Hindawi Publishing Corporation
Journal of Inequalities and Applications
Volume 2007, Article ID 21640, 8 pages
doi:10.1155/2007/21640
Research Article
Bessel’s Differential Equation and Its Hyers-Ulam Stability
Byungbae Kim and Soon-Mo Jung
Received 23 August 2007; Accepted 25 October 2007
Recommended by Panayiotis D. Siafarikas
We solve the inhomogeneous Bessel differential equation and apply this result to obtain a
partial solution to the Hyers-Ulam stability problem for the Bessel differential equation.
Copyright © 2007 B. Kim and S.-M. Jung. This is an open access article distributed under
the Creative Commons Attribution License, which permits unrestricted use, distribution,
and reproduction in any medium, provided the original work is properly cited.
1. Introduction
In 1940, Ulam gave a wide ranging talk before the Mathematics Club of the University of
Wisconsin, in which he discussed a number of important unsolved problems (see [1]).
Among those was the question concerning the stability of homomorphisms: let G1 be a
group and let G2 be a metric group with a metric d(·, ·) . Given any δ > 0, does there exist
an ε > 0 such that if a function h : G1 →G2 satisfies the inequality d(h(xy), h(x)h(y)) < ε
for all x, y ∈ G1 , then there exists a homomorphism H : G1 →G2 with d(h(x),H(x)) < δ
for all x ∈ G1 ?
In the following year, Hyers [2] partially solved the Ulam problem for the case where
G1 and G2 are Banach spaces. Furthermore, the result of Hyers has been generalized by
Rassias (see [3]). Since then, the stability problems of various functional equations have
been investigated by many authors (see [4–6]).
We will now consider the Hyers-Ulam stability problem for the differential equations:
assume that X is a normed space over a scalar field K and that I is an open interval,
where K denotes either R or C. Let a0 ,a1 ,...,an : I →K be given continuous functions,
let g : I →X be a given continuous function, and let y : I →X be an n times continuously
differentiable function satisfying the inequality
an (t)y (n) (t) + an−1 (t)y (n−1) (t) + · · · + a1 (t)y (t) + a0 (t)y(t) + g(t) ≤ ε
(1.1)
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Journal of Inequalities and Applications
for all t ∈ I and for a given ε > 0. If there exists an n times continuously differentiable
function y0 : I →X satisfying
an (t)y0(n) (t) + an−1 (t)y0(n−1) (t) + · · · + a1 (t)y0 (t) + a0 (t)y0 (t) + g(t) = 0
(1.2)
and y(t) − y0 (t) ≤ K(ε) for any t ∈ I, where K(ε) is an expression of ε with lim ε→0 K(ε) =
0, then we say that the above differential equation has the Hyers-Ulam stability. For more
detailed definitions of the Hyers-Ulam stability, we refer the reader to [4–8].
Alsina and Ger were the first authors who investigated the Hyers-Ulam stability of
differential equations. They proved in [9] that if a differentiable function f : I →R is a
solution of the differential inequality | y (t) − y(t)| ≤ ε, where I is an open subinterval of
R, then there exists a solution f0 : I →R of the differential equation y (t) = y(t) such that
| f (t) − f0 (t)| ≤ 3ε for any t ∈ I.
This result of Alsina and Ger has been generalized by Takahasi et al. They proved in
[10] that the Hyers-Ulam stability holds true for the Banach space valued differential
equation y (t) = λy(t) (see also [11, 12]).
Moreover, Miura et al. [13] investigated the Hyers-Ulam stability of nth order linear differential equation with complex coefficients. They [14] also proved the HyersUlam stability of linear differential equations of first order, y (t) + g(t)y(t) = 0, where
g(t) is a continuous function. Indeed, they dealt with the differential inequality y (t) +
g(t)y(t) ≤ ε for some ε > 0.
Recently, Jung proved the Hyers-Ulam stability of various linear differential equations
of first order (see [15–18]) and further investigated the general solution of the inhomogeneous Legendre differential equation and its Hyers-Ulam stability (see [14, 19]).
