable through the continuum into the domain G. We enclose each of the points in a circle with the center at and radius so small that none of the pairs of discs and will have common points. In the domain G we fix a subdomain lying outside all circles ,
Let
denote the sum of angles at which the continuum
K is visible from point
z in domain
G; by
we denote the angle at which the disc
is visible from
z in
G. Also, let
It is easy to see that
can be chosen so that
We will assume, in addition, that the
decreases so quickly to zero that the numbers
,
, and the function
already defined according to the assumption in
for
is bounded in the domain
obtained from
G by removing
by a number
M independent of
n. On the boundary continuum
K of the domain
we have
Now let us choose
so that
According to the well-known Lindelöf theorem, if in some domain
B there is an analytic function
such that on some arc
of the boundary
B visible from the point
at an angle
and on the remaining part of the boundary
B
then
Applying this theorem to our case and considering as
B the domain
, and
the function
, we obtain
that is, the series
in the domain
represents the analytic continuation of
.
But it is easy to see that if we consider the sum of a series outside , then for it the boundary of D is a cut, that is, we have obtained a contradiction, since, on the other hand, this sum represents in a function that can be analytically extended into the domain D. □
Now consider the case when
In this case,
may already be non-differentiable and not satisfy any Lipschitz condition of positive order; however, its modulus of continuity
in
satisfies the following constraint.
Theorem 2.3. For some independent of δ It will be shown later that, in the general case, this estimate cannot be improved.
Proof. Indeed, let
be a polynomial that least deviates from
in
of degree
n. We have
But
is easy to estimate using the Cauchy integral so that
In particular, if
and
, we have
□
Let us note one corollary of Theorem 2.1.
Corollary 2.3. Let the domain D be convex or bounded by a polygonal line with a finite number of segments. If can be approached in with the ratethen . Indeed, the inequality follows for convex regions and polygons from Lindelöf’s principle, since there always exists a segment located in and having one of its ends at the boundary point ; it is enough now to compare the lines of the external level of D and the complement to the mentioned segment.
Note that these theorems apply not only to Jordan domains, but to any domains of the Carathéodory class.
3. Estimation of the Rate of Approximation for Domains with a Smooth Boundary
Theorem 3.1. If the domain D is bounded by a smooth curve and is analytic in D and continuous in , and the derivative of the function satisfies a Lipschitz condition of order α, , in , then for any ,where const does not depend on n. Proof. It is sufficient to provide the proof for the case . The general case is derived from here by a well-known trick.
Let be an arbitrarily small fixed number; let us assume that the projection of D onto the axis contains the interval and the straight lines are drawn so that they do not touch the boundary of D and, therefore, intersect it at a finite number of points.
The part of
D located to the right of
will be denoted by
, and the part of
D located to the left of
by
. The open sets
and
obviously consist of a finite number of simply connected regions located at a positive distance from each other:
Let
denote the parametric equation of the boundary of the domain
D, and the parameter
s represent the length of the arc
from some fixed point
to
. We choose the number
so close to unity that the angle made by the normal to the boundary
at a point
with the internal level line
at the point closest to
would differ from
by less than
for any value of
s. The ring-shaped region enclosed between
and
we denote by
l.
The part of l located by the inner normal to at and the inner normal to at are drawn until their intersection with , denoted by .
Let us choose
so that the set
represents a star-shaped domain with respect to some point
for all values
s. (For
, it is sufficient to take the length of the segment of the internal normal to
at
, enclosed between
and the points closest to
).
is a curvilinear quadrilateral with angles close to right angles and depends on . We denote by the line passing through and orthogonal at to , and by the line passing through and parallel to . It is obvious that the numbers , and can, in addition, be chosen so that the part of l located between and would belong to the intersection of the domains and for all s.
Consider one of the components —for example, . We denote the part of located in by . Suppose that lies outside ; let be the largest of the s for which has no common points with , and let be the smallest of those s for which the intersection of l and coincides with .
Let denote the largest interval that has the property that belongs to .
The part located on the same side of the line as will be denoted by . The set of points located from at a distance less than will be denoted by ; is the set of points separated from by a distance less than .
Suppose that there exists a polynomial
of degree
n with the following properties:
where
M does not depend on
n and
s,
s is fixed,
.
