- Research Article
- Open Access
- Published:
Fixed Points, Inner Product Spaces, and Functional Equations
Fixed Point Theory and Applications volume 2010, Article number: 713675 (2010)
Abstract
Rassias introduced the following equality ,
, for a fixed integer
. Let
be real vector spaces. It is shown that, if a mapping
satisfies the following functional equation
for all
with
, which is defined by the above equality, then the mapping
is realized as the sum of an additive mapping and a quadratic mapping. Using the fixed point method, we prove the generalized Hyers-Ulam stability of the above functional equation in real Banach spaces.
1. Introduction and Preliminaries
The stability problem of functional equations originated from a question of Ulam [1] concerning the stability of group homomorphisms. Hyers [2] gave a first affirmative partial answer to the question of Ulam for Banach spaces. Hyers' theorem was generalized by Aoki [3] for additive mappings and by Rassias [4] for linear mappings by considering an unbounded Cauchy difference. The paper of Rassias [4] has provided a lot of influence on the development of what we call the generalized Hyers-Ulam stability or Hyers-Ulam-Rassias stability of functional equations. A generalization of the Rassias theorem was obtained by Gvruţa [5] by replacing the unbounded Cauchy difference by a general control function in the spirit of Rassias' approach.
A square norm on an inner product space satisfies the parallelogram equality

The functional equation

is called a quadratic functional equation. A generalized Hyers-Ulam stability problem for the quadratic functional equation was proved by Skof [6] for mappings , where
is a normed space and
is a Banach space. Cholewa [7] noticed that the theorem of Skof is still true if the relevant domain
is replaced by an Abelian group. Czerwik [8] proved the generalized Hyers-Ulam stability of the quadratic functional equation. The generalized Hyers-Ulam stability of the above quadratic functional equation and of two functional equations of quadratic type was obtained by C
dariu and Radu [9].
By a square norm on an inner product space, Rassias [10] introduced the following equality:

for a fixed integer . By the above equality, we can define the following functional equation:

for all with
, where
is a real vector space.
A square norm on an inner product space satisfies

for all with
(see [10]).
From the above equality we can define the following functional equation:

which is called a functional equation of quadratic type. In fact, in
satisfies the functional equation of quadratic type. In particular, every solution of the functional equation of quadratic type is said to be a quadratic-type mapping. One can easily show that if
satisfies the quadratic functional equation then
satisfies the functional equation of quadratic type. The stability problems of several functional equations have been extensively investigated by a number of authors, and there are many interesting results concerning this problem (see [11–24]).
Let be a set. Then, a function
is called a generalized metric on
if
satisfies the following:
(1) if and only if
,
(2) for all
,
(3) for all
.
We recall a fundamental result in fixed point theory.
Let be a complete generalized metric space, and let
be a strictly contractive mapping with the, Lipschitz constant
. Then, for each given element
, either

for all nonnegative integers or there exists a positive integer
such that
(1) for all
,
(2)the sequence converges to a fixed point
of
,
(3) is the unique fixed point of
in the set
,
(4) for all
.
In 1996, Isac and Rassias [27] were the first to provide applications of stability theory of functional equations for the proof of new fixed point theorems with applications. By using the fixed point method, the stability problems of several functional equations have been extensively investigated by a number of authors (see [17, 28–31]).
Throughout this paper, assume that is a fixed integer greater than
. Let
be a real normed vector space with norm
, and let
be a real Banach space with norm
.
In this paper, we investigate the functional equation (1.4). Using the fixed point method, we prove the generalized Hyers-Ulam stability of the functional equation (1.4) in real Banach spaces.
2. Fixed Points and Functional Equations Associated with Inner Product Spaces
We investigate the functional equation (1.4).
Lemma 2.1.
Let and
be real vector spaces. If a mapping
satisfies

for all with
, then the mapping
is realized as the sum of an additive mapping and a quadratic-type mapping.
Proof.
Let and
for all
. Then,
is an odd mapping and
is an even mapping satisfying
and (2.1).
Letting , and
in (2.1) for the mapping
, we get

