Pacific Journal of Applied Mathematics Volume 1, Number 2, pp. 69 75 ISSN PJAM c 2008 Nova Science Publishers, Inc. A MULTI-COMPONENT LAX INTEGRABLE HIERARCHY WITH HAMILTONIAN STRUCTURE Wen-Xiu Ma Department of Mathematics and Statistics, University of South Florida, Tampa, FL 33620-5700, USA Abstract A Lax integrable multi-component hierarchy is generated from a matrix spectral problem involving two arbitrary matrices, within the the framework of zero curvature equations. Its Hamiltonian structure is established be means of the trace variational identity. An example with five components is presented, together with the first two Hamiltonian systems in the resulting soliton hierarchy. 2000 Mathematics Subject Classification: 37K05, 37K10, 35Q58. Keywords: Lax pair, Hamiltonian structure, soliton hierarchy. 1. Introduction It is important to explore integrability 1 for nonlinear differential and/or difference equations. One of the problems contributing to the theory is to search for completely integrable differential and/or difference equations and to accumulate characteristics of integrability. To the end, we would here like to present a Lax integrable multi-component hierarchy from a matrix spectral problem involving two arbitrary matrices, within the the framework of zero curvature equations. It is known that the multiplicity of a given system of differential and/or difference equations causes difficulties in determining its integrable properties. Our result will provide a useful supplement to existing integrable theories of scalar differential and/or equations, of which a few criteria such as the inverse scattering transform, the Painlevé test, Bäcklund transformation are available for testing integrability 2, 3. The paper is structured as follows. In Section 2., starting from a new higher-order matrix spectral problem, the associated Lax integrable equations of multi-components are furnished. In Section 3., one representative hierarchy of the resulting Lax integrable equations is presented and shown to possess Hamiltonian structure based on the trace variational identity. Moreover, a concrete example with five components is computed, together with two first Hamiltonian systems in the resulting Lax integrable hierarchy. A few concluding remarks are given in Section 4.. E-mail address: mawx@cas.usf.edu
70 Wen-Xiu Ma 2. Lax Pairs and Associated Zero Curvature Equations We consider a higher-order matrix spectral problem involving two arbitrary matrices: λj2 q 0 1 φ x = Uφ = U(u, λ)φ, U =, J 2 =, (2.1) q T r 1 0 where λ is a spectral parameter, r is a skew-symmetric (i.e., r T = r) matrix and u = p(q, r) is a vector potential. Here p is an arrangement of variables in two arbitrary matrices q and r into a vector. To derive associated Lax integrable equations, let us first solve the stationary zero curvature equation V x = U, V (2.2) of the spectral problem (2.1). We try a solution V of the following type: aj2 b V =, (2.3) b T c where a is scalar and c T = c. Therefore, based on qb T + bq T λj 2 b aj 2 q + qc br U, V = λb T J 2 aq T J 2 rb T + cq T q T b + b T q + r, c, the stationary zero curvature equation (2.2) becomes a x J 2 = bq T qb T, b x = λj 2 b aj 2 q + qc br, c x = b T q q T b + r, c. As usual, we seek and specify a formal solution as follows: aj2 b ak J V = = V b T k λ k 2 b k, V k = c (b k ) T k=0 c k (2.4), (2.5) where (c k ) T = c k, k 0. Then, the three equations in (2.4) recursively define all a k, b k and c k, k 0, while the initial values need to satisfy b 0 = 0, a 0,x = 0, c 0,x = r, c 0. From now on, for any integer n 1, we take n 0 0 V (n) = (λ n V ) + + n = V j λ n j + n, n = 0 n j=0, (2.6)