The domain
is star-shaped with respect to
, so the function
is analytic in the domain
obtained from
by stretching with respect to the point
times; there is a constant
for which the image of the circle
when mapping
onto the complement of
is located entirely in
(we are only interested in small values
).
This follows from the fact that the distance of the points of the level line of the domain D, bounded by a finite number of smooth curves making angles with each other, the internal openings of which do not exceed , up to the boundary D does not exceed uniformly with respect to the points .
According to the remark (property 1), there is a polynomial
of degree
n such that
(by
we mean the level line of the complement to
).
At the same time, in
, the inequality will be satisfied,
since for
Let us cover the domain with the domain , and by the domain so that and would be at a positive distance from each other exceeding , as well as the distance of any point to ; likewise, the distance of any point to would exceed K (it is obviously possible to find such , independent of ).
As is well known, there exists a polynomial
of degree
n such that
where
depends on
K. □
Now let G be a bounded domain with a simply connected complement, whose projection onto the axis is greater than one, and be regular in G and continuous in . is the domain bounded by the outer line of level of the domain G; and are two polynomials of degree n with the following properties:
- (1)
in the part , which is located above the straight line ; similarly in the part , located below the straight line ; we assume that the segment belongs to the projection of G onto the axis , and .
- (2)
In the part
G located in the half-plane
,
in the part
G located in the half-plane
,
Let us additionally assume that the distance of any point to is less than .
Lemma 3.1. There exists a polynomial of degree n for which
Proof. The proof is based on the averaging method of Academician M.V. Keldysh [
6].
Let us denote
Let
. The formula
was obtained in [
6].
Let us split the integral on the right side (
3.2) into two parts: over the domain
G and over the remaining domain
. By adding and subtracting the fractions under the integral on the right side (
3.2)
in the numerator, we make sure that the first term does not exceed
We estimate the second term based on property 1, which is satisfied by the polynomials
and
:
But obviously
so that in the domain
G
The function
is analytic in
and, as follows from (
3.2), is bounded there by a constant independent of
n, so we can find a polynomial
of degree
n so that
Consequently, the polynomial
satisfies all the conditions of the lemma. □
Note that the assumption is not significant and was made for simplicity. In the case of , only the constant , which is included in the estimate as a factor, will change. It is also unimportant that the parallels of the axis are taken as two straight lines; the general case can be reduced to this case by rotation.
After this remark, we can apply the lemma to the case when as G we have the domain , as straight lines – straight lines and polynomials and – polynomials and , respectively.
Thus, assuming the existence of a polynomial that approximates the function in with the rate and is bounded in a certain neighborhood , we come to the existence of a polynomial approximating in with rate and bounded in the corresponding neighborhood .
But it is easy to see that
is star-shaped with respect to one of its points and for it the corresponding polynomial
exists; the same can be stated with respect to
; next we divide the interval
into parts of the corresponding length:
and, successively applying the averaging process described above to the domains
,
, …,
, we finally obtain a polynomial
of degree
n satisfying the inequalities
Carrying out a similar reasoning for each of the components
and keeping in mind that they are all located at a positive distance from each other, independent of
a, we can assume that the inequalities (
3.3) are satisfied in part
of domain
l located to the right of
; we also find the polynomial
that approximates
in part
of domain
l located to the left of
, with a rate
and bounded in the appropriate neighborhood
. Finally, we apply the averaging process to the domains
and
and the polynomials
and
.
It is easy to see that, slightly changing the proofs of formula (
3.2), we can obtain the formula for our case:
(the notation is similar to the notation above; the distance between
b and
a is set equal to one for simplicity).
Let
; since
for
, then, due to the analyticity of
in the domain
D, we have
We estimate the double integral on the right-hand side of (
3.4) in the same way as was shown in the proof of Lemma 3.1.
The polynomial
of degree
n is found so that
As a result we have
where
does not depend on
n.
Choosing sufficiently small, we arrive at the proof of Theorem 3.1.
If denotes the modulus of continuity of in D, then the following proposition can be proven in a completely analogous manner.
Theorem 3.2. For any there is a constant C such that Let us now consider the connection between the rate of best approximation and the properties of functions under some additional restrictions on the smoothness of the boundary.
Let denote the modulus of continuity of the function ( is the parametric equation of the boundary; s is the length of the arc ).
