Abstract
In this paper, we study the nonlinear equation of the form <img border=0 src="../../../../../../img/revistas/cam/v28n2/a02ent02.gif"> where is <img border=0 src="../../../../../../img/revistas/cam/v28n2/a02tex03.gif" align=absmiddle>the ultra-hyperbolic operator iterated k-times, defined by <img border=0 src="../../../../../../img/revistas/cam/v28n2/a02ent03.gif" align=absmiddle>, p + q = n is the dimension of the Euclidean space <img border=0 src="../../../../../../img/revistas/cam/v28n2/a02tex02.gif" align=absmiddle>n, (x, t) = (x1, x2,..., xn, t) <img border=0 src="../../../../../../img/revistas/cam/v28n2/a01ent09.gif" align=absmiddle><img border=0 src="../../../../../../img/revistas/cam/v28n2/a02tex02.gif" align=absmiddle>n× (0, <img border=0 src="../../../../../../img/revistas/cam/v28n2/a06tex01.gif">), k is a positive integer and c is a positive constant. On the suitable conditions for f , u and for the spectrum of the heat kernel, we can find the unique solution in the compact subset of <img border=0 src="../../../../../../img/revistas/cam/v28n2/a02tex02.gif" align=absmiddle>n × (0, <img border=0 src="../../../../../../img/revistas/cam/v28n2/a06tex01.gif">). Moreover, if we put k = 1 and q = 0 we obtain the solution of nonlinear equation related to the heat equation. Mathematical subject classification: 35L30, 46F12, 32W30.
ultra-hyperbolic heat equation; the Dirac delta distribution; the spectrum; Fourier transform
On the generalized nonlinear ultra-hyperbolic heatequation related to the spectrum
Amnuay KananthaiI; Kamsing NonlaoponII,* * Supported by The Commission on Higher Education and the Thailand Research Fund(MRG5180058).
IDepartment of Mathematics, Chiang Mai University, Chiang Mai 50200, Thailand E-mail: malamnka@science.cmu.ac.th
IIDepartment of Mathematics, Khon Kaen University, Khon Kaen 40002, Thailand E-mail: nkamsi@kku.ac.th
ABSTRACT
In this paper, we study the nonlinear equation of the form
where is the ultra-hyperbolic operator iterated k-times, defined by
,
p + q = n is the dimension of the Euclidean space n, (x, t) = (x1, x2,..., xn, t)
n× (0, ), k is a positive integer and c is a positive constant.On the suitable conditions for f , u and for the spectrum of the heat kernel, we can find the unique solution in the compact subset of n × (0, ). Moreover, if we put k = 1 and q = 0 we obtain the solution of nonlinear equation related to the heat equation.
Mathematical subject classification: 35L30, 46F12, 32W30.
Key words: ultra-hyperbolic heat equation, the Dirac delta distribution, the spectrum, Fourier transform.
1 Introduction
It is well known that for the heat equation
with the initial condition
where Δ = is the Laplace operator and (x, t) = (x1, x2,..., xn, t) n × (0, ), and is a continuous function, we obtain the solution
as the solution of (1.1).
Now, (1.2) can be written as u(x, t) = E(x, t) *(x) where
E(x, t) is called the heat kernel, where |x|2 = x12 + x22 +...+ xn2and t > 0, see [1, p. 208-209].
Moreover, we obtain E(x, t) δ as t 0, where δ is the Dirac-delta distribution. We also have extended (1.1) to be the equation
where is the ultra-hyperbolic operator, defined by
.
We obtain the ultra-hyperbolic heat kernel
where p + q = n is the dimension of the Euclidean space n and i =. For finding the kernel E(x, t) see [4].
In this paper, we extend (1.4) to be the general of the nonlinear form
for (x, t)
n ×(0,) and with the following conditions on u and as follows,(1) u(x, t) C(2k)(n) for any t > 0 where C(2k) (n ) is the space of continuous function with 2k-derivatives.
(2) satisfies the Lipchitz condition, that is
where A is constant and 0 < A < 1.
(3)
for x = (x1, x2,..., xn)
n, t (0, ) and u(x, t) is continuous function on n × (0, ).
Under such conditions of , u and for the spectrum of E(x, t), we obtain the convolution
as a unique solution in the compact subset of
n × (0, ) and E(x, t) is an elementary solution defined by (2.5).2 Preliminaries
Definition 2.1.Let (x) -the space of integrable function in n. The Fourier transform of (x) is defined by
where ξ = (ξ1,ξ2,...,ξn),x = (x1, x2,..., xn)
n, (ξ, x) = ξ1x1 + ξ2x2 + ...+ ξnxn is the usual inner product in n and dx= dx1dx2 ... dxn.Also, the inverse of Fourier transform is defined by
Definition 2.2. The spectrum of the kernel E(x, t) defined by (2.5) is the bounded support of the Fourier transform for any fixed t > 0.
Definition 2.3. Let ξ = (ξ1,ξ2,...,ξ0) be a point in
n and we write.
Denote by
and
the set of an interior of the forward cone, and + denotes the closure of Γ+.
Let Ω be spectrum of E(x, t) defined by Definition 2.2 for any fixed t > 0 and Ω +. Let
be the Fourier transform of E(x, t) and define
Lemma 2.1. Let L be the operator defined by
where
is the ultra-hyperbolic operator iterated k-times defined by k,
p + q = n is the dimension of
n, (x1, x2,..., xn) n, t (0, ), k is a positive integer and c is a positive constant. Then we obtain
as a elementary solution of (2.4) in the spectrum Ω
n for t > 0.Proof. Let LE(x, t) = δ(x, t) where E(x, t) is the kernel or the elementary solution of operator L and δ is the Dirac-delta distribution. Thus
Take the Fourier transform defined by (2.1) to both sides of the equation, weobtain
.
Thus
where H(t) is the Heaviside function. Since H(t) = 1 for t > 0. Therefore,
which has been already defined by (2.3). Thus
where Ω is the spectrum of E(x, t). Thus from (2.3)
Definition 2.4. Let us extend E(x, t) to
n × Rby setting
3 Main Results
Theorem 3.1.The kernel E(x, t) defined by (2.5) have the following properties:
(1) E(x, t) C-the space infinitely differentiable.
(2)
E(x, t) = 0 for t > 0.(3)
, for ,
where M(t) is a function of t in the spectrum Ω and Γ denote the Gamma function. Thus E(x, t) is bounded for any fixed t > 0.
(4)
Proof.
(1) From (2.5), since
.
Thus E(x, t) Cfor x
n, t > 0.(2) By computing directly, we obtain
.
(3) We have
By changing to bipolar coordinates
and
where and . Thus 1 1
where dξ = rp-1sq-1 dr ds dΩp dΩq , dΩp and Ωq are the elements of surface area of the unit sphere in p and q respectively. Since 1
n is the spectrum of E(x, t) and we suppose 0 < r < R and 0 < s < L where R and L are constants. Thus we obtain(4) By (2.5), we have
Theorem 3.2. Given the nonlinear equation
for (x, t)
n × (0, ), k is positive number and with the following conditions on u and f as follows,(1) u(x, t) C(2k)(n) for any t > 0 where C(2k)(n) is the space of continuous function with 2k-derivatives.
(2) satisfies the Lipchitz condition, that is
where A is constant and 0 < A < 1.
(3)
for x = (x1, x2,..., xn)
n, t (0, ) and u(x, t) is continuous function on n × (0, ).
Then, for the spectrum of E(x, t) we obtain the convolution
as a unique solution of (3.3) for x 10 where 10 is an compact subset of
n, 0 < t < T with T is constant and E(x, t) is an elementary solution defined by (2.5) and also u(x, t) is bounded.In particular, if we put k = 1 and q = 0 in (3.3) then (3.3) reduces to the nonlinear heat equation.
Proof. Convolving both sides of (3.3) with E(x, t) and then we obtain the solution
or
where E(r, s) is given by Definition 2.4.
We next show that u(x, t) is bounded on n × (0, ). We have
by the condition (3) and (3.1) where
Thus u(x, t) is bounded on n × (0, ).
To show that u(x, t) is unique, suppose there is another solution w(x, t) of equation (3.3). Let the operator
then (3.3) can be written in the form
.
Thus
.
By the condition (2) of the Theorem,
Let Ω0 × (0, T ] be compact subset of n × (0, ) and L : C(2k)(Ω0) C(2k)(10) for 0 < t < T .
Now (C(2k)(Ω0), ) is a Banach space where u(x, t) C(2k)(10) for 0 < t < T , given by
Then, from (3.5) with 0 < A < 1, the operator l is a contraction mapping on C(2k)(Ω0). Since (C(2k)(10), )is a Banach space and L : C(2k)(Ω0) C(2k)(Ω0) is a contraction mapping on C(2k)(Ω0), by Contraction Theorem, see [3, p. 300], we obtain the operator L has a fixed point and has uniqueness property. Thus u(x, t) = w(x, t). It follows that the solution u(x, t) of (3.3) is unique for (x, t) Ω0 × (0, T ] where u(x, t) is defined by (3.4).
In particular, if we put k = 1 and q = 0 in (3.3) then (3.3) reduces to the nonlinear heat equation
which has solution
where E(x, t) is defined by (2.5) with k = 1 and q = 0. That is complete of proof.
Acknowledgement. The authors would like to thank The Thailand ResearchFund for financial support.
Received: 07/III/08. Accepted: 08/III/09.
#752/08.
- [1] F. John, "Partial Differential Equations", 4th Edition, Springer-Verlag, New York (1982).
- [2] R. Haberman, "Elementary Applied Partial Differential Equations", 2nd Edition, Prentice-Hall International, Inc. (1983).
- [3] E. Kreyszig, "Introductory Functional Analysis with Applications", John Wiley & Sons Inc. (1978).
- [4] K. Nonlaopon and A. Kananthai, On the Ultrahyperbolic Heat Kernel, International Journal of Applied Mathematics, 13 (2) (2003), 215-225.
Publication Dates
-
Publication in this collection
14 July 2009 -
Date of issue
2009
History
-
Received
07 Mar 2008 -
Accepted
08 Mar 2009