- About this Journal
- Abstracting and Indexing
- Aims and Scope
- Annual Issues
- Article Processing Charges
- Articles in Press
- Author Guidelines
- Bibliographic Information
- Citations to this Journal
- Contact Information
- Editorial Board
- Editorial Workflow
- Free eTOC Alerts
- Publication Ethics
- Reviewers Acknowledgment
- Submit a Manuscript
- Subscription Information
- Table of Contents
Journal of Applied Mathematics
Volume 2012 (2012), Article ID 765361, 12 pages
Wave Equations in Bianchi Space-Times
1Centre for Differential Equations, Continuum Mechanics, and Applications, School of Mathematics, University of the Witwatersrand, Johannesburg 2001, South Africa
2School of Mathematical Sciences, University of KwaZulu-Natal, Durban 4041, South Africa
Received 31 July 2012; Accepted 26 September 2012
Academic Editor: Fazal M. Mahomed
Copyright © 2012 S. Jamal et al. 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.
We investigate the wave equation in Bianchi type III space-time. We construct a Lagrangian of the model, calculate and classify the Noether symmetry generators, and construct corresponding conserved forms. A reduction of the underlying equations is performed to obtain invariant solutions.
The study of partial differential equations (PDEs) in terms of Lie point symmetries is well known and well established [1–5], where these symmetries can be used to obtain, inter alia, exact analytic solutions of the PDEs. In addition, Noether symmetries are also widely investigated and are associated with PDEs that possess a Lagrangian. Noether  discovered the interesting link between symmetries and conservation laws showing that for every infinitesimal transformation admitted by the action integral of a system there exists a conservation law. Investigations have been devoted to understand Noether symmetries of Lagrangians that arise from certain pseudo-Riemannian metrics of interest [7, 8]. Recently, a study was aimed at understanding the effect of gravity on the solutions of the wave equation by solving the wave equation in various space-time geometries .
In , the Bianchi universes were investigated using Noether symmetries. The authors of  studied the Noether symmetries of Bianchi type I and III space-times in scalar coupled theories. Therein, they obtained the exact solutions for potential functions, scalar field, and the scale factors, see also .
We pursue an investigation of the symmetries of the wave equation in Bianchi III space-time. We construct solutions of these equations and find conservation laws associated with Noether symmetries. The plan of the paper is as follows.
In Section 2, we discuss the procedure to obtain an expression representing Noether symmetries and conservation laws. In Section 3, we derive and classify strict Noether symmetries of the Bianchi III space-time. Also in Section 3, we briefly describe the relation of Noether symmetries to conservation laws. We then illustrate the reduction of the wave equation and obtain invariant solutions.
2. Definitions and Notation
We briefly outline the notation and pertinent results used in this work. In this regard, the reader is referred to .
The convention that repeated indices imply summation is used. Let be independent variable with , and let be the dependent variable with coordinates . Furthermore, let be the projection map . Also, suppose that is a smooth map such that , where is the identity map on . The -jet bundle is given by the equivalence classes of sections of . The coordinates on are denoted by , where and corresponds to the partial derivatives of with respect to . The partial derivatives of with respect to are connected by the operator of total differentiation as The collection of all first-order derivatives will be denoted by . Similarly, the collections of all higher order derivatives will be denoted by .
The -jet bundle on U will be written as . We now review the space of differential forms on . To this end, let be the vector space of differential -forms on with differential . A smooth differential -form on is given by where each component . Note that for differential functions , where is the total differential or the total exterior derivative. Moreover, the total exterior derivative of is and by invoking (2.4) one has The total differential has properties analogous to the algebraic properties of the usual exterior derivative : for a -form and an -form and . Also, it is known that if , then is a locally exact -form, that is, for some -form , .
2.1. Action of Symmetries
Consider an th-order system of partial differential equations of independent variables and dependent variables:
Remark 2.2. When Definition 2.1 is satisfied, (2.10) is called a conservation law for (2.8).
It is clear that (2.10) evaluated on the surface (2.8) implies that on the surface given by (2.8), which is also referred to as a conservation law of (2.8). The tuple , , , is called a conserved vector of (2.8).
Let for some . Then is the universal space of differential functions of finite orders.
Consider a symmetry operator given by the infinite formal sum: where , , and the additional coefficients are determined uniquely by the prolongation formulae In (2.13), is the Lie characteristic function given by In particular, a symmetry operator of the form , where , is called a canonical or evolutionary representation of , and is called its characteristic.
An operator is said to be a Noether symmetry corresponding to a Lagrangian , if there exists a vector , such that If , then is referred to as a strict Noether symmetry corresponding to a Lagrangian . This case is also obtained by setting the Lie derivative on the -form in the direction of to zero, that is, where is the Lie derivative operator.
In view of the above discussions and definitions, the Noether theorem  is formulated as follows.
For any Noether symmetry corresponding to a given Lagrangian , there corresponds a vector , defined by which is a conserved current of the Euler-Lagrange equations , , where is the Euler-Lagrange operator given by and the Noether operator associated with the operator is given by in which the Euler-Lagrange operators with respect to derivatives of are obtained from (2.18) by replacing by the corresponding derivatives, for example,
3. Bianchi III Space-Time
In , some aspects of the wave equation on the Bianchi metric were studied. The multiplier method  was adopted to determine some of the conserved densities. This lengthy procedure ultimately leads to the construction of only three symmetries and its associated conserved vectors.
In this paper, we investigate the wave equation on the Bianchi III metric using Noether’s theorem and the method of differential forms. We obtain a wide range of results and also perform symmetry reductions of the wave equation for some cases to obtain invariant solutions. For the purposes of Sections 3.1 and 3.2, we denote the Lagrangian by .
