MXfEMknCAS
INVERSE FINITETYPE RELATIONS BETWEEN SEQUENCES OF POLYNOMIALS By
Francisco Marcellan I & Ridha SfaxP Abstract Marcellan, F. & R. Sfaxi: Inverse finitetype relations between sequences of polynomials. Rev. Acad. Colombo Cienc. 32( 123): 245255, 2008. ISSN 03703908.
Let be a monic polynomial, with deg = t 2: O. We say that there is a finitetype relation between two monic polynomial sequences {Bn }n~O and {Qn }n~O with respect to , if there exists (s,r) E N2 , r 2: s, such that n+t
(X)Qn(X)
=
2:::
An,..,B..,(x), n 2: s, with Ar,rs =I O.
(*)
v=ns
The corresponding inverse finitetype relation of (*) consists in a finitetype relation as follows: n+s
n:(x;n)Bn(x)
=
2:::
e~,..,Qv(x), n 2: t, with e;+t,r =I 0,
v=nt
=
where degn;(x;n) s, n 2: t. When the orthogonality of the two previous sequences is assumed, the inverse finitetype relation is always possible [11]. This work essentially studies the case when only the sequence {Bn}n~o is orthogonal. In fact, we find necessary and sufficient conditions leading to inverse finitetype relations. In particular, the structure relation characterizing semiclassical sequences is a special case of the general situation. Some examples will be analyzed.
2
Departamento de Matematicas, Universidad Carlos III de Madrid, Avenida de la Universidad 30, 28911 Leganes, Spain. Correo electr6nico:
[email protected] Departement des Methodes Quantitatives, Institut Superieur de Gestion de Gabes, Avenue lilani Habib 6002, Gabes, Tunisie. Correo electr6nico:
[email protected] 2000 Mathematics Subject Classification: 42C05, 33C45.
REV. ACAD. COLOMBo CIENC.: VOLUMEN XXXII. NUMERO 123JUNIO DE 2008
246
Key words: Finitetype relations, recurrence relations, orthogonal polynomials, semiclassical polynomials.
Resumen Sea r./> un polinomio moniea, con deg q) = t 2: O. Decimos que hay relacion de tipo finite entre dos sucesiones de polinomios monicos {Bn }n;::o Y {Qn }n2':O con respecto a :O es ortogonal. De hecho, encontramos condiciones necesarias y suficientes que conducen a relaciones de tipo finito inversas. En particular, la la relacion de estructura que caracteriza a las sucesiones semiclasicas es un caso especial de la situacion general. Se estudian varies ejemplos.
Palabras clave: Relaciones de tipe finito, relaciones de recurrencia, polinomios ortogonales, polinomios semi clcisices.
1. Introduction and background Let JP' be the linear space of complex polynomials in one variable and JP" its topological dual space. We denote by (u,!) the action of u E II''' on j E JP' and by (u}n:= (u,x n ), n 2: 0, the moments ofu with respect to the polynomial sequence {xn }n>O. We will introduce some useful operations in JP'. For any linear funct.ional u and any polynomial h, let Du = u' and hu be the linear functionals defined by duality
(u',f):=  (u,f'), (hu, f) :=(u, hf),
j E JP',
n,
Let recall the following results [11], Lemma 1.1. For any u E ll" and any integer m 2: 1, the following statements are equivalent, (u, Bn) = 0, n 2: rn,
)..mI
of 0,
As a consequence, the dual sequence {u~1 }n>O of the sequence {B~II}n"O' where B),!](x} = (n+ l)lB~+l(X), n 2: 0, satisfies
Definition 1.2. The linear functional u is said to be regular if there exists a monic polynomial sequence {Bn}n>o such that
(u, BnBml = bnon,m,
j, h E JP'.
