Research Article On a Class of Integrals Involving a Bessel Function Times Gegenbauer Polynomials

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1 Hindawi Publishing Corporation International Journal of Mathematics and Mathematical Sciences Volume 27, Article ID 7375, 5 pages doi:1.1155/27/7375 Research Article On a Class of Integrals Involving a Bessel Function Times Gegenbauer Polynomials Stamatis Koumandos Received 18 August 26; Revised 1 November 26; Accepted 16 November 26 Recommended by Feng Qi We provide information and explicit formulae for a class of integrals involving Bessel functions and Gegenbauer polynomials. We present a simple proof of an old formula of Gegenbauer. Some interesting special cases and applications of this result are obtained. In particular, we give a short proof of a recent result of A. A. R. Neves et al. regarding the analytical evaluation of an integral of a Bessel function times associated Legendre functions. These integrals arise in problems of vector diffraction in electromagnetic theory. Copyright 27 Stamatis Koumandos. 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 the recent article [1], Neves et al. gave an analytical evaluation of the integral In m := sinθ exp(ircosαcosθ)pn m (cosθ)j m (Rsinαsinθ)dθ, (1.1) where P m n (cosθ) is the associated Legendre function and J m (z) is the Bessel function of the first kind and order m. The authors of [1] encountered this integral in their work [2] dealing with the calculation of the optical force of the optical tweezers in a complete electromagnetic treatment for any beam shape focused on an arbitrary position. The integral (1.1) appears also in fields related to vector diffraction theory where computationally intensive methods or approximations are employed. As the authors of [1] pointed out, an explicit formula for the integral (1.1)wouldbea useful result, making unnecessary any numerical approximations of it. More specifically,

2 2 International Journal of Mathematics and Mathematical Sciences Nevesetal.showedin[1]thatforallintegersn, m such that n and n m n,one has I m n = 2in m P m n (cosα)j n (R), (1.2) where j n (R) is the spherical Bessel function of order n, that is, π j n (R) = 2R J n+1/2(r). (1.3) The authors mention in [1] that the integral (1.1) has not been reported in a closed form and it is not shown in any integral tables. In [3, page 379, formula (1)], however, the following closely related formula is given: exp(ircosαcosθ)j ν 1/2 (Rsinαsinθ)C ν n(cosθ)sin ν+1/2 θdθ ( ) 2π 1/2 = i n sin ν 1/2 αc ν R n(cosα)j ν+n (R), (1.4) where C ν n(x) is the Gegenbauer (or ultraspherical) polynomial of degree n and order ν defined by the generating function 1 ( 1 2xt + t 2 ) ν = Ck ν(x)tk. (1.5) The formula (1.4) is due to Gegenbauer and holds for all real numbers ν such that ν > 1/2. An equivalent form of (1.4) can also be found in the integral tables [4, pages , formula (1) and (2)]. In this note we show that (1.2) follows easily from (1.4) using properties of the associated Legendre functions. A proof of (1.4) isgivenin[3, pages ], using a method which is based on integration over a unit sphere. Since formula (1.4) is important in applications in many different fields in physics, especially for those that require partial wave decomposition, we giveinthenextsectionanewproofof(1.4) which is simpler than the one given in [3]. We will also present some other consequences of (1.4). 2. Proofs and additional comments We first show how to obtain (1.2) from(1.4). Suppose first that m, n are nonnegative integers such that m n. It is well known that the relation between associated Legendre functions P m n (cosθ) and Gegenbauer polynomials is given by the formula k= P m n (cosθ) = ( 1) m (2m)! 2 m m! sinm θc m+1/2 n m (cosθ), (2.1) see for example[4, page 152, formula (2)]. Using the relation (2.1) in(1.1) and applying (1.4) forν = m +1/2 1/2, we immediately obtain (1.2). When the integer m