A Multi-component Lax integrable Hierarchy with Hamiltonian Structure 71 n being skew-symmetric and of the same size as c, and introduce the time evolution law for the eigenfunction φ: φ tn = V (n) φ = V (n) (u, λ)φ. (2.7) Defining U 1 = U λ=0, we can compute that 0 bn,x + a n J 2 q qc n + b n r ((λ n V ) + ) x U,(λ n V ) + = V n,x U 1, V n =, 0 0 q n n,x U, n = n,x U 1, n =, n q T n,x r, n where n 1. Note that it is not necessary to compute the (2, 1)-entry, which actually is the negative of the transpose of the (1, 2)-entry. It now follows that the compatibility conditions of (2.1) and (2.7) for all n 1, i.e., the zero curvature equations U tn V (n) x + U, V (n) = 0, n 1, associated with the Lax pairs of U and V (n), n 1, engender a Lax integrable hierarchy of multi-component evolution equations q tn = b n,x + a n J 2 q qc n + b n r q n, r tn = n,x r, n, n 1. (2.8) A large number of soliton hierarchies have been presented this way (see, for example, 4-9). In the following section, we would like to single out a soliton hierarchy with Hamiltonian structure among the above multi-component Lax integrable equations. 3. Hamiltonian Soliton Hierarchy Let us take n = αc n, n 1, (3.1) where α is a constant. Then, due to (2.4), the Lax integrable hierarchy (2.8) becomes q tn = b n,x + a n J 2 q qc n + b n r αqc n, r tn = α(b T nq q T b n ), n 1. (3.2) Furthermore, the resulting Lax integrable hierarchy can be written as q bn = J c (q, r), (3.3) r c t n n where J c denotes the matrix compact form of the integro-differential operator J depending on q and r: u tn = (p(q, r)) tn = J(q, r)p(b n+1, c n+1 ), n 1. (3.4) Namely, J is skew- First, it is direct to show that the operator J is Hamiltonian. symmetric and satisfies the Jacobi identity 10: (A, JB) = (JA, B), (A, J JBC) + cycle(a, B, C) = 0, (3.5)
72 Wen-Xiu Ma where the inner product reads (A, B) = tr(a T 1 B 1 + 1 2 AT 2 B 2 )dx, J denotes the Gateaux derivative J v = ε J(u + εv) ε=0, and A = p(a 1, A 2 ), B = p(b 1, B 2 ) and C = p(c 1, C 2 ) are arbitrary vectors with A 1, B 1, C 1 and A 2, B 2, C 2 are matrices of the same size as q and r, respectively, Second, let us recall the trace variational identity: V, U dx = λ γ u λ λ λγ V, U u, (3.6) where A, B = tr(ab) is the Killing form and γ is a constant 11-13. In our case, we have V, U U U = 2b, V, = 2c, V, q r λ = 2a. It follows from the trace variational identity that q r H n = bn c n, Hn = ( a ) n+1 dx, n 1. (3.7) n This leads to the Hamiltonian structure of the Lax integrable hierarchy (3.2): q q = J c (q, r) H n, n 1. (3.8) r t n When r is a 2 2 matrix, let us introduce u1 u 3 0 u5 q =, r =, (3.9) u 2 u 4 u 5 0 dn f n 0 hn b n =, c n =, n 0, (3.10) h n 0 e n g n where u i, 1 i 5, is scalar variables, and d n, e n, f n, g n and h n are scalar functions. Then, setting u = (u 1, u 2, u 3, u 4, u 5 ) T, the resulting Lax integrable hierarchy (3.8) can be rewritten as d n,x + u 2 a n u 5 f n (α + 1)u 3 h n d n e n,x u 1 a n u 5 g n (α + 1)u 4 h n e n r u tn = f n,x + u 4 a n + u 5 d n + (α + 1)u 1 h n = J g n,x u 3 a n + u 5 e n + (α + 1)u 2 h n f n g n = J H n, n 1, (3.11) u αh n,x h n