Let {Bn}n>o be a monic polynomial sequence (MPS), degBn = n 2: 0, and {un}n>O its dual sequence, Un E JP", n ::> 0, defined by (un, Bm) := On,m' n, m::> 0, where on,m is the Kronecker symbol.
i) (u, BmI) of 0,
ii) There exist)... E C, 0 o satisfies the threeterm recurrence relation
B n+2 (x) = (x  ;3n+!)Bn+1(x)  "in+1Bn(X), n 2: 0, B , (x) = x  ;30, Bo(x) = 1, (1.5) where "in+!
# 0, n 2:
°
(see [4]).
(n,l/,m) E 1\1"'
(1.6)
In particular, one has
bV _ n,m 
{o,(bn/b
if 1/ + m < n, if v = n  rn,
m ),
o :s: m
< n,
m
=" n.
°="
v ;::: 0,
Let q, be a monic polynomial, with deg q, = t 2: 0, For any MPS {Bn }n20 and {Qn}n20 with dual sequences {un}n>O and {vn }n>O respectively, the following formula always holds
n+t q,(x)Qn(x) = L An,vBv(x), n 2: 0,
(1.7)
is
" )n+' (0 n,v v=nt. n > _ t,
a
MPS, (1.11)
a system of complex numbers (SCN), with O~,n+' = 1, n
In the sequel and under the assumption of the previous definition, we need to put b~m = b:;;,'(u,xVBmBn),
O;+t,r # 0, where {n;(x; n)}n2t degn;(x; n) = s, n 2: t, and
247
2: t,
When both two sequences are orthogonal, the inverse relation is always possible, In this case, the polynomials n;(x; n), n 2: 0, are independent of n, (see [12], Proposition 2.4), As a current example, we can mention the two structure relations characterizing the classical polynomials, (Hermite, Laguerre, Bessel, Jacobi, see [11]), which could solely be two inverse finitetype relations, In other studies, we find several situations where one of the two sequences is orthogonal. For example, the structure relations characterizing semiclassical sequences associated with Hahll's operators Lq,w, with parameters q and w, [9], The Coherent pairs and Diagonal sequences are also examples of finite typerelations [7,12,13,14], But the inverse relations corresponding to other finitetype relations are not yet considered,
",=0
where An,v = (uv, q,Qn) ,
°="
1/
=" n + t,
n 2: 0.
Definition 1.3, ([12]) If there exists an integer s 2: such that
q,(x)Qn{.'r)
n+t L An,vBv(x), n 2: s,
=
°
(1.8)
v=ns
and
:3 r 2:
S,
A1' ,1'8
=f=.
0,
(1.9)
then, we shall say that (1.8)  (1.9) gives a finitetype relation between {Bn }n20 and {Qn}n20, with respect to lP, When instead of (1. 9), we take An,ns
i= 0, n 2:
s,
(1.9')
we shall say that (1.8)  (1.9') is a strictly finitetype relation. The corresponding inverse finitetype relation of (1.8)  (1.9) consists in establishing, whenever it is possible, a finitetype relation between {Qn}n>O and {B n }n20, as follows
n;(x;n)Bn(x) =
n+. L O~vQv(x), n 2: t, v=nt
The paper essentially gives a necessary and sufficient condition allowing the existence of the inverse finitetype relations when the orthogonality of the sequence {Bn }n2>:O is assumed. From now on, it would be necessary to study the case where the sequence {Qn}n>O is orthogonal. It would be very useful to deal with many other situations like General Coherent pairs, see [6,8] in the framework of Sobolev inner products,
(1.10)
2, A basic result
'V'wre use this section to introduce some auxiliary result for the proof of the main theorem in section 3. Lemma 2,1. Suppose {Bn}n>o is a MOPS and {Qn}n>O fulfils (1.8)  (1.9), where t = and s 2: 1. For any SCN (On,v)~;t~, n 2: 0, where On,n+, = 1, n 2: 0, and Br,r # 0, there exist a unique MPS {n,(x; n)}n20, degn,(x;n) = s, n 2: 0, and a SCN (d~L)~;;;~_" n 2: 0, such that
°
n+s
n+s
LOn,vQv(X)
=
O~liBi(x)
L i=ns
= n,(x; n)Bn(x)
+
n1 L (~LBv(x), n 2: 0, (2,1) v=ns
REV. ACAD. COLOMBo CIENC.: VOLUMEN XXXII. NUMERO 12]·JUNIO DE 2008
248
Multiplying by Bm(x) and using the orthogonality of
where
{Bnk::o,
min(n,i)+s
L
O~li =
On,vAv,i, n8::; i::; n+8,n:': 0,
(2.2)
v=max(n,i)
[01
Br .r 
s
= Br.rAr,rs i= 0,
(2.3)
0n,vAv,m
b;:;,I (u, n,(x; n)BnBm) + (~~\n,
=
lI=n
n  8::; m::; n l,n:': 0, (2.4) n+,
L On,vAv,m = b;;,1(u, n,(x; n)BnBm),
v=m
n ::; m ::; n +
8 
1, n:': O.