3 Stamatis Koumandos 3 is negative such that n m<, we cannot apply (1.4) as above, because the condition ν = m +1/2 > 1/2 is not fulfilled. This case can be handled using the formulae see [3, page 15, formula (2)], and J m (z) = ( 1) m J m (z), (2.2) Pn m m Γ(n + m +1) (x) = ( 1) Γ(n m +1) P m n (x), 1 x 1, (2.3) see [4, page 125, formula (2)]. It follows from (2.2) and(2.3) that I m n = Γ(n + m +1) Γ(n m +1) I m n. (2.4) Since n m >, applying (1.2) established in the previous case and using (2.3) and (2.4), we get I m n = Γ(n + m +1) Γ(n m +1) 2in+m P m n (cosα)j n (R) = 2i n m P m n (cosα)j n (R), (2.5) which completes the proof of (1.2). We next give a simple proof of (1.4). Let ν > 1/2. The Gegenbauer polynomials C ν n(cosθ) satisfy the following orthogonality property: Cn(cosθ)C ν m(cosθ)sin ν 2ν θdθ, n m, = πγ(2ν + n)γ(ν +1/2) πγ(ν +1/2) = (ν + n)n!γ(2ν)γ(ν) (ν + n)γ(ν) Cν n(1), n = m. (2.6) We will also use the formula J ν 1/2 (Rsinαsinθ) exp(ircosαcosθ) (Rsinαsinθ) ν 1/2 = Γ(ν) 2 (k + ν)i k J k+ν(r)ck ν(cosθ)cν k (cosα) Γ(ν +1/2) R ν Ck ν(1), k= (2.7) which holds for all ν, see[4, page 153, formula (4)]. A proof of formula (2.7) is given in [3, pages ]. It should be noted that the series above is absolutely convergent. This follows easily using the standard estimate J ν (R) R ν Γ(ν +1)2 ν, R>, ν > 1 2, (2.8)

4 4 International Journal of Mathematics and Mathematical Sciences (cf. [3, page 49, formula (1)]), and [5, Theorem ] concerning the maximum of Gegenbauer polynomials on [ 1,1]. Therefore, the series in (2.7) can be termwise integrated. Then applying the dominated convergence theorem and the orthogonality relation (2.6), we obtain J ν 1/2 (Rsinαsinθ) (Rsinαsinθ) ν 1/2 exp(ircosαcosθ)cν n(cosθ)sin 2ν θdθ = Γ(ν) 2 Γ(ν +1/2) [ = 2πi n J n+ν(r)c ν n(cosα) R ν, (k + ν)i k J k+ν(r)ck ν(cosθ)cν k (cosα) R k= ν Ck ν(1) ] C ν n(cosθ)sin 2ν θdθ (2.9) whence (1.4)followsatonce. It is a further confirmation of the importance of formula (1.4) that some other results are obtained by it as special cases. Indeed, taking the limiting case of (1.4) whenα using the dominated convergence theorem and the fact that we obtain limx λ 1 J λ (x) = x 2 λ, forλ> 1, (2.1) Γ(λ +1) J ν+n (R) = ( ) ν ( i) n R π exp(ircosθ) Cν n(cosθ) πγ(ν +1/2) 2 Cn(1) ν sin 2ν θdθ. (2.11) Formula (2.11) is Gegenbauer s generalization of the Poisson integral representation of Bessel functions. For n =, it reduces to Poisson s formula J ν (R) = ( ) ν 1 R π exp(ircosθ)sin 2ν θdθ, whenν > 1 πγ(ν +1/2) 2 2, (2.12) see [4, page 48, formula (6)]. Since C lim n(cosθ) ν ν Cn(1) ν = cosnθ, (2.13) taking the limit in (2.11)asν applying once more the dominated convergence theorem, we deduce that for all nonnegative integers n,wehave i n J n (R) = 1 π = 1 2π 2π exp(ircosθ)cosnθ dθ exp(ircosθ)exp(inθ)dθ. (2.14) In view of (2.2), these equalities hold also for all negative integers n,hence(2.14)givesall the Fourier coefficientsof the functionexp(ircosθ).