for all . So,
is an additive mapping.
Letting , and
in (2.1) for the mapping
, we get

for all . So,
is a quadratic-type mapping.
For a given mapping , we define

for all .
Let and
for all
. Then,
is an odd mapping and
is an even mapping satisfying
. If
, then
and
.
Using the fixed point method, we prove the generalized Hyers-Ulam stability of the functional equation in real Banach spaces.
Theorem 2.2.
Let be a mapping with
for which there exists a function
such that there exists an
such that


for all with
. Then, there exists a unique quadratic-type mapping
satisfying

for all .
Proof.
Consider the set

and introduce the generalized metric on :

By the same method given in [17, 28, 32], one can easily show that is complete.
Now we consider the linear mapping such that

for all .
It follows from the proof of Theorem of [25] that

for all .
Letting , and
in (2.6), we get

for all . It follows from (2.12) that

for all . Hence,
.
By Theorem 1.1, there exists a mapping satisfying the following.
-
(1)
is a fixed point of
; that is,
(2.14)
for all . The mapping
is a unique fixed point of
in the set

This implies that is a unique mapping satisfying (2.14) such that there exists a
satisfying

for all .
-
(2)
One has
as
. This implies the equality
(2.17)
for all . Since
is an even mapping,
is an even mapping.
Moreover,one has (3) , which implies the inequality

This implies that inequality (2.7) holds.
It follows from (2.5), (2.6), and (2.17) that

for all with
. So,
for all
with
. By Lemma 2.1, the mapping
is a quadratic-type mapping.
Therefore, there exists a unique quadratic-type mapping satisfying (2.7).
Corollary 2.3.
Let and
be real numbers, and let
be a mapping such that

for all with
. Then, there exists a unique quadratic-type mapping
satisfying

for all .
Proof.
The proof follows from Theorem 2.2 by taking

for all . Then, we can choose
, and we get the desired result.
Remark 2.4.
Let be a mapping for which there exists a function
satisfying (2.6) and
such that there exists an
such that

for all with
. By a similar method to the proof of Theorem 2.2, one can show that there exists a unique quadratic-type mapping
satisfying

for all .
For the case , one can obtain a similar result to Corollary 2.3: let
and
be positive real numbers, and let
be a mapping satisfying (2.20). Then, there exists a unique quadratic-type mapping
satisfying

for all .
Theorem 2.5.
Let be a mapping for which there exists a function
such that there exists an
such that


for all with
. Then, there exists a unique additive mapping
satisfying

for all .
Proof.
Consider the set

and introduce the generalized metric on :

By the same method given in [17, 28, 32], one can easily show that is complete.
Now we consider the linear mapping such that

for all .
It follows from the proof of Theorem of [25] that

for all .
Letting and
in (2.27), we get

for all . It follows from (2.33) that

for all . Hence,
.
By Theorem 1.1, there exists a mapping satisfying the following.
-
(1)
is a fixed point of
; that is,
(2.35)
for all . The mapping
is a unique fixed point of
in the set

This implies that is a unique mapping satisfying (2.35) such that there exists a
satisfying

for all .
-
(2)
One has
as
. This implies the equality
(2.38)
for all . Since
is an odd mapping,
is an odd mapping;
-
(3)
Moreover
, which implies the inequality
(2.39)
This implies that inequality (2.28) holds.
It follows from (2.26), (2.27), and (2.38) that

for all with
. So,
for all
with
. By Lemma 2.1, the mapping
is an additive mapping.
Therefore, there exists a unique additive mapping satisfying (2.28), as desired.
Corollary 2.6.
Let and
be real numbers, and let
be a mapping satisfying (2.20). Then, there exists a unique additive mapping
satisfying

for all .
Proof.
The proof follows from Theorem 2.5 by taking

for all . Then, we can choose
, and we get the desired result.
Remark 2.7.
Let be a mapping for which there exists a function
satisfying (2.27) such that there exists an
such that