(2.5)
Proof. Let (On,v)~;!;~, n :': 0, where On,n+s = 1, n :': 0, and (Jr,r # 0, be a SCN. From (1.8)  (1.9), with t = 0 and 8 :': 1, we get n+s
n+s
v
v=n
v=n
i=vs
n+s
n+s
L (In,vQv(X) = L (In,v L =
L (In,v L
v=n
=
L
(J~li8m,i = b;;,1(u, n,(x; n)BnBm) +
L d~L8m,v.
i=ns
Finally, for n ::; m ::; n (2.5).
+8
1 and n :': 0, we deduce

0
Proposition 2,2, Assume {Bn}n>o is a MOPS and {Qn}n~O fulfils (1.8)  (1.9), with t :': 1. For any SCN (Bn,v)~:!~_t' n 2: 0, where Bn,n+s = 1, n :2: 0 a.nd (Jr+t,r # 0, there exist a unique MPS {n,+t(x; 11)}n~O, where degn,+t(x; n) = 8 + t, 11 :': 0, and a SCN (~!v)~;;;~,t' n:': 0, such that for every integer n:': 0 n+s
q,(x)
L
n+s+t
On,vQv(x)
=
v=nt
L
(J~I,Bi(X)
i=nst
Av"Bi(x) nI
+
Xi,vAv" Bi(X), n:': 0,
L
(~,tBv(x),
(2.7)
v=nst
i=ns
where, for each pair of integers (i,l/) such that n . i ~ n + s and n ~ v ~ n + s, we took Xi,v
nI
In particular, for 0 ::; m ::; n  8  1 and n :': 8 + I, it follows that d~'m = O. Hence, (2.1) holds. Moreover, for 11  8 ::; m ::; 11  1 and n :': s, we recover (2.4).
m+,
L
n+s
where 8 ::;
min(n,i)+s+t
B~~i
if v  s :S i :S v,
I, 0, {
=
L
(}n,vtAvt,i ,
v=max(n,i)
otherwise.
11 
s  t ::; i ::; n
+ s + t,
(2.8)
The permutation of these two sums yields n+s
n+s
v=n
i=ns
L (In,vQv(X) = L
O~,liB,(x),
m+s+t
L
where min(i+s,n+s)
L
B~~li
b;:;,1(u, n,+t(x; n)BnBm) + (~!m' n  s  t ::; m ::; n  1, (2.10)
n+s+t
n  s S i :::; n + s, n =
=
Bn ,II Av,i,
v=max(n,i)
(J~~~_,
(In.vtAvt,m
(Jr,rAr,r,
~
0,
(2.6)
L
(In.vtAvt,m = b;;,l(u,n,+t(x;n)BnB m ),
l.I=m
# O.
n ::; m ::;
11
+ s + t  1. (2.11)
Hence, (2.2) and (2.3) are valid. The Euclidean division by Bn(x) in the right hand side in (2.6) gives n+s
L
i=n8
nI
(J~I,B,(x) = n,(x; n)Bn(x)+
L
v=O
(~~LBv(x), n:': O.