In Section 2 of this paper, by using the ideas from [19], we investigate the general
solution of the inhomogeneous Bessel differential equation of the form
x2 y (x) + xy (x) + x2 − ν2 y(x) =
∞
am x m ,
(1.3)
m=0
where the parameter ν is a given positive nonintegral number. Section 3 will be devoted to
a partial solution of the Hyers-Ulam stability problem for the Bessel differential equation
(2.1) in a subclass of analytic functions.
2. Inhomogeneous Bessel equation
A function is called a Bessel function if it satisfies the Bessel differential equation
x2 y (x) + xy (x) + x2 − ν2 y(x) = 0.
(2.1)
The Bessel equation plays a great role in physics and engineering. In particular, this
equation is most useful for treating the boundary-value problems exhibiting cylindrical
symmetries.
B. Kim and S.-M. Jung 3
In this section, we define
cm = −
[m/2]
am−2i
i =0
i
1
j =0
ν2 − (m − 2 j)2
(2.2)
for each m ∈ {0,1,2,... }, where [m/2] denotes the largest integer not exceeding m/2, and
we refer to (1.3) for the am ’s. We can easily check that cm ’s satisfy
a0 = −ν2 c0 ,
a1 = −(ν2 − 1)c1 ,
(2.3)
am+2 = cm − ν2 − (m + 2)2 cm+2
for any m ∈ {0,1,2,... }.
m
Lemma 1. (a) If
the power series ∞
ρ > 1, then
m=0 am x converges for all x ∈ (−ρ,ρ) with
∞
m
m
the power series m=0 cm x with cm ’s given in (2.2) satisfies the inequality | ∞
m=0 cm x | ≤
C1 /(1 − |x|) for some positive
constant C1 and for any x ∈ (−1,1).
m
(b) If the power series ∞
m x converges for all x ∈ (−ρ,ρ) with ρ ≤ 1, then for any
m=0 a
∞
m
<
ρ,
the
power
series
positive
ρ
m=0 cm x with cm ’s given in (2.2) satisfies the inequality
0
∞
m
| m=0 cm x | ≤ C2 for any x ∈ (−ρ0 ,ρ0 ) and for some positive constant C2 which depends
m
on ρ0 . Since ρ0 is arbitrarily close to ρ, this means that ∞
m=0 cm x is convergent for all
x ∈ (−ρ,ρ).
m
Proof. (a) Since the power
series ∞
on its interval of
m=0 am x is absolutely convergent
∞
|
a
convergence, with x = 1, m=0 am converges absolutely, that is, ∞
m=0 m | < M1 by some
number M1 . Suppose that p < ν < p + 1 for some integer p. Then for any nonnegative
integer q, 1/ |ν2 − q2 | = 1/ |ν + q|1/ |ν − q| is less than 1 except, possibly, for q = p and
q = p + 1. Therefore,
i
j =0
1
≤ max
ν2 − (m − 2 j)2 1
1
ν2 − p2 , ν2 − (p + 1)2 = M2
(2.4)
for any m and i. Now,
[m/2]
i
[m/2]
1
cm ≤
am−2i am−2i M2 ≤ M1 M2 = C1
≤
2
ν2 − (m − 2 j) i=0
j =0
(2.5)
i =0
and, therefore,
∞
∞
∞
m m
m
cm x ≤ C1
x ≤ C1
≤
c
x
m
1 − |x|
m=0
m=0
m=0
for x ∈ (−1,1).
(2.6)
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Journal of Inequalities and Applications
m
convergent on its interval of conver(b) The power series ∞
m=0 am x is absolutely m
gence, and, therefore, for any given ρ0 <ρ, the series ∞
m=0 |am x | is convergent on [−ρ0 ,ρ0]
and
∞
∞
m m
am |x| ≤
am ρ = M 3
0
m=0
(2.7)
m=0
for any x ∈ [−ρ0 ,ρ0 ].
Also for m ≥ p + 2, if we let M2 = max {1,M2 }, then
i
1
1
1
≤
2
ν2 − m2 M2 ≤ (m − p − 1)2 M2 .