Theorem 3.3. If andthenifthen, generally speaking, there is a function such that Proof. Let
be some point on
. Placing the origin of the polar coordinates at
z, we notice that the angle
differs from
by less than
; but the quantity
for a small
can be replaced by the vertex angle
in the triangle
; hence,
From here we conclude that
i.e.,
so that
Let us assume that
D is star-shaped with respect to the point
. Using the technique we used to prove Theorem 1.2 and keeping in mind (
3.10), we come to the proof in the case when
satisfies a Lipschitz condition of order
. The general case is deduced from here in the same way as in the proof of Theorem 1.2.
Note that the star-shaped assumption can be eliminated by applying the averaging process described above, breaking up the domain D into overlapping star-shaped parts.
Let us now show that in the case of (
3.8) the theorem is, generally speaking, incorrect, i.e., that for (
3.8) there is, generally speaking, a function
for which the inequality (
3.7) does not hold for any constant
C.
Consider a domain
D for which the modulus of continuity
does not exceed
and in some neighborhood of the point
(such domains obviously exist). Using Warschawski’s theorem, it is easy to obtain the inequality
Now suppose that for some function
regular in
D and continuous in
there exists a polynomial
of degree
n such that
Let us estimate the modulus of continuity
in
D:
From (
2.7) we find
Taking this into account, we obtain
Evaluating the sums and setting
n equal to the integer part of the solution to the equation
we find
Let us now assume that
, but does not satisfy the additional condition (
3.11) (for example,
); then, according to what has been proven, the following relation must be satisfied:
If, for example,
, then for
the estimate (
3.7) is correct; if
or
, then it ceases to be valid in the general case.
□
Theorem 3.4. If , then, generally speaking, for some if , then for some the following inequality holds: We do not present the proof, since it is similar to the proof of Theorem 3.3.
Let (
3.6) hold and
where
k is an integer,
.
Theorem 3.5. If , then ; if , then this conclusion, as is known, is not true for analytical domains. In the case of , in order for to satisfy a first-order Lipschitz condition, it is necessary and sufficient that Proof. Using Warschawski’s theorem it is easy to show that, given condition (
3.6), there is a constant
for which the inequality
holds for all boundary points of
.
From here we conclude that
Next, we proceed in exactly the same way as when proving the corresponding result in the real domain (S.N. Bernstein’s theorem).
We now prove the second part of the theorem when .
Condition (
3.13) is necessary: assuming for simplicity
, consider the function
in the unit circle, where
is an arbitrary sequence of monotonically decreasing numbers for which
The partial sums of its Taylor series approach it in
with a rate of
; however,
does not satisfy a first-order Lipschitz condition, since for any arbitrarily large
N, one can find
n and
so that
We prove the sufficiency of condition (
3.13). Taking into account (
3.14) it is easy to obtain the inequality
Based on (
2.6) we have
Let us choose
N depending on
so that
□
Let us note the following proposition, the proof of which we will not dwell on.
Lemma 3.2. Let be an arbitrary sequence of numbers monotonically decreasing towards zero, and be an increasing sequence of integers.
In order for, from the convergence of the seriesthe convergence of the serieswould always follow, it is necessary and sufficient that there exists a positive number æ such that for all k, starting from a sufficiently large one, In particular, it follows that
that is,
satisfies a first-order Lipschitz condition.
As for the general case of domains with a smooth boundary that do not satisfy condition (
3.6), then
giving an estimate
that relates to the entire class of domains with smooth boundaries and is better than
is impossible, since the following can be proven:
Proposition 3.1. Let be a monotone function decreasing to zero for and for any satisfying the conditionThere is a domain with a smooth boundary such that the rate of approximation guarantees for some boundary points ζ the inequality( and belong to ) and there is another domain , also with a smooth boundary, and a function such that ; however, for some boundary points ζ The proof of this is similar to the proof given above for the second part of Theorem 3.3.
From Theorem 2.3 it follows that if in a domain with a smooth boundary the inequality
holds, then
where
does not depend on
M and
; and vice versa, according to Theorem 3.2, from the inequality
it follows that
where
does not depend on
N and
n.
From a comparison of these facts, it is easy to conclude that there exist domains with a smooth boundary and functions
that are regular in
D, the modulus of continuity of which in
is denoted by
, for which
as well as
where
and
do not depend on
n and
.
