Open access peer-reviewed chapter - ONLINE FIRST

Effectiveness of Basic Sets of Goncarov and Related Polynomials

Submitted: April 28th 2021Reviewed: July 12th 2021Published: September 21st 2021

DOI: 10.5772/intechopen.99411

Abstract

The Chapter presents diverse but related results to the theory of the proper and generalized Goncarov polynomials. Couched in the language of basic sets theory, we present effectiveness properties of these polynomials. The results include those relating to simple sets of polynomials whose zeros lie in the closed unit disk U=z:z≤1.. They settle the conjecture of Nassif on the exact value of the Whittaker constant. Results on the proper and generalized Goncarov polynomials which employ the q-analogue of the binomial coefficients and the generalized Goncarov polynomials belonging to the Dq- derivative operator are also given. Effectiveness results of the generalizations of these sets depend on whether q<1 or q>1. The application of these and related sets to the search for the exact value of the Whittaker constant is mentioned.

Keywords

• Basic sets
• Simple sets
• Effectiveness
• Whittaker constant
• Goncarov polynomials
• Dq operator

1. Introduction

The Chapter is on the effectiveness properties of the Goncarov and related polynomials of a single complex variable. It is essentially a compendium of certain results which seem diverse but related to the theory of the proper and generalized Goncarov polynomials.

Our first set of results deals with simple sets of polynomial [1], whose zeros lie in the closed unit disk U. It is a complement of a theorem of Nassif [1] which resolved his conjecture on the value of the Whittaker constant [2]. We provide also the relation between this problem and the theory of the proper Goncarov polynomials.

Next are results on a generalization of the problem where the polynomials are of the form

p0z=1;pnz=k=0nknannkzk;n1,E1

and the points an0are given complex numbers with knthe q-analogue of the binomial coefficient kn. From the results reported, it is shown that the location of the points ak0that leads to favorable effectiveness results depends on whether q<1orq>1. The relation of this problem to the generalized Goncarov polynomials belonging to the Dq-derivative operator is also recorded.

It is shown that applying the results of Buckholtz and Frank [3] on the generalized Goncarov polynomials Qnzz0z1zn1belonging to the Dq-derivative operator whenq>1, leads to the result that, when the points zk0lie in the unit disk U, the resulting polynomials fail to be effective.

Consequently, we provide some results on the polynomials Qnzz0z1zn1when

zkqk;k0,E2

with the obtained results justifying the restriction (2) on the points zk0.

Finally, we provide other relevant and related results on the properties of the generalized Goncarov polynomials Qnzz0z1zn1belonging to the Dq-derivative operator. For a comprehensive and easy reading, background results are provided in the Preliminaries of sections 2.1–2.5.

2. Preliminaries

We record here some background information for easy reading of the contents of the presentation.

2.1 Basic sets and effectiveness

A sequence pnzof polynomials is said to be basic if any polynomial and, in particular, the polynomials 1,z,z2,,zn,, can be represented uniquely by a finite linear combination of the form.

zn=k=0πn,kpkz;n0.E3

The polynomials pnzare linearly independent.

In the representation (3), let fz=n=0anznbe an analytic function about the origin. Substituting (3) into fz, we have

fz=n=0anzn=n=0ank=0πn,kpkz.

Formally rearranging the terms, we obtain the series

k=0pkzn=0anπn,k.

We write

kf=n=0anπn,k;k0.

Hence, we obtain the series

k=0kfpkz,

which is called the basic series associated with the function fzand the correspondence is written as

fzk=0kfpkz.E4

The coefficients Πkfis the basic coefficients of fzrelative to the basic set pkzand is a linear functional in the space of functionsfz.

If pnzis of degree nthen the set is called a simple set and is necessarily a basic set.

The basic series (4) is said to represent fzin a disk zrwhere fzanalytic, if the series is converges uniformly to fzin zror that the basic set pnzrepresents fzinzr.

When the basic set pnzrepresents in zrevery function analytic inzR,Rr, then the basic set is said to be effective in zrfor the class H¯Rof functions analytic in zR.

When R=r, the basic set represents, in zr, every function which is analytic there and we say that the basic set is effective in zr.

To obtain conditions for effectiveness, we form the Cannon sum

wnr=k=0πn,kMkr,E5

where

Mkr=maxz=rpkz.E6

From (3), we have that wnrrn,

so that, if we write

λr=limnsupwnr1n,E7
λrrn.E8

The function λris called the Cannon function of the set pnzin zr.

Theorems about the effectiveness of basic sets are due to Cannon and Whittaker (cf. [2, 4, 5]).

