c o n v e r g e n t ^-g e n e r a l iz e d f ib o n a c c i sequence s 2 T he follow ing tw o are equivalent. (a) T he sequence {\y n |} ^ = 1 converg...

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Mustapha Rachidi Departement de Mathematiques, Faculte des Sciences, Universite Mohammed V, B. P. 1014, Rabat, Morocco [email protected]

Osamu Saeki Department of Mathematics, Faculty of Science, Hiroshima University, Higashi-Hiroshima 739-8526, Japan [email protected]

1. INTRODUCTION In [4], the authors have defined oo-generalized Fibonacci sequences, which are defined by recurrence formulas involving infinitely many terms and which are generalizations of weighted r-generalized Fibonacci sequences with r finite as defined in [1]. In this paper we study the convergence property of such sequences and their associated series. Let us first recall the definition of oo-generalized Fibonacci sequences. Take an infinite sequence {oj},^, of complex numbers. We set h{z) - J^Lo®*2' for z e C and u(x) - lL*L\\ai lx' f° r x G R . Let R denote the radius of convergence of the power series /?, which coincides with that ofw. We assume the following: 0

(1.1)

Let X be the set of the sequences {^}*0 of complex numbers such that there exist C> 0 and T with 0

J V = /Ov-i,y n -i,y n -^-..)-Y. a i-\yn-i

(n = \2>

\-•),

/=i

which is well defined as is shown in [4]. The sequence {>>/}, GZ is called an oo-generalized Fibonacci sequence associated with the weight sequence {a ; }^ 0 . Note that if there exists an integer r > 1 such that at = 0 for all i >r, then the sequence {a,}*^ satisfies condition (1.1) and the above definition coincides with that of weighted r-generalized Fibonacci sequences with r finite (see[l]). Note that, as far as the authors know, this is a new generalization of the usual Fibonacci sequences and almost nothing has been known about such sequences until now, except those results obtained in [4]. For example, the following questions naturally arise. 326

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CONVERGENT oo-GENERALIZED FIBONACCI SEQUENCES

(Ql) (Q2) (Q3) (Q4) (Q5)

Are they combinations of geometric progressions, as in the finite case? Are they asymptotically geometric? Do they converge to limits? Do their sums converge to limits? Is it possible to express the /1th term of such a sequence as a function of n in some nice way?

We briefly recall the results obtained in [4], which are fundamental for the present paper and which give an answer to (Q2) above. Lemma 1.2 ([4], Lemma 23): (1) Suppose that each at is a nonnegative real number and that there exists anS with 0

(1.2.1)

Then there exists a unique q GH such that q > S~l, {?~(z+1)}J0 G %-> a n d fiff1* ^~2> ^~3> ...) = 1. (2) Suppose there exists an Swith 0

(1.2.2)

Then there exists a unique q GC such that \q\ > S~\ {q~{i+l)}Zo G x ,

and

f(^~\ 9~2> 9~3> •••) = l-

Note that, in the finite case [1], the above q corresponds to the root of the characteristic polynomial of maximal modulus. As has been seen in [4], the existence of such a q plays an important role in studying the asymptotic behavior of oo-generalized Fibonacci sequences [see (Q2) above] and hence in exploring questions (Q3) and (Q4) above (see §5). More precisely, the following has been proved in [4]. Theorem 1.3 ([4], Theorem 3.10): Let {a,.}*^ be a sequence of complex numbers that satisfies (1.1) and admits an S with 0 < S < R satisfying (1.2.1) or (1.2.2), and S2u'(S)<\.