Proof The case t = 0 was analyzed in Lemma 2.1. Let us take t:': 1. Consider the MPS {Pn}n~t defined by (2.12)
MARCELLAN. F. & R. SFAXI: INVERSE FINITETYPE RELATIONS BETWEEN SEQUENCES OF POLYNOMIALS
From (1.8)  (1.9). we have
3. A matrix approach and main results
n
Pn(X)
L
=
In this section, we will work under the assumptions of the Proposition 2.2 and we will give a matrix approach to our problem.
)..n.vBv(x). n :;0: t.
v=nts
Ant,,,,) n  t  s :S v :::; n, n 2: t, and =f. O. Now, let (Bn,v)~;:~_t' n 2: 0, where 1. n :;0: 0, and Br+t.r oF 0. be a SCN. One
where ).n,v 'xt+r,rs
Bn .n +, =
has
n+,
L
. n+s,n+s+f.l )T . 
Our data are en, En, Fn , M n , Sn, T n , Kn and our unknowns are Vn and Wn .
From (3.2), we get Vn = S;;'(M n 8 n

i) t. n (t,8),. 0, n 2: p. ii) There exist a unique SCN (O~.vl~!~_t, n 2: p,
Fn ).(3.3)
Thus, substituting in (3.1) we get Kn8n  Wn  En = T nS;;'(M n n  Fn ), i.e,
e
Wn = (Kn  TnS;;lMn)8n + T n S;;' Fn  En. As a consequence, for every choice of From (3.3), we deduce Vn .
en!
Theorem 3.2. Let {Bn}n>o be a MOPS and {Qn}n>O be the MPS satisfying (1.8)  (1.9). For each fixed integer p 2: t + 1, if we suppose that p, degn;(x; n) = s, n2: p, such that
p
n+,
L
n;(x; n)Bn(x) =
(~,vQv(x), n 2: p.
(3.8)
v=nl
F(Qn, ... , QnI) = G(Bn, ... , B n ,),
where F and G are fixed functions. When F and G are linear functions, some situations dealing with the inverse problem have been analyzed in [1,2J. There, necessary and sufficient conditions in order to {Qn}n~O be orthogonal are obtained. This kind of linear relations reads as follows. There exist.s (l,s,r) E l\I3, with r 2: such that n
L
s=
Proof. From Theorem 3.2, with t = 0, there exists the corresponding inverse finitetype relation associated with the relation (3.7) if and only if tJ.n(O, s) i' 0, n 2: p. Equivalently, there exist a unique SCN (e~,v)~~~' n 2: p, where B~,n+s = 1, n 2:: p, and B;,1' 1= 0, if p :S: T, and a unique MPS {n;(x;n)}n2:p, degn;(x;n) = s, n 2: p, such that n+,
max(l,s)
L
n;(x;n)Bn(x) =
n
(n,vQv(x) =
L
>'n,vBv(x), n 2: .;,
(3.6)
v=ns
v=nl
with (r,rI>'"r,
i' O.
Here, (n,n
+ SnPn1 (x) = R~I}(X) + tnR~~1 (x), where tn i' 0, for every n 2: 1, and with the condition tl i' SI· Pn(x)
n 2: 1,
n:(x;n)Bn(x) =
v=n
i=v/
L
n 2:
s.
n+s
L
e~,v
Xi,v(v,i Qi(X), n 2: p,
i=n/
where, for each pair of integers (i, v) sHch that n  I S + sand n ::; v ::; n + s, we took
_ {1,
0,
Xi,v =
if, v  l ::; i
~
v,
otherwise,
The permutation inside these two sums yields n+s
L
n:(x; n)Bn(x) =
(~,iQi(X),
i=nl
where
n
>'n,vBv(x),
(v,iQi(X)
i ::; n
the corresponding inverse finitetype relation between two sequences satisfying (3.6) is possible under certain conditions. Indeed, let consider the MPS {Gn}n>, given by
v
L e~,v L
v=n
technical
In the same context of our contribution, we show that
n+s
n+s
=
Notice that in general {R~I}n2:0 is not. a MOPS.