A necessary and sufficient condition for a Cannon set pnzto be effective, in zr, is

λr=r.E9

2.2 Mode of increase of basic sets

The mode of increase of a basic set pnzis determined by the order and type of the set. If pnzis a Cannon set, its order is defined, Whittaker [2], by

w=limrlimsupnlogwnrnlogn.E10

where wnris given by (5). The type γis defined, when 0<w<, by

γ=limrewlimnsupwnr1nnw1w.E11

The order and type of a set define the class of entire functions represented by the set.

Theorem 2.2.1 (Cannon [6]).

The necessary and sufficient conditions for the Cannon set of polynomials to be effective for all entire functions of increase less than order ptype qis

limnsupepqn1pwnr1n1forallr>0.E12

2.3 Zeros of simple sets of polynomials

The relation between the order of magnitude of the zeros of polynomials belonging to simple sets and the mode of increase of the sets has led to many convergence results, just as that between the order of magnitude of the zeros and the growth of the coefficients has. In the case of the zeros and mode of increase, the approach to achieve effectiveness is to determine the location of the zeros while that between the zeros and the coefficients is to determine appropriate bounds (cf. Boas [7], Nassif [8], Eweida [9]).

2.4 Properties of the Goncarov polynomials

We record in what follows certain properties of the proper and generalized Goncarov polynomials together with the definitions of the q-analogues and the Dq-derivative operator.

The proper Goncarov polynomials Gnzz0.zn1associated with the sequence zn0of points in the plane are defined through the relations, Buckholtz ([10], p. 194),

G0z=1,
znn!=k=0nznknk!Gkzz0zk1;n1.E13

These polynomials generate any function fzanalytic at the origin through the Goncarov series

fzk=0fkzkGkzz0zk1,E14

which represents fzin a disk zr, if it uniformly converges to fzin zr.

In this case, if fkzk=0,k0, the Goncarov series (14) vanishes and f0.

A consideration of gz=sinπ41z, for which gn1n=0andn=0gn1nGnz11..=0cf. Nassif [8], shows that the Goncarov series does not always represent the associated function and hence certain restrictions have to be imposed on the points zk0and on the growth of the function fz.

Concerning the case where the points zk0lie in the unit disk U, the Whittaker constant W (cf. Whittaker, Buckholtz, [2, 10]), is defined as the supremum of the number c with the following property:

If fzis an entire function of exponential type less than c and if each of f,f',f",..has a zero in U then fz0.

Buckholtz [10] obtained an exact determination of the constant W. In fact, if we write

Hn=maxGk0z0zn1,E15

where the maximum is taken over all sequences zk0n1whose terms lie in U, Buckholtz ([10], Lemma 3) proved that limnHn1nexists and is equal to sup1n<Hn1n.

Moreover, if we put

limnHn1n=H=sup1n<Hn1n,E16

Buckholtz ([10], formula 2) further showed that

W=1H.E17

Employing an equivalent definition of the polynomials Gnzz0zn1as originally given by Goncarov [11] in the form

Gnzz0zn1=z0zds1z1s1ds2,,zn1sn1dsn;n1,E18

and differentiating with respect to z, we can obtain

Gnkzz0zn1=Gnkzzkzn1;1kn1.E19

Writing

Gn0z0zn1=Fnz0zn1n1,E20

then (18) yields, among other results,

Gnzz0zn1=Fnzz0zn1Fnzz1zn1;n1,E21

and

Fn0z1zn1=0;n1.E22

Applying (21) and (22) to (19) we obtain

Fkkz0z1zn1=Fnkz0zn1E23

for 1kn1, where the differentiation is with respect to the first argument.

Expanding Fnz0zn1in powers of z0, in the form

Fnz0zn1=k=0nz0kk!Fnk0z1zn1,

we arrive through (22) and (23) to the formulae of Levinson [12],

Fnz0zn1=k=1nz0kk!Fnkzkzn1.E24

Also, differentiating (18) with respect to zk, we obtain with Macintyre ([13], p. 243),

zkGnkzz0zn1=Gkzz0zk1Gnk1zkzk1zn1E25

for 0kn1.

