1. Introduction
In the present paper, we intend to study the following problem: let
v be a regular form (linear functional), and
R and
D nonzero polynomials. Find all regular forms
u satisfying
(1.1)
This problem has been studied in some particular cases. In fact the product of a linear form by a polynomial (
R(
x) = 1) is studied in [
1–
3] and the inverse problem (
D(
x) =
λ,
λ ∈
ℂ − {0}) is considered in [
4–
7]. More generally, when
R and
D have nontrivial common factor the authors of [
8] found necessary and sufficient conditions for
u to be a regular form. The case where
R =
D is treated in [
4,
9–
11]. The aim of this contribution is to analyze the case in which
R(
x) =
x2 and
D(
x) =
λx,
λ ∈
ℂ − {0}. We remark that
R and
D have a common factor and
R ≠
D (see also [
7]). In fact, the inverse problem is studied in [
12]. On the other hand, this situation generalize the case treated in [
13] (see (
2.9)). In Section
1, we will give the regularity conditions and the coefficients of the second-order recurrence relation satisfied by the monic orthogonal polynomial sequence (MOPS) with respect to
u. We will study the case where
v is a symmetric form; thus regularity conditions become simpler. The particular case when
v is a symmetric positive definite form is analyzed. The second section is devoted to the case where
v is semi-classical form. We will prove that
u is also semi-classical and some results concerning the class of
u are given. In the last section, some examples will be treated. The regular forms
u found in theses examples are semi-classical of class
s ∈ {1,2, 3} [
14]. The integral representations of these regular forms and the coefficients of the second-order recurrence satisfied by the MOPS with respect to
u are given.
2. The Problem x2u = λxv
Let
𝒫 be the vector space of polynomials with coefficients in
𝒞 and
𝒫′ its algebraic dual. We denote by 〈
u,
f〉 the action of
u ∈
𝒫′ on
f ∈
𝒫. In particular, we designate by (
u)
n∶ = 〈
u,
xn〉,
n ≥ 0, the moments of
u. For any form
u, any polynomial
g, any
c ∈
ℂ,
a ∈
ℂ − {0}, let
u′,
hau,
gu, and (
x −
c)
−1u be the forms defined by duality:
(2.1)
where (
θcp)(
x) = (
p(
x) −
p(
c))/(
x −
c); (
hap)(
x) =
p(
ax).
We define a left multiplication of a form
u by a polynomial
p as
(2.2)
Let us recall that a form
u is called regular if there exists a monic polynomial sequence {
Pn}
n≥0, deg
Pn =
n, such that
(2.3)
We have the following result.
Lemma 2.1 (see [15].)Let u ∈ 𝒫′, f ∈ 𝒫, and c ∈ 𝒞. The following formulas hold:
(2.4)
(2.5)
(2.6)
where 〈
δc,
p〉 =
p(
c),
p ∈
𝒫.
We consider the following problem: given a regular form
v, find all regular forms
u satisfying
(2.7)
with constraints (
u)
0 = 1, (
v)
0 = 1. From (
2.6) we can deduce that
(2.8)
(2.9)
Then the form
u depends on two arbitrary parameters (
u)
1 and
λ.
We notice that when (u) 1 = λ, we encounter the problem studied in [13] again.
We suppose that the form
v has the following integral representation:
(2.10)
where
V is a locally integrable function with rapid decay, continuous at the origin; then the form
u is represented by
(2.11)
where [
16,
17]
(2.12)
Let {
Sn}
n≥0 denote the sequence of monic orthogonal polynomials with respect to
v; we have
(2.13)
with
(2.14)
When
u is regular, let {
Zn}
n≥0 be the corresponding MOPS:
(2.15)
From (
2.7), we know that the existence of the sequence {
Zn}
n≥0 is among all the strictly quasi-orthogonal sequences of order two with respect to
λxv =
w (
w is not necessarily a regular form) [
15,
18–
20]. That is,
(2.16)
with
an ≠ 0,
n ≥ 0.
From (
2.16), we have
(2.17)
(2.18)
Lemma 2.2. Let {Zn} n≥0 be a sequence of polynomials satisfying (2.16) where an, bn, and cn are complex numbers such that an ≠ 0 for all n ≥ 0. The sequence {Zn} n≥0 is orthogonal with respect to u if and only if
(2.19)
Proof. The conditions (2.19) are necessary from the definition of the orthogonality of {Zn} n≥0 with respect to u.