2
j =0 ν − (m − 2 j)
(2.8)
Now,
∞
[m/2]
i
∞
1
m
m
= −
c
x
x
a
m
m
−2i
ν2 − (m − 2 j)2 m= p+2
m= p+2
∞ [m/2]
≤
i =0
am−2i ρm
m= p+2 i=0
≤
≤
=
p+1
and, therefore, if |
m=0 cm x
j =0
0
1
M2
(m − p − 1)2
∞
[m/2]
1
am−2i ρm−2i M 2
2
0
m= p+2 (m − p − 1) i=0
(2.9)
∞
1
2 M3 M2
(m
−
p
−
1)
m= p+2
∞
1
k2
k =1
m| ≤
M3 M2 ≤ 2M3 M2 ,
p+1 [m/2]
m=0
i=0
|am−2i |ρ0m−2i M2 ≤ (p + 2)M3 M2 , then
∞
m
≤ (p + 2)M2 M3 + 2M2 M3 = (p + 2)M2 + 2M2 M3 = C2
cm x (2.10)
m=0
for all x ∈ (−ρ0 ,ρ0 ).
∞
Lemma 2. Suppose that the power series m=0 am xm converges
for all x ∈ (−ρ,ρ) with some
m
c ’s given in (2.2) is
positive ρ. Let ρ1 = min {1,ρ}. Then the power series ∞
m=0 cm x with
m m
convergent for all x ∈ (−ρ1 ,ρ1 ). Further, for any positive ρ0 < ρ1 , | ∞
m=0 cm x | ≤ C for any
x ∈ (−ρ0 ,ρ0 ) and for some positive constant C which depends on ρ0 .
Proof. The first statement follows from the latter statement. Therefore, let us prove the
latter
statement. If ρ ≤ 1, then ρ1 = ρ. By Lemma 1(b), for any positive ρ0 < ρ = ρ1 ,
∞
| m=0 cm xm | ≤ C2 for x ∈ (−ρ0 ,ρ0 ) and for some positive constant C2 which depends
on ρ0 .
B. Kim and S.-M. Jung 5
If ρ > 1, then by Lemma 1(a), for any positive ρ0 < 1 = ρ1 ,
∞
C1
C1
m
≤
c
x
<
=C
m
1 − |x| 1 − ρ
0
m=0
(2.11)
for x ∈ (−ρ0 ,ρ0 ) and for some positive constant C which depends on ρ0 .
Using these definitions and the lemmas above, we will show that
ticular solution of the inhomogeneous Bessel equation (1.3).
∞
m=0 cm
xm
is a par-
Theorem 2.1. Assume that ν is a given positive nonintegral number and the radius of
m
convergence of the power series ∞
m=0 am x is ρ. Let ρ1 = min {1,ρ}. Then, every solution
y : (−ρ1 ,ρ1 )→C of the differential equation (1.3) can be expressed by
∞
y(x) = yh (x) +
cm xm ,
(2.12)
m=0
where yh (x) is a Bessel function and cm ’s are given by(2.2).
∞
m
m
Proof. We show that ∞
m=0 cm x satisfies (1.3). By Lemma 2, the power series
m=0 cm x
is convergent foreach x ∈ (−ρ1 ,ρ1 ).
m
Substituting ∞
m=0 cm x for y(x) in (1.3) and collecting like powers together, we have
x2 y (x) + xy (x) + x2 − ν2 y(x)
∞
= −ν2 c0 − ν2 − 1 c1 x +
cm − ν2 − (m + 2)2 cm+2 xm+2
m=0
= a0 + a1 x +
∞
am+2 xm+2 =
m=0
∞
(2.13)
am x m
m=0
for all x ∈ (−ρ1 ,ρ1 ) by (2.3).
Therefore, every solution y : (−ρ1 ,ρ1 )→C of the differential equation (1.3) can be expressed by
y(x) = yh (x) +
∞
cm xm ,
(2.14)
m=0
where yh (x) is a Bessel function.
3. Partial solution to Hyers-Ulam stability problem
In this section, we will investigate a property of the Bessel differential equation (2.1) concerning the Hyers-Ulam stability problem. That is, we will try to answer the question
whether there exists a Bessel function near any approximate Bessel function.