2.5 The q-analogues and Dqderivatives

Let qbe a positive number different from 1. The q–analogue of the positive integer nis given by

n=qn1q1.E26

Also, the q-analogue of n!is

n!=nn121;n1;0!=1,E27

and the q-analogue of the binomial coefficient knis

kn=n!k!nk!;0kn.E28

Moreover, the Dq– derivative operator, corresponding to the number q is defined as follows: Iffzis any function of z, then

Dqfz=fqzfzzq1,E29

so that when fz=zn, then according to (26), we have Dqzn=nzn1and if fz=n=0anzn1is any function analytic at the origin then

Dqfz=n=1nanzn1.E30

In [3] we have a generalization of the Goncarov polynomials as in (13) belonging to the operator D such that for fz=n=0anzn,

Dfz=n=1dnanzn1E31

associated with the sequence zk0, where en=d1d2dn1,e0=1and dn1is a non-decreasing sequence of numbers to obtain

p0z=1,enzn=k=0nenkzknkPkzz0.zk1;n1.E32

When dn=n, the relations (32) reduce to (6), hence the polynomials pnzreduce to the proper Goncarov polynomials Gnzz0zn1. Comparing (30) and (32), Nassif [14] investigated the class of generalized Goncarov polynomialsQnzz0zn1belonging to the Dq- derivative operator when dn=nand en=1n!given by,

Q0z=1znn!=k=0nzknknk!Qkzz0.zk1;n1,E33

and the Goncarov series associated with the function fz=n=0anznis

fzk=0DqkfkzkQkzz0zk1.E34

Writing

Rnz0zn1=Qn0z0zn1E35

so that

Rn0z1zn1=0,n1E36

then we have from, (32) that

Rnz0zn1=k=0n1znknk!Rkz0.zk1.E37

Also, Nassif ([14], Lemma 4.1), proved that

Qnzz0zn1=Rnz0zn1Rnzz1zn1.E38

We can verify, with Buckholtz ([10], Lemma 1), from the formulae (33), the following:

Qnλzλz0λzn1=λnQnzz0zn1;n1.E39
Qnz0z0zn1=0;n1.E40
DqQnzz0zn1=Qn1zz1zn1;n1.E41

And hence, by repeated application of Dq, we obtain

DqkQnzz0zn1=Qnkzzkzn1;1kn1.E42

Expressing Qnzz0zn1as a polynomial of degree nin z, then we have from (27), (29) and (42), that

Qnzz0zn1=k=0nzkn!Rnkzk.zn1.E43

The identities (39) and (43) have been obtained, in their general form, in ([3]; formulae (2.5), (2.9)). Also, a combination of (38) and (42) yields

DqkRn0z1zn1=Rnkzk.zn1,E44

for 1kn1, where the differentiation is with respect to the first argument. Expanding Rnz0z1zn1in powers of z0, then (36) and (44) imply that

Rnz0z1zn1=k=1nz0kk!Rnkzk.zn1.E45

Finally, if we put

hn=maxRnz0z1zn1,E46

where the maximum is taken over all sequences zk0n1and the terms lie in the unit disk U, then Buckholtz and Frank ([3], Corollary 5.2), proved that

limnhn1n=h=sup1n<hn1n.E47

Also, in view of the formulae (33), we can verify that, when q<1,

hh212=1+1212>3212>1.E48

3. Results on the zeros of simple sets

3.1 Zeros of simple sets of polynomials and the conjecture of Nassif on the Whittaker constant are discussed here

The following result is known for simple sets of polynomials whose zeros all lie in the unit disk.

Theorem A.([1], Theorem 1).

When the zeros of polynomials belonging to a simple set all lying within or on the unit circle the set will be of increase not exceeding order 1 type 1.378.

Using known contributions in the theory of Goncarov polynomials, we show that the alternative form of the above theorem is as follows:

Theorem 3.1.1 ([Nassif and Adepoju [15], Theorem B)

When the zeros of the polynomials belonging to a simple set all lying in the unit disk, the set will be of increase not exceeding order 1 type 1W, where Wis the Whittaker constant. It is shown also that the result in this theorem is bes t possible.

Indeed, applying the result of Buckholtz ([10], formula 2), the following theorem which resolved the conjecture of Nassif ([8], p.138), is established.

Theorem 3.1.2 ([15], Theorem B)

Given a positive number ε, a simple set pnzof polynomials, whose zeros all lie in Ucan be constructed such that the increase of the set is not less than order 1 type H–ε.

For completeness, we give the proof of Theorem 3.1.1 as a revised version of Theorem A.

Proof of Theorem 3.1.1 (Proof of alternative form of Theorem A)

Let bn1be a sequence of points lying in the unit disk and consider the set qnzof polynomials given by

q0z=1;qnz=z+bn1n;n1.E49

zn=k=0nw˜n,kqnkz.E50

Then multiplying the matrix of coefficients nkbnnkwith its inverse w˜n,k, we obtain

w˜n,0=k=1nnkbnkw˜nk,0;n1.

Write.

un=w˜n,0n!;n0,E51

then the above relation will give

un=k=1nbnkk!unk;n1.