For k ≥ 2, we have (by (2.7))
(2.20)
and from (
2.16), we get
(2.21)
Taking into account the orthogonality of {
Sn}
n≥0, we obtain
(2.22)
By (
2.19), it follows that
(2.23)
Consequently, the previous relations and (
2.22) prove that {
Zn}
n≥0 is orthogonal with respect to
u, which proves the Lemma.
Remark 2.3. When u is regular, from Theorem 5.1 in [21], there exist complex numbers rn+2 ≠ 0, tn+2 and vn+2 ≠ 0 such that
(2.24)
From (
2.16), (
2.24), and (
2.15) we obtain the following relations:
(2.25)
Taking into account (
2.16), (
2.18) and (
2.19), we get
(2.26)
with the initial conditions:
(2.27)
If we denote
(2.28)
from the Cramer rule we have
(2.29)
(2.30)
(2.31)
Lemma 2.4. The following formulas hold:
(2.32)
(2.33)
(2.34)
(2.35)
where
,
n ≥ 0, and
.
Proof. Equations (2.32) and (2.33) are deduced, respectively, from (2.9) and (2.8).
We have
(2.36)
Using (
2.4), we get
(2.37)
From (
2.9), we obtain
(2.38)
According to (
2.5) and (
2.37), we can deduce (
2.34).
We have
(2.39)
Then (by (
2.39))
(2.40)
It follows that
(2.41)
hence (
2.35).
Proposition 2.5. One has
(2.42)
where
(2.43)
with
(2.44)
Proof. Using (2.13), we, respectively, obtain
(2.45)
Taking into account previous relations, we obtain for (
2.28) the following:
(2.46)
that is,
(2.47)
Let
n ≥ 0; based on the relations (
2.32)–(
2.34), it follows that
(2.48)
From (
2.48) and (
2.47), we obtain the desired results.
Proposition 2.6. The form u is regular if and only if Δn ≠ 0, n ≥ 0. Then, the coefficients of the three-term recurrence relation (2.15) are given by
(2.49)
(2.50)
(2.51)
(2.52)
Proof
Necessity From (2.27) and Lemma 2.4, we get
(2.53)
and again with (
2.27) and (
2.42), we can deduce that
(2.54)
Moreover, {
Zn}
n≥0 is orthogonal with respect to
u, therefore it is strictly quasiorthogonal of order two with respect to
xv, and then it satisfies (
2.16) with
an ≠ 0,
n ≥ 0. This implies Δ
n ≠ 0,
n ≥ 0. Otherwise, if there exists an
n0 ≥ 1 such that
, from (
2.29), Δ
0 = 0, which is a contradiction.
Sufficiency Let
(2.55)
(2.56)
(2.57)
We get
(2.58)
We have 〈
u,
Z1〉 =
c1 + 〈
u,
θ0S2〉 = 0.
From (2.56) and (2.57) we get
(2.59)
On account of (
2.54), we can deduce that
S2(0) +
c1S1(0) +
b0 = 0.
Then we had just proved that the initial conditions (2.27) are satisfied.
Furthermore, the system (2.26) is a Cramer system whose solution is given by (2.29), (2.30), and (2.31); with all these numbers an, bn, and cn (n ≥ 0), define a sequence polynomials {Zn} n≥0 by (2.16). Then it follows from (2.26) and Lemma 2.2 that u is regular and {Zn} n≥0 is the corresponding MOPS.
Moreover, by (2.22) we get
(2.60)
Making
n = 0 in (
2.60), it follows that
(2.61)
Based on relations (
2.58), (
2.60), (
2.61), and (
2.29), we, respectively, obtain
(2.62)
We have proved (
2.49) and (
2.50).
When {Zn} n≥0 is orthogonal, we have
(2.63)
By (
2.16) and the orthogonality of {
Zn}
n≥0, we get
(2.64)
By virtue of (
2.13) and the regularity of
u we obtain
(2.65)
and consequently, we get the second result in (
2.51) from (
2.58), and (
2.64).
From (2.16), and the orthogonality of {Zn} n≥0, we have
(2.66)
Using (
2.13), (
2.16), and the the orthogonality of {
Sn}
n≥0, we have
(2.67)
Taking into account the previous relation, (
2.66) becomes
(2.68)
From (
2.60) and (
2.29), we have
(2.69)
Last equation and (
2.68) give (
2.52).
Moreover, if the form
u is regular, for (
2.29), (
2.30), and (
2.31), we get
(2.70)
(2.71)
(2.72)
where
(2.73)
In the sequel, we will assume that
v is a symmetric linear form.
We need the following lemmas.
