research article the approximate solution of fredholm ...due to the highly oscillatory kernels of...
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Research ArticleThe Approximate Solution of Fredholm Integral Equations withOscillatory Trigonometric Kernels
Qinghua Wu
School of Mathematics and Statistics, Central South University, Changsha, Hunan 410083, China
Correspondence should be addressed to Qinghua Wu; [email protected]
Received 9 March 2014; Accepted 4 April 2014; Published 17 April 2014
Academic Editor: Turgut OΜzisΜ§
Copyright Β© 2014 Qinghua Wu.This is an open access article distributed under the Creative Commons Attribution License, whichpermits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited.
A method for approximating the solution of weakly singular Fredholm integral equation of the second kind with highly oscillatorytrigonometric kernel is presented. The unknown function is approximated by expansion of Chebychev polynomial and thecoefficients are determinated by classical collocation method. Due to the highly oscillatory kernels of integral equation, thediscretised collocation equation will give rise to the computation of oscillatory integrals. These integrals are calculated by usingrecursion formula derived from the fundamental recurrence relation of Chebyshev polynomial. The effectiveness and accuracy ofthe proposed method are tested by numerical examples.
1. Introduction
The Fredholm integral equations of second kind,
π¦ (π₯) + πβ«
π
π
πΎ (π₯, π‘)
|π₯ β π‘|πΌπ¦ (π‘) ππ‘ = π (π₯) ,
π₯ β [π, π] , 0 < πΌ < 1,
(1)
where πΎ(π₯, π‘) is a continuous function and π(π₯) is a givenfunction, have many applications in mathematical physicsand engineering, such as heat conduction problem, potentialproblems, quantum mechanics, and seismology image pro-cessing [1β4]. Particularly, when πΎ(π₯, π₯) ΜΈ= 0 and 0 < πΌ < 1,(1) is called weakly singular.
In most of the cases, the integral equation cannot bedone analytically and one has to resort to numericalmethods.Many numerical methods, such as collocation method andGalerkinmethod, have been developed to solve (1); for detailssee [2, 5, 6]. These methods are well-established numericalalgorithms; however, standard version of these classicalmeth-ods may suffer from difficulty for computation of (1), con-taining highly oscillatory kernels since the computation of thehighly oscillatory integrals by standard quadrature methodsis exceedingly difficult and the cost steeply increases withthe frequency. Furthermore, Galerkin method requires manydouble integrals when approximating the solution of integral
equation. Specially, when the kernel is highly oscillatory, itrequires the evaluation of many highly oscillatory doubleintegrals, which can become computationally expensive.
Recently, for weakly singular Volterra integral equationsof the second kind with highly oscillatory Bessel kernels, itwas found that the collocationmethods aremuchmore easilyimplemented and can get higher accuracy than discontinuousGalerkin methods under the same piecewise polynomialsspace; for details see [7β10]. In addition, collocation methodonly involves single integrals which are a little easier to evalu-ate. Motivated by this fact, here we investigate the applicationof collocation method for the solution of Fredholm integralequation
π¦ (π₯) + πβ«
π
π
πππ(π₯βπ‘)
|π₯ β π‘|πΌπ¦ (π‘) ππ‘ = π (π₯) ,
π₯ β [π, π] , πΌ < 1, π β« 1.
(2)
Here π¦(π₯) is the unknown function and π(π₯) is a givenfunction and assume that π is not the eigenvalue of integralequation, since for any finite interval [π, π] it can be trans-formed to interval [β1, 1] by linear transformation. In thispaper, we only consider the case of [π, π] = [β1, 1].
On the other hand, when the unknown function isapproximated byChebychev polynomial, the discretization of
Hindawi Publishing CorporationJournal of Applied MathematicsVolume 2014, Article ID 172327, 7 pageshttp://dx.doi.org/10.1155/2014/172327
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2 Journal of Applied Mathematics
the integral equation (2) by collocation method will give riseto highly oscillatory integral which can be formulated as
β«
π
π
π(ππ(π₯βπ‘))
|π₯ β π‘|πΌππ (π‘) ππ₯, π₯ β [π, π] , 0 < πΌ < 1, (3)
where ππ(π₯) denotes the Chebychev polynomials of the firstkind.