for all . By a similar method to the proof of Theorem 2.5, one can show that there exists a unique additive mapping
satisfying

for all .
For the case , one can obtain a similar result to Corollary 2.6: let
and
be positive real numbers, and let
be a mapping satisfying (2.20). Then, there exists a unique additive mapping
satisfying

for all .
Since

Note that

Combining Theorems 2.2 and 2.5, we obtain the following result.
Theorem 2.8.
Let be a mapping satisfying
for which there exists a function
satisfying (2.5) and

for all with
. Then, there exist an additive mapping
and a quadratic type mapping
such that

for all .
Corollary 2.9.
Let and
be positive real numbers, and let
be a mapping such that

for all with
. Then, there exist an additive mapping
and a quadratic-type mapping
such that

for all .
Proof.
Define , and apply Theorem 2.8 to get the desired result.
Note that

Combining Remarks 2.4 and 2.7, we obtain the following result.
Remark 2.10.
Let be a mapping for which there exists a function
satisfying (2.48) and
such that there exists an
such that

for all . By a similar method to the proof of Theorem 2.8, one can show that there exist an additive mapping
and a quadratic-type mapping
such that

for all .
For the case , one can obtain a similar result to Corollary 2.9: let
and
be positive real numbers, and let
be a mapping satisfying (2.50). Then, there exist an additive mapping
and a quadratic-type mapping
satisfying