4. This Section Is Missing from the Manuscript
5. Some Quasi-Analytic Classes of Functions
Academician S.N. Bernstein in 1923 showed [
7] that the class of functions defined on
, for which
where
is a particular sequence of integers, is a quasi-analytic class, in the sense that if any two of its functions coincide on any part of the interval
, then they are identical.
Below we give some other quasi-analytic classes of functions defined using best approximations.
Let
be a positive function of the integer argument
n. The class of functions for which
where
is some sequence of integers, is denoted by
.
Let
be a monotone function satisfying the condition
where
means the root of the equation
for some fixed
.
Theorem 5.1. Ifthen the class is quasi-analytic in the sense that if with respect to any two of its functions and it is known thatwhere , then . If, in particular, a function of class decreases around some point as , then is required.
Proof. Let
and
since on the interval
, then
Let
and
be the level line of the complement of
, passing through
. We choose a point
so close to
that the following inequality holds:
(a simple calculation shows that
depends on
in such a way that this can always be performed). Then, as is known,
Let
; hence, it follows that
The same can be said regarding the points
, so
on the segment
and
If
x is any point in
, and
is the level line of the complement of
, passing through
x, then
and since for sufficiently large
n the inequality
holds, then
, that is,
.
If, in particular,
, then
□
Let be a closed bounded set that does not break up the plane, and M be an infinite set of points belonging to ( is assumed to be infinite).
We denote by
the class of functions continuous on
for which
Theorem 5.2. For any infinite set M there is a positive function for which the class is quasi-analytic in the sense that from the coincidence of any two of its functions and on the set M their identity on follows.
Proof. Let
be a polynomial of best approximation of the function
of degree
n on the set
. On
M we choose some countable part of
,
and represent the polynomial
in the form of an interpolation polynomial with nodes at points
:
Let
denote
The numbers
depend, obviously, only on
M.
It is enough now to set
equal to
, since from
follows
If, in particular,
is the segment
, and the set
M consists of points of the form
,
,
, then it is easy to calculate that
If
M is more sparse, and consists of points
,
,
then
Thus, the class of functions for which
has two properties characteristic of the class of analytic functions.
Let be the circle , and M be the set of zeros located on the circumference , and let be regular in , continuous in . Applying the theorem to this case, we conclude that the class of functions for which any infinite set is a set of uniqueness includes functions with an arbitrarily bad (in the sense of slowness of decrease) modulus of continuity. □
Let
be the modulus of continuity of
in
. Suppose that the zeros of
located on
are condensed to the point
. To characterize the rate of condensation to the limit point, we introduce the function
;
is the distance
to
M; if
, then
If
, then
Proof. Indeed, we have
But it is known that if the circumference
is divided into
n arcs
with lengths
, respectively, and
then for some constant
C and any point
z located in the disc
,
In our case, on the arc
; hence,
But if
, then the left-hand side can be made as small as desired, i.e.,
.
If, in particular, satisfies a Lipschitz condition of some positive order in and vanishes at points , , then . □
Theorem 5.4. If can be approached in with the ratethen the cardinality of the set of zeros is at most . Proof. Suppose that this is not so, that is, the set M is irreducible. We denote by the derived set of the set .
From each we choose one point , which can be done since M is irreducible. Let be one of the limit points of the set .
It is easy to see that the set
has the property that
vanishes on
. From (
5.1) it follows that
is infinitely differentiable in
, and
But
. Consequently, at point
all derivatives of
vanish; hence, according to the mentioned Carleman theorem, it follows that
. □
Remark 5.1. From the proof it is clear that more can be stated, namely, if is regular in , andthen has only a finite number of zeros in . 6. Best Approximation on Closed Sets
Let
E be a closed set of points located in the plane of the complex variable
z, and let
be a function defined and continuous on
E. We denote by
the infimum of the numbers
by any polynomials of degree
.
Suppose that
E is bounded, is not dense anywhere, and does not break up the plane; according to Lavrentiev’s theorem [
8],
always for
.
Theorem 6.1. Let be an arbitrary function growing to as faster than any power of r, i.e.,and E be an unbounded set. There is a function such that from the inequalityit follows that the function can be continued from the set E to the entire plane so that the function obtained as a result of the continuation will be an entire function satisfying the condition Proof. Without loss of generality, it can be assumed that the origin is a limit point of the set E. The polynomial of best approximation of the function on E of degree n will be denoted by .