And to show the dependence of unon the points bn, this relation can be rewritten as

unb1b2bn=k=1nbnkk!unkb1b2bnk.

Comparing this relation with the identify

Fnz0z1zn1=k=1nz0kk!Fnzkzn1,

of Levinson [12], we infer that

unb1b2bn=Fnbnbn1b1.E52

Differentiating (50)ktimes, k=1,2,,n1, we obtain that

w˜n,k=nkw˜nk,0bk+1bk+2bn.E53

Hence, a combination of (15), (16), (20), (51)-(53) leads to the inequality.

w˜n,kn!k!Hnk;0kn.E54

Observing that Mqkr1+rkfor any value of r0, then the Cannon sum of the set qnzfor z=rwill, in view of (54), be

wnr=k=0nw˜n,kΜqkrn!Ηnexp1+rΗ.

It follows from (17) that the set qnzis of increase not exceeding order 1 type 1W. The proof is now completed by applying the results of Walsh and Lucas, cf. Marden ([16], pp. 15,46), with (54) and following exactly the same lines of argument as in ([1], pp.109–110), to arrive at the inequality.

πn,kn!k!Hnk.E55

Since pnz1+rnin zr, it follows that the set pnzis of increase not exceeding order 1 type H = 1W.

This completes the proof of the theorem.

3.2 Background and the proof of the conjecture

Before the proof of Theorem 3.2.1, we note that we can take, ε<H1.(In fact, according to Macintyre ([13]; p. 241), we have H > 10.7378). Hence it follows from (16) that corresponding to ε, there exists an integer msuch that

m>logΗ/log1+2ΗE56

such that

Hm1m>H2.E57

Moreover, from (20), the definition (15) ensures the existence of the points ak1mlying in z1such that

Hm=Fmamam1a1.E58

Having fixed the integer mand the sequence ak1m, the following Lemma is to be first established.

Lemma 3.2.1 ([15], Lemma 3.2).

For any integer j1, write

fjz1ƶ2ƶj=Fj+1m+jamaiƶjamaiƶj1amaiƶ1ama1E59

Then, the complex numbers ξk1can be chosen so that

ξk=1;k1,E60

and

fjξ1ξ2ξj=Hmj+1;j1.E61

Proof.

The proof is by induction.

When j=1, we have from (59) that

f1z1=F2m+1ama1z1ama1.

Then the value ξ1will be chosen so that

ξ1=1;f1ξ1=supz11f1z1.E62

Applying the identify (25) of Macintyre to

F2m+1ama1z1ama1,we obtain

ddz1f1z1=Fmama1Gmz1ama1,

so that (20) and (58) imply that

f1''0=Hm2,

where the prime denotes differentiation with respect to z1.

Hence, in view of (62), Cauchy’s inequality yields

f1ξ1Hm2,

and the inequality (61) is satisfied for j=1. Suppose then that, for some value j=k,the complex numbers ξ1,ξ2,,ξkhave been chosen satisfying (60) and (61).

The numbers ξk+1will be fixed so that

ξk+1=1Fk+1ξ1ξ2ξk+1=supzk+11Fk+1ξ1ξ2ξkzk+1.E63

Proceeding in a similar manner as for the Case j=1and applying the identity (25) of Macintyre with (58), (59) and (61),we can obtain the inequality.

fk+11ξ1ξ2ξk0Hmk+2,E64

where the prime denotes differentiation with respect to zk+1..

Applying Cauchy’s inequality to the polynomial Fk+1ξ1ξ2ξkzk+1,we can deduce, using (63) and (64), that

Fk+1ξ1ξ2ξkzk+1Hmk+2.

Hence, by induction, the inequality (61) of the Lemma is established.

We now prove theorem 3.1.2.

The required simple set pnzof polynomials is constructed as follows:

P0z=1,pjm+1z=z+ξjjm+1;j1,pjm+1+iz=z+ajjm+1+i;1im;j0,E65

where the points ak1mare chosen to satisfy (63) and the numbers ξk1are fixed as in the Lemma.

It follows that the zeros of the polynomials pnzall lie in the unit disk U.

Also, if znadmits the unique linear representation.

zn=k=0nπn,kpkz,E66

and if we write

un=πn,0n!;n0,E67

then from the relation (52), we deduce from (59) and (65), that

uj+1m+j=fjξ1ξ2ξj;j1.E68

Now, in view of (66), the Cannon sum of the set pnzfor z=r, is wnr>πn,0.