Theorem 3.1. Let y : (−ρ,ρ)→C be a given analytic function which can be represented by
a power-series expansion centered at x = 0. Suppose there exists a constant ε > 0 such that
2 x y (x) + xy (x) + x2 − ν2 y(x) ≤ ε
(3.1)
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Journal of Inequalities and Applications
number ν. Let ρ1 = min {1,ρ}. Suppose,
for all x ∈ (−ρ,ρ) and for some positive nonintegral
m
further, that x2 y (x) + xy (x) + (x2 − ν2 )y(x) = ∞
m=0 am x satisfies
∞
∞
m
am x m ≤ K a
x
m
m=0
(3.2)
m=0
for all x ∈ (−ρ,ρ) and for some constant K. Then there exists a Bessel function yh : (−ρ1 ,ρ1 )→
C such that
y(x) − yh (x) ≤ Cε
(3.3)
for all x ∈ (−ρ0 ,ρ0 ), where ρ0 < ρ1 is any positive number and C is some constant which
depends on ρ0 .
Proof. We assumed that y(x) can be represented by a power series and
x2 y (x) + xy (x) + x2 − ν2 y(x) =
∞
am x m
(3.4)
m=0
also satisfies
∞
∞
m
am x m ≤ K ≤ Kε
a
x
m
m=0
(3.5)
m=0
for all x ∈ (−ρ,ρ) from (3.1).
m
According to Theorem 2.1, y can be written as yh + ∞
m=0 cm x for x ∈ (−ρ1 ,ρ1 ), where
yh is some Bessel function and cm ’s are given by (2.2). Then by Lemmas 1 and 2 and their
proofs (replace M1 and M3 with Kε in Lemma 1),
∞
m
y(x) − yh (x) = c
x
m
≤ Cε
(3.6)
m=0
for all x ∈ (−ρ0 ,ρ0 ), where ρ0 < ρ1 is any positive number and C is some constant which
depends on ρ0 . This completes the proof of our theorem.
4. Example
In this section, our task is to show that there certainly exist functions y(x) which satisfy
all the conditions given in Theorem 3.1.
Example 1. Let y : (−1,1)→R be an analytic function given by
y(x) = J1/2 (x) + b x2 + x4 + · · · + x2n ,
(4.1)
where J1/2 (x) is the Bessel function of the first kind of order 1/2, n is a given positive
integer, and b is a constant satisfying
0≤b≤
2
17
n 2n2 + 3n +
3
8
−1
ε
(4.2)
B. Kim and S.-M. Jung 7
for some ε ≥ 0. Since J1/2 (x) is a particular solution of the Bessel differential equation
(2.1) with ν = 1/2, we then have
n 2 3
15
1
2n+2
y(x) = bx
+
2m + bx2m + bx2 .
x y (x) + xy (x) + x −
4
4
4
m=2
2 2
(4.3)
If we set
⎧
⎪
b
⎪
⎪
⎪
⎪
⎪
⎪
⎪
3
⎪
⎪
b
⎨ m2 +
am = ⎪ 4
⎪
15
⎪
⎪
b
⎪
⎪
⎪
4
⎪
⎪
⎪
⎩
0
form = 2n + 2,
for m ∈ {4,6,...,2n},
(4.4)
for m = 2,
otherwise,
then we obtain
x2 y (x) + xy (x) + x2 −
∞
1
y(x) =
am x m
4
m=0
(4.5)
for all x ∈ (−1,1). It further follows from (4.2) and (4.4) that
∞
∞
m
am x m = ≤ε
am x m=0
(4.6)
m=0
for any x ∈ (−1,1).
Indeed, if we choose the J1/2 (x) as a Bessel function, then we have
−1
y(x) − J1/2 (x) = bx2 + x4 + · · · + x2n ≤ nb ≤ n 2 n 2n2 + 3n + 17
ε
3
8
(4.7)
for all x ∈ (−1,1), which is consistent with the assertion of Theorem 3.1.
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and Applied Analysis.
Byungbae Kim: Mathematics Section, College of Science and Technology, Hong-Ik University,
Chochiwon 339-701, South Korea
Email address: bkim@hongik.ac.kr
Soon-Mo Jung: Mathematics Section, College of Science and Technology, Hong-Ik University,
Chochiwon 339-701, South Korea
Email address: smjung@hongik.ac.kr