Lemma 2.7. If {yn} n≥0 and {bn} n≥0 are sequences of complex numbers fulfilling
(2.74)
then
(2.75)
Lemma 2.8. When {Sn} n≥0 given by (2.13) is symmetric, one has
(2.76)
Proof. As v is symmetric, then ξn = 0, n ≥ 0, and therefore from (2.13) we have
(2.77)
Now, it is sufficient to use Lemma
2.7 in order to obtain the desired results.
Corollary 2.9. If v is a symmetric form, one has
(2.79)
where
(2.80)
Proof. Following Lemma 2.8, for (2.43) we have
(2.81)
As a consequence, relations (
2.81) and (
2.42) yield (
2.79).
Theorem 2.10. The form u is regular if and only if (ω − 1)Λn + 1 ≠ 0, n ≥ 0, where Λn is defined in (2.80).
In this case one has
(2.82)
(2.83)
(2.84)
(2.85)
where
,
n ≥ 0.
Proof. From Proposition 2.6 and Corollary 2.9, we can deduce that u is regular if and only if (ω − 1)Λn + 1 ≠ 0, n ≥ 0.
Moreover, from (2.70) we can deduce (2.82).
By (2.49), (2.51), (2.78), and (2.79), for (2.55), (2.56), and (2.57) we get
(2.86)
When
n ≥ 0 by Lemma
2.8, for (
2.73) we get
(2.87)
Taking into account (
2.79), (
2.80), and (
2.86)-(
2.87), relations (
2.70), (
2.71) and (
2.72) give (
2.82)–(
2.84).
As a result of relations (2.82)–(2.84) and Proposition 2.6 we get (2.85).
Corollary 2.11. (1) If v is a symmetric positive definite form, then the form u is regular when ω ∈ ℂ−] − ∞, 1[.
(2) When u is regular, it is positive definite form if and only if
(2.88)
Proof. (1) If v is positive definite, then σn+1 > 0, n ≥ 0, therefore Λn > 0, n ≥ 0 and so (ω − 1)Λn + 1 ≠ 0, n ≥ 0 under the hypothesis of the corollary.
(2) If u is regular, it is positive definite if and only if γn+1 > 0, n ≥ 0. By Theorem 2.10, we conclude the desired results.
3. Some Results on the Semiclassical Case
Let us recall that a form
v is called semiclassical when it is regular and its formal Stieltjes function
S(·;
v) satisfies [
15]
(3.1)
where
ϕ monic,
C, and
D are polynomials with
(3.2)
The class of the semi-classical form
v is
s = max (deg
ϕ − 2, deg
C − 1) if and only if the following condition is satisfied [
22]:
(3.3)
where
c ∈ {
x :
ϕ(
x) = 0}, that is,
ϕ,
C, and
D are coprime.
In the sequel, we will suppose that the form v is semi-classical of class s satisfying (3.1).
Proposition 3.1. When u is regular, it is also semi-classical and satisfies
(3.4)
where
(3.5)
Moreover, the class of
u depends on the zero
x = 0 of
ϕ.
Proof. We need the following formula:
(3.6)
From (
2.7), we have
S(
z;
x2u) =
λS(
z;
xv). Using (
3.6), we get
(3.7)
Differentiating the previous equation, we obtain
(3.8)
By (
3.1) we can deduce (
3.4) and (
3.5).
Since v is a semi-classical, S(z; v) satisfies (3.1) where ϕ, C and D are coprime.
Let c be a zero of different from 0, which implies that ϕ(c) = 0. We know that |C(c)|+|D(c)| ≠ 0.
If C(c) ≠ 0, then . if C(c) = 0, then . Hence .
Corollary 3.2. Introducing
(3.9)
- (1)
if ϑ1 ≠ 0, then ;
- (2)
if ϑ1 = 0 and ϑ2 ≠ 0, then ;
- (3)
if ϑ1 = ϑ2 = 0 and ϕ(0) ≠ 0 or ϑ3 ≠ 0, then .
Proof. (1) From (3.9) and (3.5), we obtain , . Therefore, it is not possible to simplify, which means that the class of u is s + 3.
(2) If ϑ1 = 0, then from (3.5) we have . Consequently, (3.4)–(3.6) is divisible by z. Thus, u fulfils (3.4) with
(3.10)
If
, it is not possible to simplify, which means that the class of
u is
s + 2.
(3) When ϑ1 = ϑ2 = 0, then it is possible to simplify (3.4)–(3.10) by z. Thus, u fulfils (3.4) with
(3.11)
Since we have
,
, then we can deduce that if
ϕ(0) ≠ 0 or
ϑ3 ≠ 0, it is not possible to simplify, which means that the class of
u is
s + 1.