Hence, combining (57), (61), (67) and (68) yields

wj+1m+jrj+1m+j!H2j+1m;j1.

vIt follows from this inequality and Theorem 3.1 that the order of the set Pnzis exactly 1 and since H2>1, the type of the set will be

γH2mm+1H2m=1m.E69

In view of the inequality (56), we deduce from (69) that

γ>H

and Theorem 3.1.2 is established.

This settles the conjecture.

4. Generalization

4.1 As a generalization of the above problem, we consider the simple set pnzngiven by

p0z=1;pnz=pnza=k=0nnkannkzk;n1,E70

where nkis the q-analogue of the binomial coefficient nkand ak1is a sequence of given complex numbers. The set pnzis in fact, the q-analogue of the set qnzin (49). This study is motivated by the fact that this set is related to the generalized Goncarov polynomials belonging to the Dq-derivative operator. Our results show that effectiveness properties of the set.

pnzdepend on whether q<1or q>1.

We establish the following:

Theorem 4.1.1 ([17], Theorem 1.1)

When the points ak1all lie in the unit disk U, the corresponding set pnzfor q<1,will be effective in zrfor rh1q, where his as in (47).

Theorem 4.1.2. ([17], Theorem 3.1)

Given >0, the points ak1lying in z1can the chosen so that the correspondence set pnzof (70) with q<1will not be effective in z<rfor r<h1q..

Theorem 4.1.3 ([17], Theorem 1.2)

When q>1and

akqk;q1E71

the corresponding set pnzof (70) will be effective in zrfor r>q1, where 1γis the least root of the equation.

n=0qn2xn=2.E72

Theorem 4.1.2 shows that the result in Theorem 4.1.1 is best possible. Also, the restriction (71) on the sequence ak1when q>1, is shown to be justified in the sense that if the restriction is not satisfied, the corresponding set pnzmay be of infinite order and not effective.

Proof.

Proof of Theorem 4.1.1 is similar to the first part of Theorem 3.1.1.

zn=k=0nπn,ka1a2anpkz,E73

then multiplying the matrix of coefficients nkannkof the set pnzwith the inverse matrix πn,kwe obtain

k=0nnkannkπk,0a1a2ak=0;n1.

Putting

v0=1,vk=vka1a2..ak=1k!πk,oa1ank,E74

the above relation yields

vna1..an=k=1nakk!vnka1ankE75

Comparing the formulae (45) and (75) we infer that

vka1..ak=Rkak..a1.E76

Moreover, operating Dqon the polynomials pnz, we can deduce, from (28) and (29), that

Dpkzakq=Kpk1zak;k1.E77

Hence, when the operator Dqacts on the representation (73), then (77) leads to the equality

πn,ka1.an=nkπn1,k1a1an,

which, on reduction, yields

πn,ka1.an=nkπnk,0aK+1an;0kn.E78

Applying (74), (76) and (78), we obtain

πn,ka1.an=n!k!RnkanaK+1;0kn.E79

Identify (79) is the bridge relation between the set pnzand the Goncarov polynomials mentioned earlier.

Suppose q<1and assume that

rh1q.E80

Since h>1as in (47), and restricting the points ak1to lie in the unit disk Uas in the theorem, it follows from (28) and (80) that

Μpkrk+1rk;k0.E81

The Cannon sum of the set pnzfor z=r,is evaluated from (46), (47), (79), (80) and (81) to obtain

wnr=k=0nπn,kΜpkrn+12rn,E82

from which it follows that the set pnzis effective in zrfor rh1qand the theorem is established.

5. Proof

5.1 Proof of Theorem 4.1.2

We argue as in the Proof of Theorem 3.1.2. We first obtain an identity similar to (25) of Macintyre using the following Lemma:

Lemma 5.1.1.

For n1and k0, the following identity holds.

Dq,zkQk+nzz0zk+n1=Qkzzozk1Qn1zkzk+1zk+n1,E83

where Dq,zkdenote the Dq-derivative with respect to zk.

Proof of Lemma

The proof is by induction.

For n=1,k0, we have from the construction formulae (33),

QK+1zz0zk=zk+1k+1!j=0k1zjk+1jk+1j!Qjzz0zj1zkQkzz0zj1.

Hence, operating Dq,zkon this equality, we have that

Dq,zkQk+1zz0zk1=Qkzz0zk1,

so that the identity (83) is satisfied for n=1,k0. Suppose that (83) is satisfied forn=1,2,,m;k0. The formulae (33) can be written for k+m+1in the form,

Qk+m+1zz0zk+m=zk+m+1k+m+1!j=0k1zjk+m+1jk+m+1j!Qjzz0,,zj1zm+1km+1!Qkzz0zk1j=1mzk+jm+1jm+1j!Qk+jzz0zk+j1.