In last few years many efficient methods have beendevised for the evaluation of oscillatory integral, such asasymptotic method [11], Filon-type method [12, 13], Levinβscollocation method [14], modified Clenshaw-Curtis method,Clenshaw-Curtis-Filon-type method [15], and generalizedquadrature rule [16], although some of these methods do notinvolve Chebyshev polynomial. Piessens and Poleunis [17]consider a simpler case of πΌ = 0 and use Chebyshev polyno-mials to evaluate the integral by somewhat indirect methodinvolving a truncated infinite series of Bessel functions. Theevaluation of this infinite summation suggests a defect of thismethod. An alternative procedure which avoids this infiniteseries is the original Bakhvalo and Vasilβeva-Legendre workinvolved in the evaluation of the integral
ππ (π) = β«
1
β1
ππ (π₯) ππππ₯ππ₯, (4)
by recurrence relation, whereππ(π₯) denotes Legendre polyno-mial. Sadly, this relation proved to be unstable in the forwardsdirection for small π. In [18], Alaylioglu et al proposeda simple alternative approach analogous to Newton-Cotesbased formulae. This method avoids the instability to acertain degree compared with these two methods mentionedabove. We used the same idea and generalized it to the caseof weakly singular and calculated oscillatory integral (3).
This paper is organized as follows: in Section 2 we derivesome basic formulae and introduce some mathematicalpreliminaries of the proposed method. In Section 3 we dis-cuss the evaluation of the integrals occurring in collocationequation. In Section 4 numerical experiments are conductedto illustrate the performance of the proposed method.
2. Fundamental Relations
Firstly, by separation of real and imaginary part of π¦(π₯) andπππ€(π₯βπ‘), we transform the integral equation (2) into equivalentsystems of two linear integral equations of Fredholm in theforms
π¦1 (π₯) + πβ«
1
β1
cos (π (π₯ β π‘))|π₯ β π‘|
πΌπ¦1 (π‘) ππ‘
β πβ«
1
β1
sin (π (π₯ β π‘))|π₯ β π‘|
πΌπ¦2 (π‘) ππ‘ = π1 (π₯) ,
π¦2 (π₯) + πβ«
1
β1
sin (π (π₯ β π‘))|π₯ β π‘|
πΌπ¦1 (π‘) ππ‘
+ πβ«
1
β1
cos (π (π₯ β π‘))|π₯ β π‘|
πΌπ¦2 (π‘) ππ‘ = π2 (π₯) ,
(5)
where π¦(π₯) = π¦1(π₯) + ππ¦2(π₯), π(π₯) = π1(π₯) + ππ2(π₯), and π =ββ1.
Assume that y(π₯) = (π¦1(π₯), π¦2(π₯))π and f(π₯) = (π1(π₯),
π2(π₯))π; then, systems of linear integral equations (5) can be
written in the matrix form
Iy (π₯) + πβ«1
β1
K (π₯, π‘) y (π‘) ππ‘ = f (π₯) , (6)
where
I = (1 00 1) ,
K (π₯, π‘) = (
cos (π (π₯ β π‘))|π₯ β π‘|
πΌ
β sin (π (π₯ β π‘))|π₯ β π‘|
πΌ
sin (π (π₯ β π‘))|π₯ β π‘|
πΌ
cos (π (π₯ β π‘))|π₯ β π‘|
πΌ
).
(7)
Set π¦π(π₯) = T(π₯)bπ, π = 1, 2, where T(π₯) = [π0(π₯),π1(π₯), . . . , ππ(π₯)], bπ = [ππ0, ππ1, . . . , πππ]
π.Hence, unknown functions can be expressed by
y (π₯) = Tπ΅, (8)where
y (π₯) = (π¦1 (π₯)π¦2 (π₯)) , T (π₯) = (
T (π₯) 00 T (π₯)) ,
π΅ = (b1b2) .
(9)
Then, the aim is to find Chebyshev coefficients, that is,the matrix π΅. We first substitute the Chebyshev collocationpoints, which are defined by π₯π = cos(ππ/π), π = 0, . . . , π,into (6) and then rearrange a new matrix form to determineπ΅:
IY + πK = F (10)
in whichK is the integral part of (6) and
I = (
I 0 β β β 00 I β β β 0...
... d...
0 0 β β β I
), Y =(
y (π₯0)
y (π₯1)...
y (π₯π)
),
F =(
f (π₯0)
f (π₯1)...
f (π₯π)
), K =(
IK (π₯0)
IK (π₯1)...
IK (π₯π)
).
(11)
By substituting (8) into (10), the unknown coefficients canbe easily computed from this linear algebraic equations andtherefore we find the solution of integral equation (2).