4. Some Examples
In the sequel the examples treated generalize some of the cases studied in [13].
4.1. v the Generalized Hermite Form
Let us describe the case
v∶ =
ℋ(
τ), where
ℋ(
τ) is the generalized Hermite form. Here is [
1]
(4.1)
From (
4.1), we get
(4.2)
We want
,
n ≥ 0.
But from (
4.1) and (
4.2), we have
, with
(4.3)
fulfilling
(4.4)
and so
(4.5)
Then we get Table
1.
Δn |
,
|
. |
|
an |
. |
bn |
. |
cn |
,
|
. |
|
, |
γn+1 |
, |
|
. |
|
, |
βn |
, |
|
. |
Proposition 4.1. If v = ℋ(τ) is the generalized Hermite form, then the form u(τ, ω, λ) given by (2.9) has the following integral representation:
(4.6)
It is a quasi-antisymmetric ((
u(
τ,
ω,
λ))
2n+2 = 0,
n ≥ 0) and semi-classical form of class
s satisfying the following functional equation:
(4.7)
(4.8)
Proof. It is well known that the generalized Hermite form possesses the following integral representation [1]:
(4.9)
Following (
2.11), we obtain (
4.6). Also the form
u is quasi-antisymmetric because it satisfies
(4.10)
since
v is symmetric by hypothesis.
When τ = 0, v is classical and satisfies (3.4) with [22]
(4.11)
Then,
ϑ1 =
λ(
ω − 1),
ϑ2 = 0.
Now, it is sufficient to use Corollary 3.2 and Proposition 3.1 in order to obtain (4.7).
If τ ≠ 0, the form v is semi-classical of class one and satisfies (3.4) with [23]
(4.12)
Therefore
ϑ1 = 0,
ϑ2 =
λ(
ω − 1)(2
τ + 1),
ϑ3 = 2
τ.
By Proposition 3.1 and Corollary 3.2 we can deduce (4.8).
4.2. v the Corecursive of the Second Kind Chebychev Form
Let us describe the case
v∶ =
𝒥(−1/2,1/2); it is the corecursive of the second kind Chebychev functional. Here is [
1]
(4.13)
In this case we have the following result.
Lemma 4.2. For n ≥ 0, one has
(4.14)
Proof. The proof is analogous for the demonstration of Lemma 2.8.
Following Lemma
4.2, for (
2.44) we have
(4.15)
Therefore, we get for (
2.42)
(4.16)
Then we obtain
(4.17)
where
(4.18)
On account of Proposition
2.6, we can deduce that the form
u given by (
2.9) is regular if and only if
tn −
xi ≠ 0,
n ≥ 0, 1 ≤
i ≤ 4.
In the sequel, we suppose that the last condition is satisfied.
By virtue of (4.17) and Lemma 4.2, relations (2.49)–(2.52), and (2.55)–(2.57), (2.70)–(2.72) give Table 2.
an |
, n ≥ 0. |
bn |
, |
|
. |
|
cn |
, |
|
. |
|
|
, |
γn+1 |
, |
|
. |
|
|
,
|
|
|
βn |
, |
|
. |
Proposition 4.3. If v = 𝒥(−1/2,1/2) is the corecursive of the second kind Chebychev form, then the form u(t, λ) given by (2.9) has the following integral representation:
(4.19)
It is a semi-classical form of class
s satisfying the following functional equation:
(4.20)
Proof. It is well known that v = 𝒥(−/2,1/2) possesses the following integral representation [1]:
(4.21)
From (
2.11) we easily obtain (
4.19).
The form v satisfies (3.4) with [15]
(4.22)
Therefore,
ϑ1 = −
t,
ϑ2 =
t,
ϕ(0) ≠ 0.
Now, we can simply use Proposition 3.1 and Corollary 3.2 in order to obtain (4.20).
Corollary 4.4. When t = 0 and λ = −1, one has
(4.23)
Proof. From Table 2, we reach the desired results.
Remarks 4.2. (1) One has the form h−1u(0, −1) = ℒ(−3/2,1/2), where ℒ(α, β) is studied in [24].
(2) Let [15, 19] be the first associated sequence of {Zn} n≥0 orthogonal with respect to u(0, −1) and , the coefficients of the three-term recurrence relations; we have
(4.24)
The sequence is a second-order self-associated sequence; that is, is identical to its associated orthogonal sequence of second kind (see [25]).
Acknowledgments
Sincere thanks are due to the referee for his valuable comments and useful suggestions and his careful reading of the manuscript. The author is indebted to the proofreader the English teacher Hajer Rebai who checked the language of this work.