Hence, the derivative Dq,zkoperating on this equation gives, in view of (83),

Dq,zkQk+m+1zz0zk+m=zmm!Qkzz0zk1+j=1mzk+jm+1jm+1j!QKzz0zK1×Qj1zkzk+1zK+j1.

Or equivalently,

Dq,zkQk+m+1zz0zk+m=Qkzz0zk1×zkmm!j=0m1zk+j+1m+jmj!Qjzkzk+1zK+j.

Hence, formulae (33) imply that

Dq,zkQk+m+1zz0zk+m=Qkzz0zk1Qmzkzk+1zK+m,

and the relation (83) is also valid for n=m+1

The Lemma is thus proved by induction. Now, following similar lines paralleling those of the proof of Theorem 3.1.2, we need to establish a Lemma similar to that used for Theorem 3.1.2.

Indeed, observing that h>1as in (39), the >0of Theorem 4.1.2 can always be picked less than h1. Also, from (39) it follows that, corresponding to the number ∈, there exists an integer mfor which

m>logh/log1+2h,E84

such that

hm1m>h2.E85

Also, from the definition (46) of hm, the points αi1mlying in Ucan be chosen so that

hm=Rmαm.α1.E86

With this choice of the integer mand the points αi1m,the Lemma to be established is the following:

Lemma 5.1.2.

With the notation

ujz1z2,,zj=Rj+1m+jαmα1zjαmα1zj1αmα1αmα1,E87

we can choose a sequence ξj1mof points on z=1such that

ujξ1ξ2ξjmj+1;j1.E88

Proof.

We first observe, from a repeated application of (30), that an analytic function fzregular at the origin, can be expanded in a certain disk z1in a series of the form

fz=n=0znn!Dqnf0.

Hence, by Cauchy’s inequality, we have

ΜfrrDqf0.E89

Applying the usual induction process, we obtain, from (87) for the case j=1, that

u1z1=R2m+1αmα1ziαmα1

Hence the identity (83) yields

Dqu1z1=Dq,z1Q2m+10αmα1ziαmα1=Rmαmα1Qmziαmα1.

Therefore, we obtain

Dqu10=Rm2αmα1,E90

where the Dqis operating with respect to z1.

Pick the number ξ1, with ξ1=1, such that

u1ξ1=supu1z1:z1=1;

hence, a combination of (86), (89) and (90) yields

u1ξ1hm2,

and the inequality (88) is satisfied for j=1. The similarity with the proof of Lemma 3.2.1 shows that the proof of this Lemma can be completed in the same manner as that for ealier Lemma.

We can now prove Theorem 5.1.4.

We note that the points ak1lying in Uwhich define the required set pnzof polynomials (70), are chosen as follows:

ajm+1=ξjajm+1+i=αi;1im;j0,E91

where the points αi1mare fixed as in (86) and the sequence ξj0of points is determined as in Lemma 5.1.2; and the integer mis chosen as in (84) and (85).

If znadmits the representation (86), then applying (79), (87) and (91) we have that

πj+1m+j,o=j+1m+i!ujξ1ξ2ξj,j1,E92

so that, for the Cannon sum of the set pnzfor z=r,we obtain, from (85), (88) and (92),

wj+1m+jr>j+1m+j!h2j+1m;r>0..E93

Since q<1, we have that

limnn!1n=11q.E94

Hence, (93) and (94) yield, for the Cannon function,

λr=limsupnwnr1nlimsupjwj+1m+jr1j+1m+j11qh2mm+1;r>0.

Noting that h2>1, we conclude, from (84), as in the proof of Theorem (50), that

λrh1q;r>0,

and pnzwill not be effective in zrfor r<h1q. This completes the proof.

5.2 Proof of Theorem 4.1.3

Let pnzbe the basic set in (70) with q>1. We first justify the statement that if the restriction (71) is not satisfied the corresponding set pnzmay be of infinite order.

For this, we put

ak=tk;k1,E95

and let tbe such that

t=β,1q<β<qE96

We claim that, in this case, the corresponding set pnzwill be of infinite order and hence the effectiveness properties of the set will be violated.

Now, in the identity (37), we let

zk=ank=tnk;0kn1,

to obtain

k=0ntnkk!Rnktnkt=0;n>0.E97

Put

Rjtjt=t12jj1uj;j1,E98

so that (97) yields

k=0nt12kk+1k!unk=0;n>0.E99

Hence, if we put

uz=n=0unzn,E100

then (97) implies that

uz=1φz,E101

where

ϕzt=n=0t12nn1n!zn.

Since t=β<q, the function ϕztis entire of zero order and hence it will have zeros in the finite part of the plane.