5 Stamatis Koumandos 5 Replacing θ with π/2 θ in (2.14) and using the 2π-periodicity of the integrand, we get Bessel s formula J n (R) = 1 π cos(nθ Rsinθ)dθ, (2.15) see [3, page 2, formula (2)]. Finally, we note that the Gouesbet and Lock result [6] for the integral ( ) n + sin m +1 m! j θ exp(±ircosθ)p n m (cosθ)dθ = 2(±i) n+ m ( ) n (R), (2.16) n m! R m where n, m are integers such that m n,canbederivedfrom(2.11)inexactlythesame way as (1.2)is obtained from(1.4). References [1] A. A. R. Neves, L. A. Padilha, A. Fontes, et al., Analytical results for a Bessel function times Legendre polynomials class integrals, Journal of Physics. A. Mathematical and General, vol. 39, no. 18, pp. L293 L296, 26. [2] A. A. R. Neves, A. Fontes, L. A. Padilha, et al., Exact partial wave expansion of optical beams with respect to an arbitrary origin, Optics Letters, vol. 31, no. 16, pp , 26. [3] G. N. Watson, A Treatise on the Theory of Bessel Functions, Cambridge University Press, Cambridge, UK, 2nd edition, [4] I. S. Gradshteyn andi. M. Ryzhik,Table of Integrals, Series, and Products, Academic Press, Boston, Mass, USA, 5th edition, [5] G. Szegő, Orthogonal Polynomials, vol. 23 of American Mathematical Society, Colloquium Publications, American Mathematical Society, Providence, RI, USA, 4th edition, [6] G. Gouesbet and J. A. Lock, Rigorous justification of the localized approximation to the beamshape coefficients in generalized Lorenz-Mie theory. II. Off-axis beams, Journal of the Optical Society of America A, vol. 11, no. 9, pp , Stamatis Koumandos: Department of Mathematics and Statistics, University of Cyprus, P. O. Box 2537, 1678 Nicosia, Cyprus address: skoumand@ucy.ac.cy

6 Mathematical Problems in Engineering Special Issue on Modeling Experimental Nonlinear Dynamics and Chaotic Scenarios Call for Papers Thinking about nonlinearity in engineering areas, up to the 7s, was focused on intentionally built nonlinear parts in order to improve the operational characteristics of a device or system. Keying, saturation, hysteretic phenomena, and dead zones were added to existing devices increasing their behavior diversity and precision. In this context, an intrinsic nonlinearity was treated just as a linear approximation, around equilibrium points. Inspired on the rediscovering of the richness of nonlinear and chaotic phenomena, engineers started using analytical tools from Qualitative Theory of Differential Equations, allowing more precise analysis and synthesis, in order to produce new vital products and services. Bifurcation theory, dynamical systems and chaos started to be part of the mandatory set of tools for design engineers. This proposed special edition of the Mathematical Problems in Engineering aims to provide a picture of the importance of the bifurcation theory, relating it with nonlinear and chaotic dynamics for natural and engineered systems. Ideas of how this dynamics can be captured through precisely tailored real and numerical experiments and understanding by the combination of specific tools that associate dynamical system theory and geometric tools in a very clever, sophisticated, and at the same time simple and unique analytical environment are the subject of this issue, allowing new methods to design high-precision devices and equipment. Authors should follow the Mathematical Problems in Engineering manuscript format described at Prospective authors should submit an electronic copy of their complete manuscript through the journal Manuscript Tracking System at mts.hindawi.com/ according to the following timetable: Guest Editors José Roberto Castilho Piqueira, Telecommunication and Control Engineering Department, Polytechnic School, The University of São Paulo, São Paulo, Brazil; piqueira@lac.usp.br Elbert E. Neher Macau, Laboratório Associado de Matemática Aplicada e Computação (LAC), Instituto Nacional de Pesquisas Espaciais (INPE), São Josè dos Campos, São Paulo, Brazil ; elbert@lac.inpe.br Celso Grebogi, Center for Applied Dynamics Research, King s College, University of Aberdeen, Aberdeen AB24 3UE, UK; grebogi@abdn.ac.uk Manuscript Due December 1, 28 First Round of Reviews March 1, 29 Publication Date June 1, 29 Hindawi Publishing Corporation