(1.3.1)

Then X\mn^O0ynl qn exists and is equal to 00

Yfinffy-m

^

oo

n

with bm = Y.^k-

Y.K

,=m q

m=Q

In the following, we always assume that the conditions of Theorem 1.3 are satisfied. These conditions demand that the modulus of the leading weight coefficient a0 should be sufficiently large (see also §5). There are many sequences that satisfy these conditions. For example, take an arbitrary holomorphic function hx(z) defined in a neighborhood of zero. Then the sequence appearing as the coefficients of the power series expansion of the holomorphic function h(z) = hx(z) + a at z - 0 satisfies the above conditions for all a eC with sufficiently large modulus \a\. In this paper we consider questions (Q3) and (Q4) mentioned above and prove the following results, which give answers to the questions in certain situations. Theorem 1.4: Suppose that each at is a nonnegative real number and that E^ =0 hmqmy_m ^ 0. 2000]

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CONVERGENT oo-GENERALIZED FIBONACCI SEQUENCES

1.

The following three are equivalent. (a) The sequence {y„}*=1 does not converge.

(b) sr=o«,>i(e) 2.

q>\.

The following three are equivalent. (a) The sequence {y„}"=1 converges to a nonzero real number. (e) q = \.

Furthermore, in case 2(a), we have 00

2-4 ^ms-m

limJn = - < L

oo

with

bm = Yjai.

/w=0

3.

The following three are equivalent. (a) The sequence {y„}J|Li converges to zero.

(b) sr= 0 ^

q

Theorem 1.5: Suppose that each at is a nonnegative real number and that Y£^bmqmy_m * 0. L The following three are equivalent. (a) The series S*=1 >>„ does not converge.

(b) zr=o«^ift) <7>1. 2.

The following three are equivalent. (a) The series Z*=iJ>w converges.

(b) zr= 0 ^

Z

v •^ «

oo

oo /

_ y=o >=y OO

w =o

oo

\

V/=m

y

00

When a, are general complex numbers, we have the following theorem. Theorem 1.6: Suppose that Z* =0 K^y-m * °1. We have the implications (b) => fcj <=> fa) among the following: (a) The sequence {\yn |}^ =1 converges to oo.

(b) Ki-zr=iKi>i. (c) 328

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2

The following two are equivalent. (a) The sequence {\yn |}^ =1 converges to a nonzero real number. (h) |*| = 1.

Furthermore, in case 2(a), we have

lim[)/J=

2XO-

m=Q

TK

m=0

J.

We have the implications (b) => (c) <=> fa) among the following: (a) The sequence {yn}™=i converges to zero.

m sr=oKi

The following two are equivalent. (a) The series Z"=il.y„ | does not converge. (b) \q\>\.

2.

The following two are equivalent. (a) The series Y%=1\yn | converges. (b) \q\

Furthermore, in case 2(a), we have oo

Z

y

oo

_ 7=0 *=J °°

i-5> /=0

oo

f

oo

A

m=0 \i=m

J oo

i-5> /=0

Note that the above results generalize some of the results of Gerdes [2], [3], concerning weighted r-generalized Fibonacci sequences with r = 2 and 3. The paper is organized as follows: In §2 we prove the convergence result, Theorem 1.4, for the nonnegative real case. In §3 we prove the convergence result, Theorem 1.6, for the general case. In §4 we give an explicit formula for the generating functions of oo-generalized Fibonacci sequences that generalize a result of Raphael [5], and prove the convergence results for the series, i.e., Theorems 1.5 and 1.7. Finally, in §5 we give some remarks concerning questions (Q1)-(Q5) mentioned above. 2. CONVERGENCE OF SEQUENCE—NONNEGATIVE MEAL CASE In this section, we prove Theorem 1.4. Lemma 2.1: J£K = TZ=obnflmy-m * 0 and |^| > 1, then lim^Jj/,,| = oo. Proof: By Theorem 1.3, there exists an integer N such that, for all n > N, we have 2000]

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CONVERGENT oo-GENERALIZED FIBONACCI SEQUENCES

\{ynlq")-K\<\K\l2. In particular, we have \yn\>\K\\q"\-\K-{ynlq")\\q"\>\K\\q\"l2 for all n> N. Then the result is obvious. • Lemma 2.2: If \q\ < 1, then l i m ^ J y n | = 0. Proof: By Theorem 1.3, there exists an integer N such that, for all n> TV, we have

IO„/<7")-*l

Then we have

hence, \yn \ < (\K\ + l)\q\n for all n> N. Then the result is obvious. • Lemma 2.3: Suppose that each at is a nonnegative real number.