for all .
References
Ulam SM: Problems in Modern Mathematics. John Wiley & Sons, New York, NY, USA; 1960.
Hyers DH: On the stability of the linear functional equation. Proceedings of the National Academy of Sciences of the United States of America 1941, 27: 222–224. 10.1073/pnas.27.4.222
Aoki T: On the stability of the linear transformation in Banach spaces. Journal of the Mathematical Society of Japan 1950, 2: 64–66. 10.2969/jmsj/00210064
Rassias ThM: On the stability of the linear mapping in Banach spaces. Proceedings of the American Mathematical Society 1978,72(2):297–300. 10.1090/S0002-9939-1978-0507327-1
Găvruţa P: A generalization of the Hyers-Ulam-Rassias stability of approximately additive mappings. Journal of Mathematical Analysis and Applications 1994,184(3):431–436. 10.1006/jmaa.1994.1211
Skof F: Proprietà locali e approssimazione di operatori. Rendiconti del Seminario Matematico e Fisico di Milano 1983,53(1):113–129. 10.1007/BF02924890
Cholewa PW: Remarks on the stability of functional equations. Aequationes Mathematicae 1984,27(1–2):76–86.
Czerwik S: On the stability of the quadratic mapping in normed spaces. Abhandlungen aus dem Mathematischen Seminar der Universität Hamburg 1992, 62: 59–64. 10.1007/BF02941618
Cǎdariu L, Radu V: Fixed points and the stability of quadratic functional equations. Analele Universităţii de Vest din Timişoara 2003,41(1):25–48.
Rassias ThM: On characterizations of inner product spaces and generalizations of the H. Bohr inequality. In Topics in Mathematical Analysis. Volume 11. Edited by: Rassias ThM. World Scientific, Teaneck, NJ, USA; 1989:803–819.
Hyers DH, Isac G, Rassias ThM: Stability of Functional Equations in Several Variables, Progress in Nonlinear Differential Equations and Their Applications, 34. Birkhäuser, Boston, Mass, USA; 1998:vi+313.
Moslehian MS: On the orthogonal stability of the Pexiderized quadratic equation. Journal of Difference Equations and Applications 2005,11(11):999–1004. 10.1080/10236190500273226
Park C-G: Homomorphisms between Poisson -algebras. Bulletin of the Brazilian Mathematical Society 2005,36(1):79–97. 10.1007/s00574-005-0029-z
Park C, Cho YS, Han M-H: Functional inequalities associated with Jordan-von Neumann-type additive functional equations. Journal of Inequalities and Applications 2007, 2007:-13.
Park C, Cui J: Generalized stability of -ternary quadratic mappings. Abstract and Applied Analysis 2007, 2007:-6.
Park C, Najati A: Homomorphisms and derivations in -algebras. Abstract and Applied Analysis 2007, 2007:-12.
Radu V: The fixed point alternative and the stability of functional equations. Fixed Point Theory 2003,4(1):91–96.
Rassias ThM: Problem 16; 2, Report of the 27th International Symp. on Functional Equations. Aequationes Mathematicae 1990, 39: 292–293; 309.
Rassias ThM: On the stability of the quadratic functional equation and its applications. Studia Universitatis Babeş-Bolyai. Mathematica 1998,43(3):89–124.
Rassias ThM: The problem of S. M. Ulam for approximately multiplicative mappings. Journal of Mathematical Analysis and Applications 2000,246(2):352–378. 10.1006/jmaa.2000.6788
Rassias ThM: On the stability of functional equations in Banach spaces. Journal of Mathematical Analysis and Applications 2000,251(1):264–284. 10.1006/jmaa.2000.7046
Rassias ThM: On the stability of functional equations and a problem of Ulam. Acta Applicandae Mathematicae 2000,62(1):23–130. 10.1023/A:1006499223572
Rassias ThM, Šemrl P: On the Hyers-Ulam stability of linear mappings. Journal of Mathematical Analysis and Applications 1993,173(2):325–338. 10.1006/jmaa.1993.1070
Rassias ThM, Shibata K: Variational problem of some quadratic functionals in complex analysis. Journal of Mathematical Analysis and Applications 1998,228(1):234–253. 10.1006/jmaa.1998.6129
Cădariu L, Radu V: Fixed points and the stability of Jensen's functional equation. Journal of Inequalities in Pure and Applied Mathematics 2003.,4(1, article 4):
Diaz JB, Margolis B: A fixed point theorem of the alternative, for contractions on a generalized complete metric space. Bulletin of the American Mathematical Society 1968, 74: 305–309. 10.1090/S0002-9904-1968-11933-0
Isac G, Rassias ThM: Stability of -additive mappings: applications to nonlinear analysis. International Journal of Mathematics and Mathematical Sciences 1996,19(2):219–228. 10.1155/S0161171296000324
Cădariu L, Radu V: Fixed point methods for the generalized stability of functional equations in a single variable. Fixed Point Theory and Applications 2008, 2008:-15.
Mirzavaziri M, Moslehian MS: A fixed point approach to stability of a quadratic equation. Bulletin of the Brazilian Mathematical Society 2006,37(3):361–376. 10.1007/s00574-006-0016-z
Park C: Fixed points and Hyers-Ulam-Rassias stability of Cauchy-Jensen functional equations in Banach algebras. Fixed Point Theory and Applications 2007, 2007:-15.
Park C: Generalized Hyers-Ulam stability of quadratic functional equations: a fixed point approach. Fixed Point Theory and Applications 2008, 2008:-9.
Cădariu L, Radu V: On the stability of the Cauchy functional equation: a fixed point approach. In Iteration Theory (ECIT '02), Grazer Mathematische Berichte. Volume 346. Karl-Franzens-Universitaet Graz, Graz, Austria; 2004:43–52.
Acknowledgment
This paper was supported by Basic Science Research Program through the National Research Foundation of Korea funded by the Ministry of Education, Science and Technology (NRF-2009-0070788).
Author information
Authors and Affiliations
Corresponding author
Rights and permissions
Open Access This article is distributed under the terms of the Creative Commons Attribution 2.0 International License (https://creativecommons.org/licenses/by/2.0), which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited.
About this article
Cite this article
Park, C. Fixed Points, Inner Product Spaces, and Functional Equations. Fixed Point Theory Appl 2010, 713675 (2010). https://doi.org/10.1155/2010/713675
Received:
Revised:
Accepted:
Published:
DOI: https://doi.org/10.1155/2010/713675
Keywords
- Banach Space
- Functional Equation
- Stability Problem
- Additive Mapping
- Positive Real Number