3. Evaluation of the Integral
πΌ[π, πΌ, π, π₯]= β«1
β1(π(ππ(π₯βπ‘))
/|π₯βπ‘|πΌ)ππ(π‘)ππ‘
The discretization of integral equation will lead to thecalculation of integral πΌ[π, πΌ, π, π₯]. In this section, we willderive the recursion formula to compute it efficiently from thefundamental recurrence relation of Chebyshev polynomial.
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Journal of Applied Mathematics 3
100
10β2
10β4
10β6
10β8
10β10
β1 β0.8 β0.6 β0.4 β0.2 0 0.2 0.4 0.6 0.8 1
x
Abso
lute
erro
rApproximate the solution of example 4.1 with
πΌ = 0.5, = 1000, N = 4π
(a)
100
10β2
10β4
10β6
10β8
10β10
β1 β0.8 β0.6 β0.4 β0.2 0 0.2 0.4 0.6 0.8 1
x
Abso
lute
erro
r
Approximate the solution of example 4.1 withπΌ = 0.5, = 1000, N = 8π
(b)
100
10β2
10β4
10β6
10β8
10β10
β1 β0.8 β0.6 β0.4 β0.2 0 0.2 0.4 0.6 0.8 1
x
Abso
lute
erro
r
Approximate the solution of example 4.1 withπΌ = 0.5, = 1000, N = 16π
(c)
Figure 1: Example 1: the absolute error versus the π₯-coordinate forπ = 4 (a),π = 8 (b), andπ = 16 (c) with negative data ignored.
The Chebyshev polynomials are of the form
π0 (π₯) = 1
π1 (π₯) = π₯
...ππ (π₯) = 2π₯ππβ1 (π₯) β ππβ2 (π₯) , π = 2, 3, . . . ,
(12)
and the coefficients πΆπ,j of π₯π in ππ(π₯) can be easily calculated
by the means of the recurrence relation
πΆπ,π = 2πΆπβ1,πβ1 β πΆπβ2,π, π β₯ 2, π β€ π. (13)
By expanding the Chebyshev polynomial ππ(π₯) in terms ofpowers of π₯, the integral πΌ[π, πΌ, π, π₯] would be transformedinto the form of
πΌ [π, πΌ, π, π₯] =
π
β
π=0
πΆπ,ππΌ1 [π, πΌ, π, π₯] , (14)
where πΌ1[π, πΌ, π, π₯] = β«1β1(π(ππ(π₯βπ‘))
/|π₯ β π‘|πΌ)π‘πππ‘.
By separation of real and imaginary part of πππ€(π₯βπ‘), (14) istransformed into
πΌ1π [π, πΌ, π, π₯] = β«
1
β1
π‘π
|π₯ β π‘|πΌcos (π (π₯ β π‘)) ππ‘,
πΌ1π [π, πΌ, π, π₯] = β«
1
β1
π‘π
|π₯ β π‘|πΌsin (π (π₯ β π‘)) ππ‘,
(15)
whose main difficulty now is turning to the evaluation of thefollowing basic integrals:
π1π [π, πΌ, π, π₯] = β«
1
β1
π‘π
|π₯ β π‘|πΌcos (ππ‘) ππ‘,
π1π [π, πΌ, π, π₯] = β«
1
β1
π‘π
|π₯ β π‘|πΌsin (ππ‘) ππ‘.
(16)
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4 Journal of Applied Mathematics
100
10β2
10β4
10β5
10β6
10β7
10β3
10β1
β1 β0.8 β0.6 β0.4 β0.2 0 0.2 0.4 0.6 0.8 1
x
Abso
lute
erro
rApproximate the solution of example 4.2 with
πΌ = 1/3, = 1000, N = 4π
(a)
100
10β2
10β4
10β6
10β8
10β10
β1 β0.8 β0.6 β0.4 β0.2 0 0.2 0.4 0.6 0.8 1
x
Abso
lute
erro
r
Approximate the solution of example 4.2 withπΌ = 1/3, = 1000, N = 8π
(b)
100
10β2
10β4
10β6
10β8
10β10
β1 β0.8 β0.6 β0.4 β0.2 0 0.2 0.4 0.6 0.8 1
x
Abso
lute
erro
r
Approximate the solution of example 4.2 withπΌ = 1/3, = 1000, N = 16π
(c)
Figure 2: Example 2: the absolute error versus the π₯-coordinate forπ = 4 (a),π = 8 (b), andπ = 16 (c) with negative data ignored.