(1) IfZSo^ >l,then?>l. (2) lfZT=o^i=^,thmq

= l.

(3) I f Z ^ o ^ < l , t h e n ^ < l . Proof: Let #>: [0, R) -> R be the function defined by #?(x) = x/?(x), which is strictly increasing. Note that

S~l. When S > 1, we have 1 < S < R by our assumption and

\

=

/=0

l

by Lemma 1.2. Thus, we have 1 > q~ , which implies that q > 1. (2) Since Hf=oai converges, we have i? > 1. If R > 1, then x = 1 is the unique solution of the equation

1 for some 5 e [0, R). (3) Since Z J 0 a i converges, we have i? > 1. By the same argument as in (2), we have R > 1. Then we have #>(1) < 1 =

1. If all at are nonnegative real numbers and if SJlo a / = 1> *hen ^ e sequence {&•},*! converges to

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3, CONVERGENCE OF SEQUENCE—GENERAL CASE In this section we prove Theorem 1.6. Lemma3.1: If |a 0 | - I ^ a , . | > 1, then \q\>\. Proof; When S < 1, we have \q\ > 1 by Lemma 1.2. When S > 1, we have 1 < 5 < i?. Consider the function t: [0, i?) -» R defined by f(x) = xV(x), which is strictly increasing. Then we have r(l) < t(S) < 1 by condition (1.3.1). Furthermore, by our assumption, we have \a0 |> 1 + u(l). Hence, in (1.2.2) and (1.3.1), we may assume that £ = 1. Thus, we have \q\ > 1 by Lemma 1.2. This completes the proof. • Lemma 3.2: If Z^oK-1 < 1> then \q\ < 1. Proof; We have oo

oo

0+1)

/=o

i=0

- i i/+i ^IM^Y

Thus, we have \q l \ > 1. This completes the proof. • Theorem 1.6 follows from the above lemmas together with the lemmas in §2 and Theorem 1.3. Corollary 3.3: Let {g^fLi be the oo-generalized Fibonacci sequence associated with the initial sequence {giJJo* where g0 = l and g_t = 0 for / > 1. Then the sequence {g}^ converges to a nonzero complex number if and only if | q | = 1. 4 GENERATING FUNCTION AND CONVERGENT SERIES First, we prove the following formula for the generating function of oo-generalized Fibonacci sequences, which generalizes a result of Raphael [5]. Theorem 4.1: Suppose that the sequence m ^ 0 satisfies the condition in Lemma 1.2. Then the generating function of the sequence \y^=\ is equal to •zh(zY where h(z) = J^L^a^1 and \

(,

More precisely, the above equality holds for all z e C with \z\ < \q\~l. Proof: First, consider the power series k(z). Let the radius of convergence of k be denoted by R!. Then we have V1 ( R! lim sup n I>/^-/ '=; s an

Since the sequence {y0, y-i, y-2> • •-} * element of X, there exist C > 0 and T with 0< T

331

CONVERGENT oo-GENERALIZED FIBONACCI SEQUENCES

lim sup j \ /->oo y

< lim sup y EK|C7*-> = lim sup l ^ W ? y->oo y i=j y-»oo y i /=y < lim sup ^-{/u(T)

= —.