Let the points
belonging to
E converge to
and
We denote by
Representing the difference
in the form of an interpolation polynomial with nodes at
, from here we find
We have
Let
denote the integer part
, and
be the function inverse to
. Assuming
we obtain
that is, the theorem is proven. □
Theorem 6.2. Whatever the functions and , as , there exists a set and so thathowever,where is the modulus of continuity of on the portion contained in the segment (, if ). Proof. We denote by
a positive function satisfying the inequalities
Let
,
and
,
. Suppose that the points
and the numbers
are constructed for
,
.
Let us define and , .
By
we denote a polynomial of degree
taking in
the values
, and by
we denote the modulus of continuity of
. We also assume that for
We determine the number
from the conditions
As
,
, we take a set of points satisfying the condition
It follows that if (
6.1) was fulfilled for any
, then it will be fulfilled for
. But the numbers
are defined and for
(
6.1) holds, so
are defined and (
6.1) holds. We determine the numbers from the relations
Since
and
are defined, then for every
,
, and from (
6.3) it follows that
constitutes a monotonically increasing sequence
Let us denote
is a segment with ends at
a and
b .
obviously represents a perfect set, which is the closure of the set .
At the point let us set the function equal to for , ; at the limit points of the set x we define as the supremum of the numbers for some .
From the monotonicity of it follows that is monotonically increasing and continuous on .
Let us prove that satisfies the required conditions. Let be an arbitrary segment located at and containing the points .
Let
i be sufficiently large and let
. Since
and
, then
Now let
be an arbitrary, sufficiently small number. Let us choose
i so that
Obviously, based on (
6.2) we have
i.e.,
Now, let
x be an arbitrary point of
,
be an arbitrary integer,
k be determined from the condition
. Due to the monotonicity of
we have
But since
Hence,
Let
be an arbitrary integer and
. We have
that is, the theorem is proven. □
The above two qualitative results determine the formulation of the problem of studying the best approximation on closed sets depending on the properties of these sets and on the behavior of the approximated functions on them.
Without dwelling here on all sorts of problems in this direction, let us consider one of them.
According to a well-known classical result, if
is a segment or closed domain and the function
can be approached with a progression rate on
, i.e.,
then
is analytic at every point of
.
Let us consider the question for which, more generally, set the previous result continues to be valid.
Theorem 6.3. Suppose the set is perfect, with the connected complement, and for any function continuous on , there exists a function harmonic on the complement of , taking at the point value . If for some function defined on then is analytic at every point of . Remark 6.1. Theorem 6.3 can also be formulated in local form: if a given point is regular (in the sense of the Dirichlet problem), then (6.4) implies the analyticity of at this point.
Remark 6.2. Thus, the possibility of extending the above-mentioned classical result to a more general type of set is influenced not by the measure (linear or flat), but by a more subtle characteristic of the set—its capacity.
Proof of Theorem 6.3. Let
denote a polynomial of degree
n whose maximum modulus on
is
M. Taking into account the fact that
is a subharmonic function, non-positive on
and behaving at infinity as
, we obtain
where
is the Green’s function of the complement to
, which vanishes, according to the regularity criterion for the Bouligan point, on the set
.
For any
we denote by
an open set containing
and such that
Thus,
Taking into account (6.4), we have
Choosing
so that
, we notice that the series
converges uniformly on
, i.e.,
is analytic on the set
. □
In the case where has only one limit point, the capacity of is zero; hence, the previous theorem says nothing.
Theorem 6.4. For any positive function satisfying the conditionthere exists a countable set E on , having only one limit point and such that if the inequalityholds for some function defined on E, it follows that takes its values from some entire function . Proof. Let us denote
,
,
We denote the set of points
and
by
. Let us show that the set
is what we are looking for.
Indeed, from (
6.5) it follows that
E has only one limit point
. In addition, if by
we denote the polynomial of best approximation
of degree
n on
E, then, representing the difference
in the form of an interpolation polynomial with nodes at points
we find
But
The distance of any point on the segment
to
E does not exceed
; therefore,
Let z be any point in the plane, and let be the level line of the complement of passing through z .
It is easy to see that , where depends only on z.
It follows from this that
that is, the series
converges in the entire plane, and its sum is an entire function coinciding on
E with
, i.e., the theorem is proven. □