With substitution for definite integral and binomial the-orem, (16) is equivalent to
π1π [π, πΌ, π, π₯]
=
π
β
π=0
πΆπ
ππ₯πβπ[(β1)
πβ«
π₯+1
0
π‘πβπΌ cos (π (π₯ β π‘)) ππ‘
+β«
1βπ₯
0
π‘πβπΌ cos (π (π₯ β π‘)) ππ‘] ,
(17)
where πΆππis number of combination andπ1π [π, πΌ, π, π₯] can
be formulated analogy.Efficient evaluation of (18) is based on accurate calcu-
lation of integrals in the formula which can be computed
explicitly by the incomplete Gamma function Ξ(π§, πΌ) [19];that is,
β« π‘πβ1 cos (ππ‘) ππ‘
=1
2[(ππ)βπΞ (π, πππ‘) + (βππ)
βπΞ (π, βπππ‘)] ,
(18)
β« π‘πβ1 sin (ππ‘) ππ‘
=π
2[(ππ)βπΞ (π, πππ‘) β (βππ)
βπΞ (π, βπππ‘)] ,
(19)
in which π > 0, π₯ > 0; for details see ([20], pp 215).Once the integral πΌ[π, πΌ, π, π₯] is obtained, by substituting
into (10), the coefficient matrix is derived; then, we cancompute the solution of integral by solving these linearalgebraic equations.
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Journal of Applied Mathematics 5
Table 1: Approximations for π¦(π₯) + β«1β1(πππ(π₯βπ‘)/|π₯ β π‘|
0.5)π¦(π‘)ππ‘ = π
π₯ with π = 103.
π₯ cos(π) cos(3π/4) cos(0)π¦4
10.351696035495271 0.459037927481918 0.924223909091303
π¦8
10.351696947581043 0.459033811940511 0.924224547405338
π¦16
10.351741610883315 0.459036527929146 0.924226969620880
π¦4
20.0141286409694892 0.000146310591227839 β0.00107727880701281
π¦8
20.0141193392298137 0.000145019893750479 β0.00107593754108643
π¦16
20.0140583682955762 0.000144178555797097 β0.00106881635211410
Table 2: Approximations for π¦(π₯) + β«1β1(πππ(π₯βπ‘)/|π₯ β π‘|
0.5)π¦(π‘)ππ‘ = π
π₯ with π = 103.
π₯ cos(π/4) 1π¦4
11.88249404678421 2.61059423521207
π¦8
11.88248275737555 2.61059120470589
π¦16
11.88248361661370 2.61073031676337
π¦4
20.00323202027752064 β0.0995508315886191
π¦8
20.00321634589900894 β0.0994917961267987
π¦16
20.00319500282444326 β0.0993143176430846
4. Numerical Examples
In this section, we give some numerical examples to illustratethe performance of proposed method. In all the followingexamples,π+1 is the number of mesh points, π¦π
π(π₯) denotes
the approximate solution, where π is the number of termsof the Chebyshev series, and π¦(π₯) denotes the exact solution,respectively. All the computations have been performed byusing Matlab R2012a on a 2.5GHz PC with 2GB of RAM.
Example 1. We consider π¦(π₯) + β«1β1(πππ(π₯βπ‘)/|π₯ β π‘|
πΌ)π¦(π‘)ππ‘ =
π(π₯) and set π(π₯) = 1 + β«1β1(πππ(π₯βπ‘)/|π₯ β π‘|
πΌ)ππ‘; then, the
solution of which is
π¦ (π₯) = 1. (20)
We plot the absolute error |π¦(π₯) β π¦(π₯)| by Matlab 2012ainternal function ezplot. Specially, for π = 4, with π = 103and πΌ = 0.5, solve linear algebraic equations (10); it can befound that the approximate solutions are
π¦1 (π₯) = 0.9999999996 β 0.110 Γ 10β15π₯ + 0.281 Γ 10
β9π₯2
+ 0.323 Γ 10β15π₯3+ 0.563 Γ 10
β10π₯4,
π¦2 (π₯) = 0.460 Γ 10β14β 0.329 Γ 10
β9π₯ + 0.554 Γ 10
β11π₯2
+ 0.322 Γ 10β9π₯3β 0.555 Γ 10
β11π₯4.
(21)
It is easy to see from Figure 1 that the proposed method isconverging and we only needπ = 4 so we could achieve highaccuracy.