A Multi-component Lax integrable Hierarchy with Hamiltonian Structure 73 where the Hamiltonian operator is given by J = + u 2 1 u 2 u 2 1 u 1 u 2 1 u 4 u 5 u 2 1 u 3 (α + 1)u 3 u 1 1 u 2 + u 1 1 u 1 u 1 1 u 4 u 1 1 u 3 u 5 (α + 1)u 4 u 4 1 u 2 + u 5 u 4 1 u 1 + u 4 1 u 4 u 4 1 u 3 (α + 1)u 1. u 3 1 u 2 u 3 1 u 1 + u 5 u 3 1 u 4 + u 3 1 u 3 (α + 1)u 2 (α + 1)u 3 (α + 1)u 4 (α + 1)u 1 (α + 1)u 2 (3.12) Taking the initial values as follows a 0 = 1, b 0 = 0, h 0 = 0, and setting each integration constant to zero, then the first few quantities are a 1 = 0, b 1 = q, h 1 = 0; d 2 = u 2x + u 4 u 5, e 2 = u 1x u 3 u 5, f 2 = u 4x u 2 u 5, g 2 = u 3x + u 1 u 5, a 2 = 1 2 (u2 1 + u 2 2 + u 2 3 + u 2 4), h 2 = u 1 u 4 u 2 u 3. Thus, the first and second nonlinear systems of Hamiltonian equations in the Lax integrable hierarchy (3.8) with u = (u 1, u 2, u 3, u 4, u 5 ) T read u 1x u 3 u 5, u 2x u 4 u 5, u t1 = u 3x + u 1 u 5, = J H 1 u, u 4x + u 2 u 5, 0 and u t2 = u 2xx + 2u 4x u 5 + u 4 u 5x 1 2 u 2p 1 (α + 1)u 1 u 3 u 4 u 1xx 2u 3x u 5 u 3 u 5x + 1 2 u 1p 2 + (α + 1)u 2 u 3 u 4 u 4xx 2u 2x u 5 u 2 u 5x 1 2 u 4p 3 (α + 1)u 1 u 2 u 3 u 3xx + 2u 1x u 5 + u 1 u 5x + 1 2 u 3p 4 + (α + 1)u 1 u 2 u 4 = J H 2 u, α(u 1 u 4 u 2 u 3 ) x where p 1 = u 2 1 + u2 2 (2α + 1)u2 3 + u2 4 2u2 5, p 2 = u 2 1 + u2 2 + u2 3 (2α + 1)u2 4 2u2 5, p 3 = (2α + 1)u 2 1 + u2 2 + u2 3 + u2 4 2u2 5, p 4 = u 2 1 (2α + 1)u2 2 + u2 3 + u2 4 2u2 5,
74 Wen-Xiu Ma the Hamiltonian operator J is defined by (3.12), and the Hamiltonian functionals H 1 and H 2 are given by 1 H 1 = 2 (u2 1 + u 2 2 + u 2 3 + u 2 4)dx, H 2 = (u 1 u 4 u 2 u 3 )u 5 1 2 u 1u 2x + 1 2 u 1xu 2 1 2 u 3u 4x + 1 2 u 3xu 4 dx. 4. Concluding Remarks A higher-order matrix spectral problem involving two arbitrary matrices was introduced, and an associated Lax integrable hierarchy were presented through selecting appropriate temporal parts of Lax pairs. The key in the construction is to force the associated zero curvature equations to yield evolution equations. The presented multi-component Lax integrable hierarchy was proved to possess Hamiltonian structure by means of the trace variational identity. An example with five components and its first two nonlinear Hamiltonian systems were computed. We point out that if we take n = αc n+1, n 1, where α is a constant, the Lax pairs in Section 2. leads to another interesting Lax integrable hierarchy of multi-component Hamiltonian equations, and the resulting Hamiltonian structure is local 9. Note that the Hamiltonian operator J defined by (3.12) is non-local. We also mention that multi-component integrable equations can be generated through other approaches such as perturbations 5, 14, loop algebra methods 15 and enlarging spectral problems based on semi-direct sums of loop algebras 16, 17. Those approaches provide clues for classifying multi-component integrable equations, in particular, Hamiltonian equations 13, 18. References 1 W. X. Ma, Integrability, in: The Encyclopedia of Nonlinear Science, ed. A. Scott, Fitzroy Dearborn Publishers, New York, 2005, pp450 453. 2 M. J. Ablowitz and P. A. Clarkson, Solitons, Nonlinear Evolution Equations and Inverse Scattering, London Mathematical Society Lecture Note Series 149, Cambridge University Press, Cambridge, 1991. 3 A. Degasperis, Resource Letter Sol-1: Solitons, Amer. J. Phys. 66 (1988) 486 497. 4 M. J. Ablowitz, D. J. Kaup, A. C. Newell and H. Segur, The inverse scattering transform-fourier analysis for nonlinear problems, Stud. Appl. Math. 53 (1974) 249 315.
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