Let

σ=infzφz=0<,E102

then from (100) and (101), we have limsupnun1n=1σ>0.

Thus, for the Cannon sum of the set pnz, we have, from (79), (96) and (98), that

wnr>πn,0=n!β12nn1un.E103

Since q>1and β>1qthen, in view of (102), we deduce from (103) that the set pnzis of infinite order; as claimed.

To prove Theorem 4.1.3 we first note, from (72), that if we put

c=q1,E104

then

c>1q1.E105

We then multiply the matrix nkannkwith the inverseπn,kto get

πn,k=j=kn1nkannjπj,k:n>k;πk,k=1.E106

Now, imposing the restriction (71) on the points ak1, we have from (105) and (106) that

πk+1,kc.

Thus, the inequality

πmkcmk;mk,E107

is true for m=k,k+1.

To prove (107), in general, we observe that, since q>1,

njqjnjqq1nj;1jn.E108

Assume that (107) is satisfied for m=k,k+1,,n1;then a combination of (71), (72), (104), (106), (107) and (108) leads to the inequality.

πn,kcnkj=1qcq1jqj2=cnk.

Hence, it follows by induction, that the inequality (107) is true for mk.Noting that

kj=qjkjkjq>1,

where kjis the q–analogue of kj, q1=1q<1, we then deduce from (70) and (71), that

Μpkrrkj=0kkjqj2rjrkj=0kkjq12jj1qrj;q>1.

Appealing to a result of Al-Salam ([18]; formula 2.5), we deduce that

Μpkrrkj=1k1+1qjr;k1,r>0.E109

The Cannon sum of the set pnzfor z=rcan be evaluated from (107) and (109) in the form

wnrj=1n1+1qjrk=0ncnkrk.E110

Hence, when rcwe should have

wnrn+1j=1n1+1qjrrn,

from which it follows that the set pnzis effective in zrand Theorem 4.1.3 is proved.

6. Other related results

6.1 The Goncarov polynomials belonging to the Dq–derivative operator have other properties of interest and worth recording. Hence, we present, in this section, more results regarding the Goncarov polynomials Qnzz0…zn−1as defined in (84) which belong to the derivative operator Dqand whose points zn0∞lie in the unit disk Ufor which q<1or q>1

When q<1, the result of Buckoltz and Frank ([3]; Theorem 1.2) applied to the derivative operator Dqleads, in the language of basic sets, to the following theorem:

Theorem 6.1.1 ([19], Theorem 1).

The set of Gancarov polynomials Qnzz0zn1belonging to the Dqoperator, with q<1and associated with the sequence of points zn0in U, is effective in zrfor rh1q.

Theorem 1.5 of Buckholtz and Frank [3] shows that the result of Theorem 6.1.1 above is best possible. They also showed that when q>1the Goncarov polynomials fail to be effective and also, that if zqn, no favorable effectiveness results will occur, thus justifying the restriction zqnon the points zn0.

We also state and prove the following theorem.

Theorem 6.1.2 ([19], Theorem 2).

Suppose that q>1and that the points zn0satisfy the restriction (111). Then the Goncarov set Qnzz0zn1belonging to the Dq–derivative operator, will be effective in zrfor rhqq1and this result is best possible.

To prove this theorem we put, as in the proof of Theorem (72),

q1=1q,E111

so that q1<1and we differentiate between the Goncarov polynomials belonging to the operations Dqand Dq1by adopting the notation.

Qnzz0zn1andpnzz0zn1,

for these respective polynomials. Thus, the constructive formulae (33) for these polynomials will be

Qnzz0zn1=znn!k=0n1zknknk!Qkzz0zk1,E112

and

Pnzz0zn1=znn!k=0n1zknknk!Pkzz0zk1,E113

where k!and k!are the respective qand q1analogues of the factorial k. With this notation, the following Lemma is to be proved.

Lemma 6.1.1.

The following identity is true for n1and q>1:

q12nn+1Qnqnzqnz0..qzn1=pnzz0.zn1.E114

Proof.

We finish note, from the definition of the analogue k!and k!, that

qn2n!=q12nn+1n!;n1,E115

and

qnk2+nkknk!=1nk!q12nn+112kk+1;0kn.E116

Hence, applying the relations (37) and (112) toQNqnzqnz0qzn, we get

Qnqzqnz0qzn1=qn2n!znk=0n1qnk2+nkknk!zkn1Qkqkzqkz0qzk1.

Hence, the relations (115) and (116) can be introduced to yield

q12nn+1Qnqnzqnz0qzn1=znn!k=0n1zknknk!q12kk+1Qkqkzqkz0qzk1.E117

Now, since

q1Q1qzqz0=zz0=p1zz0,

the identity (114) is satisfied for n=1.