Example 2. We consider π¦(π₯) + β«1β1(πππ(π₯βπ‘)/|π₯ β π‘|
πΌ)π¦(π‘)ππ‘ =
π(π₯) and set π(π₯) = ππ₯ + β«1β1(πππ(π₯βπ‘)/|π₯ β π‘|
πΌ)ππ‘ππ‘; then, the
solution of which isπ¦ (π₯) = π
π₯. (22)
In this case, for π = 4, with π = 1000 and πΌ = 1/3, theapproximate solutions are
π¦1 (π₯) = 1.000000001 + 0.9956819990π₯ + 0.4992866904π₯2
+ 0.1795192648π₯3+ 0.04379395375π₯
4,
π¦2 (π₯) = β0.530 Γ 10β7+ 0.189 Γ 10
β7π₯ + 0.426 Γ 10
β6π₯2
β 0.246 Γ 10β7π₯3β 0.420 Γ 10
β6π₯4.
(23)
It is obvious to see from Figure 2 that the proposedmethod is efficient and could achieve high accuracy with littlenumber of collocation points.
Example 3. For a general case, we considerπ¦(π₯) + β«
1
β1(πππ(π₯βπ‘)/|π₯ β π‘|
πΌ)π¦(π‘)ππ‘ = π
π₯ and set πΌ = 0.5.Although the solution of this equation is unknown, we canverify the calculation precision of the method to a certaindegree by comparing the approximate evaluation at meshpoints.
In this case, for π = 4, with π = 1000 and πΌ = 0.5, theapproximate solutions areπ¦1 (π₯) = 0.9242239091
+ 0.8836218495π₯ + 0.4292470859π₯2
+ 0.2458272504π₯3+ 0.1276741404π₯
4,
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6 Journal of Applied Mathematics
Table 3: Approximations for π¦(π₯) + β«1β1(πππ(π₯βπ‘)/|π₯ β π‘|
0.5)π¦(π‘)ππ‘ = π
π₯ with π = 106.
π₯ cos(π) cos(3π/4) cos(0)π¦4
10.367419631536920 0.491835233010361 0.997500711679274
π¦8
10.367419631541538 0.491835233006225 0.997500711679218
π¦16
10.367419632527633 0.491835233064998 0.997500711678921
π¦4
20.000458466865252363 0.150404190 Γ 10
β5β0.219327280 Γ 10
β5
π¦8
20.000458466860662437 0.150403636 Γ 10
β5β0.219326739 Γ 10
β5
π¦16
20.000458463698666909 0.150390826 Γ 10
β5β0.219326736 Γ 10
β5
Table 4: Approximations for π¦(π₯) + β«1β1(πππ(π₯βπ‘)/|π₯ β π‘|
0.5)π¦(π‘)ππ‘ = π
π₯ with π = 106.
π₯ cos(π/4) 1π¦4
12.02304108762491 2.71487514654707
π¦8
12.02304108760947 2.71487514654071
π¦16
12.02304108780835 2.71487515460854
π¦4
20.384042582 Γ 10
β5β0.00339813438891857
π¦8
20.384040621 Γ 10
β5β0.00339813431478428
π¦16
20.3840356665 Γ 10
β5β0.00339813322237100
π¦2 (π₯) = β0.001077278807 + 0.06120358877π₯
+ 0.05269959347π₯2β 0.1180433250π₯
3
β 0.09433340997π₯4.
(24)
It is also easy to see from Tables 1, 2, 3, and 4 that thepresentedmethod is efficient and accurate, although the exactsolution is unknown.
5. Conclusion
In this paper, we explore quadrature methods for weaklysingular Fredholm integral equation of the second kindwith oscillatory trigonometric kernels and present colloca-tion methods with Chebyshev series for calculation of thesolution. For integral equationwith highly oscillatory kernels,the standard collocation methods with classical quadraturemethods are not suitable for the numerical approximationof the solution of integral equation, since the computationof the highly oscillatory integrals by standard quadraturemethods is exceedingly difficult and the cost steeply increaseswith the frequency. Based on the recursion formula derivedin Section 3, we compute the highly oscillatory integralsoccurring in collocation equation, directly and efficiently.Numerical examples demonstrate the performance of algo-rithm.
Conflict of Interests
The author declares that there is no conflict of interestsregarding the publication of this paper.
Acknowledgment
This paper was supported by NSF of China (no. 11371376).
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