L
(3.9)
But from (3.6) and (3.7), the above expreRSion becomes
= >'n,n = 1, n 2: s.
More recently, in [5], A, M_ Delgado and F. Marcelhin exhaustively describe all the set of pairs of quasidefinite (regular) linear functionals such that their corresponding sequences of monic polynomials {Pn}n>O and {Rn}n~o are related by a differential expression
Gn(x) =
e~,vGv(x), n 2: p.
v=n
min(i+l,n+s)
(~,i
(3.7)
v=ns
=
L
(J~,v(v,i
,
v=max(n,i)
With the finitetype relation between the sequences {Gn }n2:' and {Bn }n2:', we can associate the determinants tJ.n(O, s), n 2: s. So, we have.
if n  I S i S'n
Corollary 3.3. Let {Bn}n2:0 be a MOPS and {Qn}n2:0 be the MPS satisfying (3.6). For each fixed integer
ifpSr.
+ s, n 2: p, and
o
252
REV. ACAD. COLOMBo CIENC VOLUMEN XXXII, NUMERO 12JJUNIO DE 2008
4. The case: (t,s) = (0,1)
where ,
"In,
= , + An+l,n
vn,o
Let {Bn}n>o be a MOPS with respect to the linear functional u and satisfying the threeterm recurrence relation (1.5). Consider the following finitetype relation between and {Qn}n~O, with index s = 1, with respect to q,(x) = 1, {Bn}n~o
(4.1) ~r
2: 1,
Ar ,r1 =F 0.
(4.2)
From Lemma 2.1, for every set of complex numbers, On,n, n 2: 0, with Or,r =F 0, there exists a unique MPS {111 (x; n)}n>O, where 111 (x; n) = x + vn,o, n 2: 0, and a
unique set of complex numbers, Qn+I(X)
+ On,nQn(x)
(~oJn_ll n 2: 0, such that
111 (x; n)Bn(x)+
=
(~ln_IBnl(x), n 2: 0,
(4.3)
where An,nIOn,n
{ fJn,n  vn,o
_
The determinants associated with (4.1)  (4.2) are given by
Ll. 0 (0,1) = 0,
Ll. n (O,l) = An,nI, n 2: 1,
(4.5)
where Ll.r(O, 1) = Ar,rI =F 0. As a consequence of Theorem 3.2, when t = and s = 1, we have the following result
°
Proposition 4.1. Let {Bn}n>o be a MOPS and {Qn}n>O be the MPS satisijring (4.1)  (4.2). For every fixed integer p 2: 1, the following statements are equivalent i) An,nI =F 0, n 2: p. ii) There exist a. unique set of complex numbers O~,n =F 0, n 2: p, and a unique MPS {l1i(x;n)}n~p, degl1j(x;n) = 1, n 2: p, such
the functional equation u' + 1/Ju = 0, (4.9) 2 where 1/J(x) = ix + 2x  i(a  1) and with regularity condition a 'tc Un>oEn, where Eo = {a E o be a MOPS anG {Qn}n20 be tlle MPS satisfying (5.1)  (5.2). For ev· ery fixed integer p 2: 2, the following statements an equivalent
(5.5)
and s = 2,
i) An ,n2(A n +l,n1  "In) of 0, n 2: p.