Thus, we have R' >T. Since we can choose Tas close to R as we want, we have R > R. Thus, in particular, for z G C with \z\ < R, the series k(z) converges absolutely. Therefore, for z with | z\ < R, we have V/=o

)

V. ;=o

A<=o

= Ji + O2 - %Vi)* + O s " ^ 2 - " l ^ y + 0>4 - «oJ3 - «iJ2 - a2^i)z3 + • • • \ k(z), y=0 ^1=/

where we have changed the order of addition appropriately, which is allowed since all the series above converge absolutely. Thus, as long as 1 -zh(z) * 0, we have

y y ^- m On the other hand, we have q~lh(q~l) = 1 and that q~l is the solution for zh(z) - 1 which has the smallest modulus by Lemma 1.2. Hence, for \z\ < \q\~l, we have (4.1.1). This completes the proof of Theorem 4.1. D Now Theorem 1.5 follows from Theorems 1.4 and 4.1. Proof of Theorem 1.7: By Theorem 1.3 and Lemma 2.1, if\q\ > 1, then the series EJJLiLyJ does not converge. Suppose that \q\ < 1. The radius of convergence of the power series 7=0

is equal to the radius of convergence R" of the power series i=0

By Theorem 4.1 together with our assumption, we have R" > \q\~l > 1. Thus, the series c(z) for z-\ converges. Then the rest of Theorem 1.7 follows from Theorem 4.1. This completes the proof. • Corollary 4.2: Let { g } ^ be the oo-generalized Fibonacci sequence associated with the initial sequence {&_;}*()> where g0 = 1 and g_t = 0 for / > 1. If \q\ < 1, then the series T*=l gf converges to 00

z=o

332

/ /

/

V

00

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CONVERGENT ^-GENERALIZED FIBONACCI SEQUENCES

5, CONCLUDING REMARKS In this section we give some remarks about questions (Q1)-(Q5) raised in §1. About (Ql), in the finite case, the answer to this question is given by a Binet-type formula (e.g., see [1]). The question in the infinite case is also posed in [4, Problem 4.5]. In a forthcoming paper we will consider approximation of oo-generalized Fibonacci sequences by finitely generalized ones and will give an asymptotic Binet formula which will give an answer to the question in a certain sense. This study is also closely related to question (Q2). About (Q2), in the finite case, it has been shown that if the characteristic polynomial has a simple root of maximal modulus then the sequence is asymptotically geometric (see [1]). This condition is satisfied as long as the leading weight coefficient a0 has sufficiently large modulus (see Theorem 15 and Remark 16 of [1]). In [4], the authors have shown that a statement similar to this also holds in the infinite case as well, which is nothing but Theorem 1.3 of the present paper. About (Q3) and (Q4), Theorems 1.4 and 1.5, respectively, give satisfactory answers in the nonnegative real coefficient case under our assumption. In the general case, Theorems 1.6 and 1.7, respectively, give partial answers to the questions. About (Q5), in a forthcoming paper, combinatorial expressions for the general terms of an oo-generalized Fibonacci sequence will be studied. ACKNOWLEDGMENT The authors would like to thank Francois Dubeau for his helpful comments and suggestions. They would also like to thank the anonymous referee for invaluable comments and suggestions. W. Motta and O. Saeki have been partially supported by CNPq, Brazil. The work of M. Rachidi has been done in part while he was a visiting professor at UFMS, Brazil. O. Saeki has also been partially supported by the Grant-in-Aid for Encouragement of Young Scientists (No. 08740057), Ministry of Education, Science and Culture, Japan, and by the Anglo-Japanese Scientific Exchange Programme, run by the Japan Society for the Promotion of Science and the Royal Society. REFERENCES 1. F. Dubeau, W. Motta, M. Rachidi, & O. Saeki. "On Weighted r-Generalized Fibonacci Sequences." The Fibonacci Quarterly 35.2 (1997): 102-10. 2. W. Gerdes. "Convergent Generalized Fibonacci Sequences." The Fibonacci Quarterly 15.2 (1977): 156-60. 3. W. Gerdes. "Generalized Tribonacci Numbers and Their Convergent Sequences." The Fibonacci Quarterly 16.3 (1978):269-75. 4. W. Motta, M. Rachidi, & O. Saeki. "On oo-Generalized Fibonacci Sequences." The Fibonacci Quarterly 37.3 (1999):223-32. 5. B. L. Raphael. "Linearly Recursive Sequences of Integers." The Fibonacci Quarterly 12A (1974): 11-37. AMS Classification Number: 40A05 <»•><•

2000]

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