3.1. The Strict Noether Symmetries of (3.2)
We classify the cases that yield strict Noether symmetries (gauge is zero) of (3.2), via the Lagrangian Many of the calculations have been left out as they are tedious—the details are available to the reader in a number of texts that have been cited here.
The principle Noether algebra is Furthermore, specific cases of and give rise to the symmetries to from above, and some additional symmetries.
Case 1 (, ). The additional symmetries are,
Case 2 (, ). The additional symmetries are,
Case 3 (, ). The additional symmetries are,
3.2. Symmetry Reduction and Invariant Solutions
We briefly show how the order of the (1+3) wave equation (3.2) can be reduced. Ultimately the equation with four independent variables is reduced to an ordinary differential equation.
3.2.1. Reduction—Using the Principle Noether Algebra
A Lagrangian of (3.11) is It turns out that we if we let , in (3.11), we can obtain its Noether symmetries, namely, We reduce (3.11) with , and the characteristic equations are Integrating yields and (3.11) is reduced to the ordinary differential equation: with , and which has a solution in terms of special functions, that is, where , are arbitrary constants, Legendre refers to the Legendre polynomial , and refers to the Legendre function of the second kind .
3.2.2. Reduction—Case 1, ,
A Lagrangian of (3.18) is Hence, we obtain the Noether symmetries of (3.18), namely, We reduce (3.18) with , and the characteristic equations are Integrating yields and (3.18) is reduced to the ordinary differential equation: with , and which has a solution in terms of special functions, that is, where, as before, , are arbitrary constants, Legendre refers to the Legendre polynomial , and refers to the Legendre function of the second kind .
We classified the Noether symmetry generators, determined some conserved forms, and reduced some cases of the underlying equations associated with the wave equation on the Bianchi III manifold. The first reduction done above involved the principle Noether algebra, whilst the second dealt with a particular case. To obtain other reductions, one needs to conclude a three-dimensional subalgebra of symmetries to reduce to an ordinary differential equation whose solution would be an invariant solution invariant under the subalgebra. Alternatively, a lower dimensional subalgebra can be used to reduce to a partial differential equation which may be tackled using other methods. The final solution in this case will be invariant only under the lower dimensional algebra. In general, the procedure performed above is the most convenient.
- S. Anco and G. B. luman, “Direct construction method for conservation laws of partial differential equations—part I: examples of conservation law classifications,” European Journal of Applied Mathematics, vol. 13, pp. 567–585, 2002.
- N. H. Ibragimov, Ed., CRC Handbook of Lie Group Analysis of Differential Equations: Symmetries, Exact Solutions and Conservation Laws, CRC Press, Boca Raton, Fla, USA, 1994.
- N. H. Ibragimov, Elementary Lie Group Analysis and Ordinary Differential Equations, vol. 4, John Wiley & Sons, Chichester, UK, 1999.
- P. J. Olver, Applications of Lie Groups to Differential Equations, vol. 107, Springer, New York, NY, USA, 2nd edition, 1993.
- L. V. Ovsiannikov, Group Analysis of Differential Equations, Academic Press, New York, NY, USA, 1982.
- E. Noether, “Invariante variations probleme,” Nachrichten der Akademie der Wissenschaften in Göttingen, Mathematisch-Physikalische Klasse, vol. 2, p. 235, 1918, English translation in: Transport Theory and Statistical Physics, vol. 1, no. 3, p. 186, 1971.
- A. H. Bokhari and A. H. Kara, “Noether versus Killing symmetry of conformally flat Friedmann metric,” General Relativity and Gravitation, vol. 39, no. 12, pp. 2053–2059, 2007.
- A. H. Bokhari, A. H. Kara, A. R. Kashif, and F. D. Zaman, “Noether symmetries versus Killing vectors and isometries of spacetimes,” International Journal of Theoretical Physics, vol. 45, no. 6, pp. 1029–1039, 2006.
- A. H. Bokhari, A. Y. Al-Dweik, A. H. Kara, M. Karim, and F. D. Zaman, “Wave equation on spherically symmetric Lorentzian metrics,” Journal of Mathematical Physics, vol. 52, no. 6, 11 pages, 2011.
- S. Capozziello, G. Marmo, C. Rubano, and P. Scudellaro, “Nöther symmetries in Bianchi universes,” International Journal of Modern Physics D, vol. 6, no. 4, pp. 491–503, 1997.
- U. Camci and Y. Kucukakca, “Noether gauge symmetry for f(R) gravity in Palatini formalism,” Physical Review D, vol. 76, no. 8, Article ID 084023, 2007.
- A. K. Sanyal, “Bianchi type I cosmology in generalized Saez-Ballester theory via Noether gauge symmetry,” Physics Letters B, vol. 524, p. 177, 2002.
- I. M. Anderson and T. E. Duchamp, “Variational principles for second-order quasilinear scalar equations,” Journal of Differential Equations, vol. 51, no. 1, pp. 1–47, 1984.
- I. M. Anderson and T. Duchamp, “On the existence of global variational principles,” American Journal of Mathematics, vol. 102, no. 5, pp. 781–868, 1980.
- N. H. Ibragimov, Transformation Groups Applied to Mathematical Physics, D. Reidel Publishing, Dordrecht, The Netherlands, 1985.
- N. H. Ibragimov, A. H. Kara, and F. M. Mahomed, “Lie-Bäcklund and Noether symmetries with applications,” Nonlinear Dynamics, vol. 15, no. 2, pp. 115–136, 1998.
- S. Jamal, A. H. Kara, and A. H. Bokhari, “Symmetries, conservation laws and reduction of wave and Gordon-type equations on Riemannian manifolds,” in Proceedings of the International Conference on Applied Mathematics and Engineering (WASET '11), vol. 60, pp. 945–957, World Academy of Science, Engineering and Technology, Puket, Thailand, 2011.