REV. ACAD. COLOMBo CIENC VOLUMEN XXXII. NUMERO 123·JUNIO DE 2008
254
ii) There exist a unique SCN (B~.v)~i~, 71. ~ p, with B~,n+2 = 1) n ;: : p, and e;,r =I 0, if p :S r, and there exists a unique MPS {n2(x; 71.)ln:"p, where degn2(x;71.) = 2, 71. ~ p, such that, for
B~'12(x) + en,nB~I(x)
=
+ (~~~_2Bn2(X),
n 2(x; 71.)Bn(x) =Qn+2(X) +B~,n+! Qn+! (x) +e~,nQn(x) . (5.6)
= 1'1 + vo,o, = I'n+l + I'n
+ vn,o, 71. ~ 1, (5.10) = I'nl'nl + (~~~2' 71. ~ 2.
[A n ,n2(tJnl{3n+l +"\n+2,'1.+l}).n,'1.l')'nl]')'n An ,n2("\n+l,'1.l
n,n41 
0*
=
n,n
(5.9)
where
We write _
0, we
(x 2 + vn,o)Bn(x)
71. ~p,
()*
~
are symmetric, i.e, Bn(x) = (l)nB n (x), n get, for n 2: 0,
 ')'n)
Since we have An ,n21 n 2: 2, the choice (~O~_2 2, is possible and yields the inverse relati~n
/n')'nl >''1.,n2 '
=
0, n 2:
n;(x; 71.) = x 2 + V~"X + v~,o, 71. ~ p, (5.7) where
where
+ (A n+l,n  f3n)f)~,n+l I'n+l ),n+2.n + f3n(f3n+l  ),n+2.n+l), B~,n+l  i3n+l  !3n + An+2,n+l.
v~,o = e~,n
V~,l =
71.+1
=
j
+OO
00
p(x)e x ' dx.
This sequence of polynomials was introduced by P. Nevai (see [15]) in the framework of the socalled Freud measures. These polynomials satisfy the threeterm recurrence relation (1.5), with coefficients f3n = 0, 71. ~ 0, and where ')'n+l, n 2: 0, are given by a nonlinear recur
rence relation (see [3] and [15])
71. = 41'nbn+l
+ I'n + I'nl), 71.
~ 1,
with '1'0 = 0 and '1'1 = r(3/4)r(1/4). The sequence {Bn}n>o satisfies the following structure relation ( see [3]) B~I(x) = Bn(x) + ),n,n2Bn2(X),
71.
~ 2,
(5.8)
where
),n,n2
4
= 71.
+ 1 'In+l'lnl'nl io 0,
71.+1
, 4')'n+l
v~,o = _   I'n  'Yn+l 4 ')'n+l
Example. Let {B n }n2:0 be the sequence of monic polynomials, orthogonal with respect to the linear functional u such that (u,p)
e~,n=
l'n+
71. ~ 2.
From (5.3), with Qn(x) = B~ll(x), 71. ~ 0, and the fact that the polynomial sequences {B n }n2:0 and {B~I }n2:0
4
+ + 31'n+1'l'n+2'l'n+3' n
Here, the determinants associated with (5.8) are
4 1 I'n+1'l'nl'nl 2 [4 b.n(O, 2) = 71. + 71. + 2 I'n+21'n+l  1] , (5.12) 71. ~ 2, with b.o(O, 2) = b. , (0, 2) = O. From Proposition 5.1, we deduce that the uniqueness of the previous inverse relation requires that
An+1.nl 
I'n = I'n [n!21'n+21'n+l  1] io 0, n ~ 2. Equivalently, 41'n+21'n+l io 71.+2, 71. ~ 2. Indeed, by using (5.8), where n is replaced by 71. + 1 and taking into account the orthogonality of the polynomial sequence {Bn}n>o, we get B~L (x) = xBn(x) + (),n+1.nl  I'n)Bn l (xl, 71. ~ 1. On the other hand, if we suppose that there exists an integer no 2: 2 such that Ano +l,nol  "'tno = 0, then B~~+I (x) = xBn" (x). In this case (5.11), with 2 71. = no will be written as (x + ax + v~ o)Bn(x) = I'l [11 • [11 ' . B n+2(x) + aBn+l (x) + en,nBn (x), for all a E