Moreover, if (114) is valid for k=1,2,,n1, the relations (113) and (117) will give

q12nn+1Qnqnzqnz0qzn1=znn!k=0n1zknknk!Pkzz0zk1=Pnzz0zn1,

and hence the Lemma is established.

Proof of Theorem 6.1.2.

Write

zk=qkak;k0,E118

so that the restriction (111) implies that

ak1;k0E119

Therefore, a combination of (37), (114), (118) yields

Qnzz0zk1=q12kk+1Pkzz0zk1.E120

Also, by actual calculation we have that

n!nk!qknk12kk+1=n!nk!;0knE121

Inserting (118), (120) and (121) into (33), we obtain

zn=k=0nn!nk!zknkQkzz0zk1
=k=0nn!nk!aknkPkza0ak1,

in the sense that each term in the sum on the left hand side of this relation is equal to the corresponding term in the sum on the right hand side.

Hence, if

Μkr=supz=rQkz;0k1
mkr=supz=rPkza0ak1

and Ωnrand wnrare the respective Cannon sums of the sets Qnzz0zn1and Pnza0an1, it follows that

Ωnr=k=0nn!nk!zknkΜkrE122
=k=0nn!nk!aknkmkr=wnr.

Since the points ak0lie in U, from (119), then applying Theorem 6.1.1 we deduce from (122) that the set Qnzz0znwill be effective in zrfor rh1q=qhq1as to be proved.

To show that the result of the Theorem is best possible we appeal to Theorem 1.5 of Buckholtz and Frank [3] to deduce that the set Pnza0an1may not be effective in zrfor r<qhq1.

In view of the relation (122), we may conclude that the set Qnzz0znwill not be effective in zrfor r<qhq1and Theorem 6.1.2 is fully established.

6.2 The case of Goncarov polynomials with Zk=atk,k≥0

Nassif [14] studied the convergence properties of the class of Goncarov polynomials Qnzz0zn1generated through the qth derivative described in (33) where now, zk=atk,k0and aand tare any complex numbers. By considering possible variations of t and q, it was shown that except for the cases t1,q<1and t>1q;q>1, all other cases lead to the effectiveness of the set Qnzaatatn1in finite circles ([14]; Theorems 1.1, 1.2, 1.3, 3.2, 3.3).

6.3 Quasipower basis (QP-basis)

Kazmin [20] announced results on some systems of polynomials that form a quasipower basis, (QP-basis), in specified spaces. These include the systems of Goncarov polynomials and of polynomials of the form:

z+αnn,n=0,i,2;αn11.E123

For full details of QP-basis and some of the results announced, cf. ([20]; Corollaries 3, 4).

Of interest is his results that the system in (123), for arbitrary sequence a0of complex numbers with an1, forms a QP- basis in the space 1σ, for 0<σ<Wand in the space 1σ, for 0<σW,where W = 0.7377 is the Whittaker constant. This value of W = 0.7377 is attributed to Varga [21]. He also added that Corollaries 3 and 4 contain known results in [5, 9, 15, 22, 23].

7. Conclusions

The chapter presents a compendium of diverse but related results on the convergence properties of the Goncarov and Related polynomials of a single complex variable. Most of the results of the author (or joint), have appeared in print but are here presented in considerable details in the proofs and in their development, for easy reading and assimilation. The results of other authors are summarized with related and relevant ones mentioned to complement the thesis of the chapter. Some recent works related to the Goncarov and related polynomials, cf. [24, 25, 26, 27, 28, 29], which provides further applications are included in the references.

The comprehensiveness of the presentation is for the needs of those who may be interested in the subject of the Goncarov polynomials in general and also in their application to the problem of the determination of the exact value of the Whittaker constant, a problem that is still topical and challenging.

Acknowledgments

I acknowledge the mentorship of Professor M. Nassif, (1916-1986), who taught me all I know about Basic Sets. I thank Dr. A. A. Mogbademu and his team for typesetting the manuscript at short notice and also the Reviewer for helpful comments which greatly improved the presentation.

No conflict of interest

The author declares no conflict of interest.

chapter PDF

More

© 2021 The Author(s). Licensee IntechOpen. This chapter is distributed under the terms of the Creative Commons Attribution 3.0 License, which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited.

How to cite and reference

Cite this chapter Copy to clipboard

Jerome A. Adepoju (September 21st 2021). Effectiveness of Basic Sets of Goncarov and Related Polynomials [Online First], IntechOpen, DOI: 10.5772/intechopen.99411. Available from: