# Electronic Journal of Differential Equations, Vol. 2009(2009), No. 77, pp.... ISSN: 1072-6691. URL: or ```Electronic Journal of Differential Equations, Vol. 2009(2009), No. 77, pp. 1–13.
ISSN: 1072-6691. URL: http://ejde.math.txstate.edu or http://ejde.math.unt.edu
ftp ejde.math.txstate.edu
REMARKS ON A 2-D NONLINEAR BACKWARD HEAT
PROBLEM USING A TRUNCATED FOURIER SERIES METHOD
DANG DUC TRONG, NGUYEN HUY TUAN
Abstract. The inverse conduction problem arises when experimental measurements are taken in the interior of a body, and it is desired to calculate
temperature on the surface. We consider the problem of finding, from the
final data u(x, y, T ) = ϕ(x, y), the initial data u(x, y, 0) of the temperature
function u(x, y, t), (x, y) ∈ U ≡ (0, π) &times; (0, π), t ∈ [0, T ] satisfying the nonlinear system
ut − ∆u = f (x, y, t, u(x, y, t)),
(x, y, t) ∈ U &times; (0, T ),
u(0, y, t) = u(π, y, t) = u(x, 0, t) = u(x, π, t) = 0,
(x, y, t) ∈ U &times; (0, T ).
This problem is known to be ill-posed, as the solution exhibits unstable dependence on the given data functions. Using the Fourier series method, we
regularize the problem and to get some new error estimates. A numerical
experiment is given.
1. Introduction
In this paper, we consider the following two dimensional problem in an rectangle
U = (0, π) &times; (0, π)
ut − ∆u = g(x, y, t, u(x, y, t))
(x, y, t) ∈ U &times; (0, T ), U = (0, π) &times; (0, π)
u(0, y, t) = u(π, y, t) = u(x, 0, t) = u(x, π, t) = 0
(x, y, t) ∈ U &times; [0, T ]
u(x, y, T ) = ϕ(x, y) x, y ∈ U.
(1.1)
(1.2)
(1.3)
where we want to determine the temperature distribution u(., ., t) for 0 ≤ t &lt; T
from the data ϕ(x, y) . The problem is called the backward heat problem (BHP),
the backward Cauchy problem or the final value problem. This is a typical ill-posed
problem. In general no solution which satisfies the heat conduction equation with
final data and the boundary conditions exists. Even if the solution exists, it will not
be continuously dependent on the final data such that the numerical simulations
are very difficult and some special regularization methods are required. In the
context of approximation method for this problem, many approaches have been
investigated. In the mathematical literature various methods have been proposed
for solving backward Cauchy problems. Such authors as Lattes and Lions, Miller
2000 Mathematics Subject Classification. 35K05, 35K99, 47J06, 47H10.
Key words and phrases. Backward heat problem; nonlinearly ill-posed problem;
Fourier series; contraction principle.
c
2009
Texas State University - San Marcos.
Submitted December 11, 2008. Published June 16, 2009.
Supported by the Council for Natural Sciences of Vietnam.
1
2
D. D. TRONG, N. H. TUAN
EJDE-2009/77
have approximated BHP by quasi-reversibility methods (QR method for short). In
1983, Showalter, in , presented a different method called the quasiboundary
value (QBV method) method to regularize that linear homogeneous problem which
gave a stability estimate better than the one of disscused method. The main ideas
of the method is of adding an appropriate “corrector” to the final data. Using the
method, Clark and Oppenheimer, in , and Denche-Bessila, very recently in ,
regularized the backward problem by replacing the final condition by u(T )+u(0) =
g and u(T ) − u0 (0) = g respectively.
Although there are many papers on the linear homogeneous case of the backward
problem, we only find a few papers on the nonhomogeneous and nonlinear cases of
BHP. We can notably mention the method of QBV and modified quasi-reversibility
to solve the one dimensional nonlinear backward heat problem (NBHP), such as
. Moreover, the two dimensional case of NBHP is very scarce and it is not
considered by QR or QBV methods. To the authors’s knowledge, in some recent
papers on the nonlinear backward heat, the error estimates of most regularization
methods are the form Ct/T . It makes difficult to solve the error in the time t = 0.
To improve this, we develop a new regularization method which is called Fourier
method for solving the Problem (1.1)-(1.3). As far as we know, there are not
any results of Fourier series method for treating NBHP until now. Meanwhile, we
will establish faster convergence results via improved error estimates. Especially,
the convergence of the approximate solution at t = 0 is also proved. This is an
improvement of known results in [15, 20, 22].
Informally, problem (1.1)-(1.3) can be transformed to an integral equation
u(x, y, t) =
∞ X
e−(t−T )(n
2
+m2 )
ϕnm
n,m=1
Z
−
T
(1.4)
−(t−s)(n2 +m2 )
e
gnm (u)(s)ds sin(nx) sin(my),
t
where
ϕ(x, y) =
g(u)(x, t) =
∞
X
ϕnm sin(nx) sin(my),
n,m=1
∞
X
gnm (u)(t) sin(nx) sin(my),
n,m=1
are the expansion of ϕ and g(u), respectively. Since t &lt; T , we know from (1.4) that,
when m2 + n2 becomes large, exp{(T − t)(m2 + n2 )} increases rather quickly. Thus,
2
2
the term e−(t−T )(m +n ) is the cause of the instability. So, we hope to recover the
stability of problem (1.4) by filtering the high frequencies with suitable method.
The essence of our regularization method is just to eliminate all high frequencies
from the solution, and instead consider (1.4) only for m2 + n2 ≤ aβ , where aβ is
an appropriate positive constant satisfying limβ→0 aβ = ∞. We note that aβ is a
constant which will be selected appropriately as regularization parameter. Then, we
get a stable and convergent iteration scheme. We have the following approximation
EJDE-2009/77
A TRUNCATED FOURIER SERIES METHOD
3
problem
v
β,aβ
m,n≥1
X
(x, y, t) =
e(T −t)(n
2
+m2 )
ϕnm
m2 +n2 ≤aβ
Z
(1.5)
T
e(s−t)(n
−
2
+m2 )
gnm (v β,aβ )(s)ds sin(nx) sin(my)
t
where
ϕnm =
4
hϕ(x, y), sin(nx) sin(my)i,
π2
4
hg(x, y, t, u(x, y, t)), sin(nx) sin(my)i,
π2
and h&middot;, &middot;i is inner product in L2 (U ).
gnm (u)(t) =
2. Fourier regularization and the main results
For clarity, we denote the solution of (1.1)-(1.3) by u(., ., t), and the solution of
Theorem 2.1. Let ϕ ∈ L2 (U ) and let g ∈ L∞ (U &times; [0, T ] &times; R) satisfy
|g(x, y, t, w) − g(x, y, t, u)| ≤ k|w − u|
for a k &gt; 0 independent of x, y, t, w, u. Then (1.5) has a unique solution v β,aβ ∈
C([0, T ]; H01 (U )) ∩ C 1 ((0, T ); L2 (U )).
Theorem 2.2. The solution of (1.5) depends continuously on ϕ in L2 (U ).
Theorem 2.3. Let ϕ, g be as in Theorem 2.1. If ∂g
∂z (x, y, t, z) is bounded on U &times;
(0, T ) &times; R then (1.1)-(1.3) has at most one solution
u ∈ C([0, T ]; H01 (U )) ∩ C 1 ((0, T ); L2 (U )).
Theorem 2.4. Let ϕ, g be as in Theorem 2.1. Suppose that (1.1)-(1.3) has a unique
solution u(x, y, t) in C([0, T ]; H01 (U )) ∩ C 1 ((0, T ); L2 (U )) which satisfies
Z T X
∞
2
2
2
e2s(n +m ) gnm
(u)(s)ds &lt; ∞.
(2.1)
0
n,m=1
Then
√
2
ku(., ., t) − v β,aβ (., ., t)k ≤ M ek T (T −t) e−taβ
for every t ∈ [0, T ], where
Z T X
∞
2
2
2
M = 4ku(0)k2 + π 2 T
e2s(n +m ) gnm
(u)(s)ds,
0
n,m=1
β,aβ
and v
is the unique solution of (1.5) corresponding to β. Moreover, if
L2 ((0, T ); L2 (U )), then there exists a tβ such that
√ q
ku(., ., 0) − v β,aβ (., ., tβ )k ≤ 2C 4 1/aβ ,
where
N=
Z
0
T
k
(2.2)
1/2
∂u
(., ., s)k2 ds
,
∂s
C = max{M, N }.
∂u
∂t
∈
4
D. D. TRONG, N. H. TUAN
EJDE-2009/77
Remark 2.5. (1) In the simple case of the function g(., ., u) = 0 (it follows that
k=0), we have
ku(., ., t) − v β,aβ (., ., t)k ≤ 2ku(., ., 0)ke−taβ .
Choosing aβ = T1 ln(1/β), we obtain the error estimate
ku(., ., t) − v β,aβ (., ., t)k ≤ 2ku(., ., 0)k.β t/T
This error is given in .
(2) In most known results, such as [15, 20, 22], the errors between the exact
solution and approximate solution can be calculated in the form Ct/T . Notice that
the convergence estimate in this Theorem does not give any useful information on
the continuous dependence of the solution at t = 0. It is easy to see that if taking
t = 0 in (2.2), the error estimate is as follows
√
2 2
ku(., ., 0) − v β,aβ (., ., 0)k ≤ M ek T
does not tend to zero when β → 0. So, the convergence of the approximate is large
when t → 0. In next Theorem, we will give a good estimate in which the error in
the case t = 0 is considered.
(3) In this Theorem, we ask for a condition on the expansion coefficient gnm in
(2.1). We note that the solution u depends on the nonlinear term g and therefore
gnm , gnm (u) is very difficult to be valued. Such a obscurity makes this Theorem
hard to be used for numerical computations. Hence, we ask the condition as follows
∞
X
e2t(n
2
+m2 )
| &lt; u(., ., t), sin(nx) sin(my) &gt; |2 &lt; ∞.
(2.3)
n,m=1
In this case, we only require the assumption of u, not need to compute the function
gnm (u). In the homogeneous case of problem (1.1)-(1.3),i.e., g = 0, then the right
hand side of (2.3) is equal to ku(., ., 0)k2 . Hence, the condition (2.3) is natural and
acceptable.
Theorem 2.6. Let ϕ, g be as in Theorem 2.1. Suppose (1.1)-(1.3) has a unique
solution u(x, y, t) satisfying (2.3). Then we have
ku(., ., t) − v β,aβ (., ., t)k ≤ Q(β, t, u)e−taβ
for every t ∈ [0, T ], where
Z
2
Q(β, t, u) = 2k 2 T e2k T (T −t)
T
P (β, s, u)ds +
0
(2.4)
1/2
π2
P (β, t, u)
2
(2.5)
and
P (β, t, u) =
X
eT (n
2
+m2 )
Z
ϕnm −
t
m,n≥1,m2 +n2 ≥aβ
=
X
e2t(n
2
T
es(n
2
+m2 )
gnm (u)(s)ds
2
(2.6)
+m2 ) 2
unm
m,n≥1,m2 +n2 ≥aβ
and v β,aβ is the unique solution of Problem (1.5).
Remark 2.7. If we let t = 0 in (2.4), we get the error at the original time,
Z T
π2
β,aβ
2
2
2k2 T 2
ku(., ., 0) − v
(., ., 0)k ≤ 2k T e
P (β, s, u)ds +
P (β, 0, u). (2.7)
2
0
EJDE-2009/77
A TRUNCATED FOURIER SERIES METHOD
5
Noting that the right hand side of (2.7) tends to zero when β → 0.
For non-exact data, we have the following result.
Theorem 2.8. Let ϕ, g be as in Theorem 2.1. Assume that the exact solution u of
(1.1)-(1.3) corresponding to ϕ be defined as in Theorem 2.4. Let ϕβ ∈ L2 (U ) be a
measured data such that
kϕβ − ϕk ≤ β.
1
Suppose the problem (1.1)-(1.3) has a unique solution u ∈
C([0, T ]; H0 (U )) ∩
1 1/T
1 −α/(2T )
1
2
C ((0, T ); L (U )). Let us select aβ = ln ( β ) (ln β )
.
(i) If u satisfies (2.1) then for t ∈ (0, T ), there exists a function v β,aβ satisfying
2
1 α
1 −α(T −t) 1 + (ln ) 2 , (2.8)
kv β,aβ (., ., t) − u(., ., t)k ≤ (M + 1)ek T (T −t) β t/T (ln ) 2T
β
β
and
q
√ kv β,aβ (., ., 0) − u(., ., 0)k ≤ 4 1/aβ 2 exp(k 2 T 2 ) + 2C
where
M = 3ku(., ., 0)k2 + 3π 2 T
Z
0
T
∞
X
e2s(n
2
+m2 ) 2
gnm (u)(s)ds
n,m=1
and C is defined in Theorem 2.4.
(ii) If u such that the condition (2.3) then for all t ∈ [0, T ]
kwβ,aβ (., ., t) − u(., ., t)k
1 α
1 −α(T −t) exp(k 2 (T − t)2 ) + Q(β, t, u)(ln ) 2 ,
≤ β t/T (ln ) 2T
β
β
(2.9)
where wβ,aβ be the solution of problem (1.5) corresponding to ϕβ .
Remark 2.9. (1) If we let α = 0 in (2.8), we have the simple error
kv β,aβ (., ., t) − u(., ., t)k ≤ (M + 1)ek
2
T (T −t) t/T
β
,
∀t ∈ (0, T ).
(2.10)
This error is similar to the one given in . Notice that the right hand side of
(2.10) does not converges to 0. This is disadvantage point of the error (2.8).
(2) In the error (2.9), if we let t = 0, we get
1 −α
kwβ,aβ (., ., 0) − u(., ., 0)k ≤ exp(k 2 T 2 )(ln ) 2 + Q(β, 0, u).
(2.11)
β
Notice that if α &gt; 0 then the right hand side of (2.11) converges to 0 and the
Theorem 2.8(ii) is a generalization of the result given in .
3. Proof of the main results
Proof of theorem 2.1. Put
G(v β,aβ )(x, y, t)
= Ψ(x, y, t) −
Z
X
m,n≥1,m2 +n2 ≤aβ
T
e(s−t)(n
2
+m2 )
gnm (v β,aβ )(s)ds sin(nx) sin(my)
t
where
Ψ(x, y, t) =
X
m,n≥1,m2 +n2 ≤aβ
e(T −t)(n
2
+m2 )
ϕnm sin(nx) sin(my).
6
D. D. TRONG, N. H. TUAN
EJDE-2009/77
We claim that
kGp (v β,aβ )(., ., t) − Gp (wβ,aβ )(., ., t)k2
≤ k 2p e2T paβ
(T − t)p C p
|||v β,aβ − wβ,aβ |||2
p!
(3.1)
for every p ≥ 1, where C = max{T, 1} and ||| &middot; ||| is sup norm in C([0, T ]; L2 (U )).
We shall prove the latter inequality by induction. For p = 1, we have
kG(v β,aβ )(., ., t) − G(wβ,aβ )(., ., t)k22
h
i2
X
2
2
π2
=
e(s−t)(n +m ) (gnm (v β,aβ )(s) − gnm (wβ,aβ )(s))ds
4
m,n≥1,m2 +n2 ≤aβ
Z T
Z T
X
2
2
π2
(e2(s−t)(n +m ) ds
≤
(gnm (v β,aβ )(s)
4
t
t
2
2
m,n≥1,m +n ≤aβ
− gnm (wβ,aβ )(s))2 ds
Z T
X
π2
≤
e2T aβ (T − t)
(gnm (v β,aβ )(s) − gnm (wβ,aβ )(s))2 ds
4
t
m,n≥1,m2 +n2 ≤aβ
Z
T
X
π 2 2T aβ
e
(T − t)
=
(gnm (v β,aβ )(s) − gnm (wβ,aβ )(s))2 ds
4
t m,n≥1,m2 +n2 ≤a
β
Z T Z πZ π
≤ e2T aβ (T − t)
(g(x, y, s, v β,aβ (x, y, s))
t
0
0
− g(x, y, s, wβ,aβ (x, y, s)))2 dx dy ds
Z T Z πZ π
≤ k 2 e2T aβ (T − t)
|v β,aβ (x, y, s) − wβ,aβ (x, y, s)|2 dx dy ds
t
0
0
≤ Ck 2 e2T aβ (T − t)|||v β,aβ − wβ,aβ |||2 .
Thus (3.1) holds. Suppose that (3.1) holds for p = j. We prove that (3.1) holds for
p = j + 1. We have
kGj+1 (v β,aβ )(., ., t) − Gj+1 (wβ,aβ )(., ., t)k2
hZ T
X
2
2
π2
(e2(s−t)(n +m ) ds gnm (Gj (v β,aβ ))(s)
=
4
t
2
2
m,n≥1,m +n ≤aβ
i2
− gnm (Gj (wβ,aβ ))(s) ds
hZ T
i2
X
π 2 2T aβ
e
|gnm (Gj (v β,aβ ))(s) − gnm (Gj (wβ,aβ ))(s)|ds
≤
4
t
2
2
m,n≥1,m +n ≤aβ
Z T X
∞
π 2T aβ
e
(T − t)
|gnm (Gj (v β,aβ ))(s) − gnm (Gj (wβ,aβ ))(s)|2 ds
4
t n,m=1
Z T
kg(., ., s, Gj (v β,aβ )(., ., s)) − g(., ., s, Gj (wβ,aβ )(., ., s))k2 ds
≤ e2T aβ (T − t)
2
≤
t
2T aβ
≤e
(T − t)k
2
Z
t
T
kGj (v β,aβ )(., ., s) − Gj (wβ,aβ )(., ., s)k2 ds
EJDE-2009/77
A TRUNCATED FOURIER SERIES METHOD
≤ e2T aβ (T − t)k 2 k 2j e2T jaβ
T
Z
t
≤k
2(j+1) 2T (j+1)aβ
e
7
(T − s)j
dsC j |||v β,aβ − wβ,aβ |||2
j!
(T − t)j+1 j+1 β,aβ
C
|||v
− wβ,aβ |||2 .
(j + 1)!
Therefore,
kGp (v β,aβ )(., ., t) − Gp (wβ,aβ )(., ., t)k2 ≤ k 2p e2T paβ
(T − t)p C p
|||v β,aβ − wβ,aβ |||2
p!
for all v β,aβ , wβ,aβ ∈ C([0, T ]; L2 (U )). We consider
G : C([0, T ]; L2 (U )) → C([0, T ]; L2 (U )).
p/2
p
Since limp→∞ k p eT paβ T √p!C = 0, there exists a positive integer number p0 , such
that Gp0 is a contraction. It follows that the equation Gp0 (u) = u has a unique
solution v β,aβ ∈ C([0, T ]; L2 (U )). We claim that G(v β,aβ ) = v β,aβ . In fact, one has
G(Gp0 (v β,aβ )) = G(v β,aβ ).
Hence
Gp0 (G(v β,aβ )) = G(v β,aβ ).
By the uniqueness of the fixed point of Gp0 , one has G(v β,aβ ) = v β,aβ , i.e., the
equation G(v β,aβ ) = v β,aβ has a unique solution v β,aβ ∈ C([0, T ]; L2 (U )).
Proof of Theorem 2.2. Let u and v be two solutions of (1.5) corresponding to the
values ϕ and ω. We have
ku(., ., t) − v(., ., t)k2
X
π2
(T −t)(n2 +m2 )
(ϕnm − ωnm )
=
e
4
m,n≥1,m2 +n2 ≤aβ
Z T
2
2
2
−
e(s−t)(n +m ) (gnm (u)(s) − gnm (v)(s)ds)
t
π2
≤
2
X
(e(T −t)(n
2
+m2 )
|ϕnm − ωnm |)2
m,n≥1,m2 +n2 ≤aβ
π2
+
2
Z
X
T
e(s−t)(n
2
+m2 )
|gnm (u)(s) − gnm (v)(s)|ds
2
t
m,n≥1,m2 +n2 ≤aβ
Then, we obtain
ku(., ., t) − v(., ., t)k2 ≤
2(T −t)aβ
≤ 2e
2
2
−2taβ
Z
kϕ − ωk + 2k (T − t)e
T
e2saβ ku(., ., s) − v(., ., s)k2 ds.
t
Hence
e2taβ ku(., ., t) − v(., ., t)k2
≤ e2T aβ kϕ − ωk2 + 2k 2 (T − t)
Z
T
e2saβ ku(., ., s) − v(., ., s)k2 ds.
t
Using Gronwall’s inequality we have
ku(., ., t) − v(., ., t)k ≤ e(T −t)aβ exp(k 2 (T − t)2 )kϕ − ωk.
8
D. D. TRONG, N. H. TUAN
EJDE-2009/77
This completes the proof of the theorem.
Proof of Theorem 2.3. Let M &gt; 0 be such that
∂g
| (x, y, t, z)| ≤ M
∂z
for all (x, y, t, z) ∈ U &times; (0, T ) &times; R. Let u1 (x, y, t) and u2 (x, y, t) be two solutions
of Problem (1.1)-(1.3) such that u1 , u2 ∈ C([0, T ]; H01 (U )) ∩ C 1 ((0, T ); L2 (U )). Put
w(x, y, t) = u1 (x, y, t) − u2 (x, y, t). Then w satisfies the equation
wt (x, y, t) − ∆w(x, y, t) = g(x, y, t, u1 (x, y, t)) − g(x, y, t, u2 (x, y, t)).
Thus
wt (x, y, t) − ∆w(x, y, t) =
∂g
(x, y, t, u(x, y, t))w(x, y, t),
∂z
for some u(x, y, t). It follows that
(wt − ∆w)2 ≤ M 2 w2 .
Now w(0, y, t) = w(π, y, t) = w(x, 0, t) = w(x, π, t) = 0 and w(x, y, T ) = 0. Hence
by the Lees-Protter theorem [8, p. 373], w = 0 which gives u1 (x, y, t) = u2 (x, y, t)
for all t ∈ [0, T ]. The proof is completed.
Proof of Theorem 2.4. The functions u(., ., t) can be written in the form
∞
X
u(x, y, t) =
(e−(t−T )(n
2
+m2 )
ϕnm
n,m=1
T
Z
e−(t−s)(n
−
2
+m2 )
gnm (u)(s)ds) sin(nx) sin(my),
t
and v β,aβ (., ., t) in the form
X
v β,aβ (x, y, t) =
e(T −t)(n
2
+m2 )
ϕnm
m,n≥1,m2 +n2 ≤aβ
Z
T
−
e(s−t)(n
2
+m2 )
gnm (v β,aβ )(s)ds sin(nx) sin(my)
t
Hence
X
v β,aβ (x, y, t) − u(x, y, t) =
(e−(t−T )(n
2
+m2 )
ϕnm
m,n≥1,m2 +n2 ≥aβ
Z
−
T
e−(t−s)(n
2
+m2 )
gnm (u)(s)ds) sin(nx) sin(my)
t
+
X
m,n≥1,m2 +n2 ≤aβ
Z
T
e(s−t)(n
2
+m2 )
(gnm (v β,aβ )(s)
t
− gnm (v)(s))ds sin(nx) sin(my)
Using the inequality (a + b)2 ≤ 2(a2 + b2 ), we obtain
ku(., ., t) − v β,aβ (., ., t)k2
Z T
2
X
2
2
2
2
π2
e−(t−T )(n +m ) ϕnm −
e−(t−s)(n +m ) gnm (u)(s)ds
≤
2
t
2
2
m,n≥1,m +n ≥aβ
EJDE-2009/77
π2
2
+
≤2
A TRUNCATED FOURIER SERIES METHOD
Z
X
π2
(T − t)
2
+
2
+m2 )
|gnm (u)(s) − gnm (v β,aβ )(s)|ds
eT (n
2
+m2 )
Z
t
2
es(n
2
+m2 )
gnm (u)(s)ds
2
0
Z
X
t
es(n
2
+m2 )
gnm (u)(s)ds
2
0
m,n≥1,m2 +n2 ≥aβ
∞
X
T
T
ϕnm −
m,n≥1,m2 +n2 ≥a
π −2t(n2 +m2 )
e
2
Z
X
2
+2
e(s−t)(n
t
m,n≥1,m2 +n2 ≤aβ
π 2 −2t(n2 +m2 )
e
2
T
9
e2(s−t)aβ (gnm (u)(s) − gnm (v β,aβ )(s))2 ds
n,m=1
≤ 4e−2taβ ku(., ., 0)k2 + π 2 T e−2taβ
Z
T
e2s(n
2
+m2 ) 2
gnm (u)(s)ds
0
+ 2(T − t)e−2taβ
Z
T
e2saβ kg(., ., s, u(., ., s)) − g(., ., s, v β,aβ (., ., s))k2 ds
t
−2taβ
≤ 4e
2
2
−2taβ
Z
T
ku(., ., 0)k + π T e
e2s(n
2
+m2 ) 2
gnm (u)(s)ds
0
+ e−2taβ 2k 2 T
T
Z
e2saβ ku(., ., s) − v β,aβ (., ., s)k2 ds.
t
Then we obtain
e2taβ ku(., ., t) − v β,aβ (., ., t)k2
Z T
−2taβ
2
≤e
M + 2k T
e2saβ ku(., ., s) − v β,aβ (., ., s)k2 ds
t
Using Gronwall’s inequality, we obtain
e2taβ ku(., ., t) − v β,aβ (., ., t)k2 ≤ M e2k
2
T (T −t)
which implies
ku(., ., t) − v β,aβ (., ., t)k ≤
√
M ek
2
T (T −t) −taβ
e
.
(3.2)
Then we have the equality
Z
u(x, y, t) − u(x, y, 0) =
0
t
∂u
(x, y, s)ds .
∂s
It follows that
2
Z
ku(., ., 0) − u(., ., t)k ≤ t
0
t
∂u
(., ., s)2 ds ≤ N 2 t.
∂s
Combining this and (3.2), we have
ku(., ., 0) − v β,aβ (., ., t)k ≤ ku(., ., 0) − u(., ., t)k + ku(., ., t) − v β,aβ (., ., t)k
√
≤ C( t + e−taβ ).
√
lnt
For every β, there exists a tβ such that tβ = e−tβ aβ ; i.e., tββ = −2aβ . Using
√
the inequality ln t &gt; − 1t for every t &gt; 0, we obtain tβ ≤ 1/(2 aβ ). Hence,
√ q
ku(., ., 0) − v β,aβ (., ., tβ )k ≤ 2C 4 1/aβ .
10
D. D. TRONG, N. H. TUAN
EJDE-2009/77
This completes the proof of Theorem 2.4.
Proof of Theorem 2.6. We recall that
Z
X
T (n2 +m2 )
P (β, t, u) =
e
ϕnm −
=
es(n
2
+m2 )
gnm (u)(s)ds
2
t
m,n≥1,m2 +n2 ≥aβ
X
T
e2t(n
2
(3.3)
2
+m ) 2
unm .
m,n≥1,m2 +n2 ≥aβ
It is easy to prove that P (β, t, u) → 0 when β → 0. As in the proof of Theorem
2.4, we have
ku(., ., t) − v β,aβ (., ., t)k2
Z T
2
X
2
2
2
2
π2
≤
e−(t−T )(n +m ) ϕnm −
e−(t−s)(n +m ) gnm (u)(s)ds
2
t
m,n≥1,m2 +n2 ≥aβ
Z T
2
X
2
2
π2
e(s−t)(n +m ) |gnm (u)(s) − gnm (v β,aβ )(s)|ds
+
2
t
2
2
m,n≥1,m +n ≤aβ
2
π −2taβ
e
P (β, t, u)
2
Z T X
∞
π2
+
(T − t)
e2(s−t)aβ (gnm (u)(s) − gnm (v β,aβ )(s))2 ds
2
t n,m=1
Z T
2
π −2taβ
−2taβ
e
P (β, t, u) + 2(T − t)e
e2saβ g(., ., s, u(., ., s))
≤
2
t
2
β,aβ
− g(., ., s, v
(., ., s)) ds
Z T
2
π −2taβ
−2taβ
2
≤
e
P (β, t, u) + e
2k T
e2saβ ku(., ., s) − v β,aβ (., ., s)k2 ds.
2
t
This implies that
≤
e2taβ ku(., ., t) − v β,aβ (., ., t)k2
Z T
π 2 −2taβ
e
P (β, t, u) + 2k 2 T
e2saβ ku(., ., s) − v β,aβ (., ., s)k2 ds
2
t
Applying Gronwall’s inequality, we obtain
Z T
2
π2
2taβ
β,aβ
2
2
−2k2 T t
(., ., t)k ≤ 2k T e
e2k T s P (β, s, u)ds + P (β, t, u).
e
ku(., ., t) − v
2
t
Finally,
Z T
π2
β,aβ
2
2
2k2 T (T −t)
(., ., t)k ≤ 2k T e
P (β, s, u)ds + P (β, t, u) e−2taβ .
ku(., ., t) − v
2
0
This completes the proof of Theorem 2.6.
≤
β,a
Proof of Theorem 2.8. (i) Let v1 β be the solution of (1.5) corresponding to ϕ and
let wβ,aβ be the solution of (1.5) corresponding to ϕβ where ϕ, ϕβ are defined in
√
Theorem 2.8. Using Theorem 2.4, there exists a tβ such that tβ = e−2tβ aβ and
√ q
β,a
kv1 β (., ., tβ ) − u(., ., 0)k ≤ 2C 4 1/aβ .
(3.4)
EJDE-2009/77
A TRUNCATED FOURIER SERIES METHOD
11
We denote
v
β,aβ
(
wβ,aβ (., ., t),
0 &lt; t &lt; T,
(., ., t) =
β,aβ
w
(., ., tβ ), t = 0 .
Using Theorems 2.2 and 2.4, we obtain
kv β,aβ (., ., t) − u(., ., t)k
β,aβ
≤ kwβ,aβ (., ., t) − v1
(T −t)aβ
β,aβ
(., ., t)k + kv1
(., ., t) − u(., ., t)k
2
exp(k (T − t) )kϕ − ϕβ k + M ek T (T −t) e−taβ
2
1 −α(T −t)
1 αt
≤ exp(k 2 (T − t)2 )β t/T (ln ) 2T + M ek T (T −t) β t/T (ln ) 2T ,
β
β
1 −α(T −t)
1 α
k2 T (T −t) t/T
≤ (M + 1)e
β (ln ) 2T
1 + (ln ) 2
β
β
2
≤e
2
for every t ∈ (0, T ). Using the results in Theorem 2.4, we have the estimate
β,aβ
kv β,aβ (., ., 0) − u(., ., 0)k ≤ kwβ,aβ (., ., tβ ) − v1
(., ., tβ )k
β,a
kv1 β (., ., tβ )
− u(., ., 0)k
√ q
≤ 2e−tβ aβ exp(k 2 T 2 ) + 2C 4 1/aβ
q
√ q
= 2 4 1/aβ exp(k 2 T 2 ) + 2C 4 1/aβ
q
√ = 4 1/aβ 2 exp(k 2 T 2 ) + 2C .
+
This completes the proof of part (i).
(ii) Using Theorems 2.2 and 2.6, we obtain
kwβ,aβ (., ., t) − u(., ., t)k
β,aβ
≤ kwβ,aβ (., ., t) − v1
(T −t)aβ
β,aβ
(., ., t)k + kv1
(., ., t) − u(., ., t)k
exp(k (T − t) )kϕ − ϕβ k + Q(β, t, u)e−taβ
1 αt
1 −α(T −t)
≤ exp(k 2 (T − t)2 )β t/T (ln ) 2T + Q(β, t, u)β t/T (ln ) 2T ,
β
β
1 −α(T −t) 1 α
≤ β t/T (ln ) 2T
exp(k 2 (T − t)2 ) + Q(β, t, u)(ln ) 2
β
β
≤e
2
2
for every t ∈ [0, T ].
4. Numerical experiments
Let us consider the simple two dimensional Allen-Cahn equation
ut − uxx − uyy = u − u3 + g(x, y, t),
(x, y, t) ∈ (0, π) &times; (0, π) &times; (0, 1)
u(0, y, t) = u(π, y, t) = u(x, 0, t) = u(x, π, t) = 0,
(x, y, t) ∈ (0, π) &times; (0, π) &times; [0, 1],
u(x, y, 1) = ϕ(x, y), x, y ∈ (0, π) &times; (0, π),
where
g(x, y, t) = 2et sin x sin y + e3t sin3 x sin3 y,
u(x, y, 1) = ϕ0 (x, y) ≡ e sin x sin y.
12
D. D. TRONG, N. H. TUAN
EJDE-2009/77
The exact solution of this equation is. u(x, y, t) = et sin x sin y In particular,
39999 39999 u x, y,
≡ u(x, y) = exp
sin x sin y.
40000
40000
Let ϕβ (x, y) ≡ ϕ(x, y) = (β + 1)e sin x sin y. Then we have
1/2
Z π Z π
π
β 2 e2 sin2 (x) sin2 ydxdy
kϕβ − ϕk2 =
= βe
2
0
0
q
Choose aβ = β1 , and let p be a natural number satisfying p = [ 12 ln( β1 )]. We find
39999
the regularized solution v β,aβ (x, y, 40000
) ≡ uβ (x, y) having the form
v β,aβ (x, y) = vm (x, y) = w11,m sin x sin y + wpp,m sin(px) sin(py)
where
v1 (x, y) = (β + 1)e sin x sin y,
w11,1 = (β + 1)e,
wpp,1 = 0.
and a = 1/400000, tm = 1 − am for m = 1, 2, . . . , 10,
Z tm
4
e2(s−tm+1 )
w11,m+1 = e2(tm −tm+1 ) wij,m − 2
π tm+1
Z π Z π
3
&times;
vm (x, y) − vm
(x, y) + g(x, y, s) sin x sin y dx dy ds ,
0
0
2p2 (tm −tm+1 )
wpp,m+1 = e
&times;
Z
0
π
Z
π
4
wpp,m − 2
π
Z
tm
2
e2p
(s−tm+1 )
tm+1
3
vm (x, y) − vm
(x, y) + g(x, y, s) sin px sin pydxdy ds.
0
β,aβ
Let aβ = kv
− uk be the error between the regularized solution v β,aβ and
the exact solution u. Let β = β1 = 10−5 (p = 2), β = β2 = 10−8 , β = β3 = 10−16 .
Then we have
β
v β,aβ
aβ
2.718241061 sin x sin y −
−5
β1 = 10
0.002039009193
0.002038827910 sin(3x) sin(3y)
2.718213894 sin x sin y −
β2 = 10−8
0.0002039162492
0.0002039162480 sin(3x) sin(3y)
2.718220664
sin
x
sin
y
−
β3 = 10−16
0.0001835495554
0.0001835495554 sin(3x) sin(3y)
Acknowledgments. The authors would like to thank Professor Julio G. Dix for
his valuable help in the presentation of this paper. The authors are also grateful to
the anonymous referees for their valuable comments leading to the improvement of
our paper.
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Dang Duc Trong
HoChiMinh City National University, Department of Mathematics and Informatics, 227
Nguyen Van Cu, Q. 5, HoChiMinh City, Vietnam