Heisenberg group and Lewy operator.1. Introduction The existence theorems, elliptic el·lip·tic or el·lip·ti·cal adj. 1. Of, relating to, or having the shape of an ellipse. 2. Containing or characterized by ellipsis. 3. a. boundary problems and boundary value problems in the theory of differential operators (with constant or variable coefficients) on [R.sup.n], have a wide applications in the whole sciences. The results for differential operators with constant coefficients In mathematics, constant coefficients is a term applied to differential operators, and also some difference operators, to signify that they contain no functions of the independent variables, other than constant functions. were first obtained by, Malgrange [11], Treves [13], Atiyah [1], Hormander [8] and many others. Elliptic operators and boundary value problems were studied by [2, 10, 12], hypoelliptic and hyperbolic hy·per·bol·ic also hy·per·bol·i·cal adj. 1. Of, relating to, or employing hyperbole. 2. Mathematics a. Of, relating to, or having the form of a hyperbola. b. equations with constant coefficients were developed by Hormander [8]. Unfortunately, L. Hormander in 1960 [8, P. 156] by his necessary condition had discovered that the situation is completely different when the coefficients are variables P(x,D)u = f on [R.sup.n], where [MATHEMATICAL EXPRESSION NOT REPRODUCIBLE IN ASCII ASCII or American Standard Code for Information Interchange, a set of codes used to represent letters, numbers, a few symbols, and control characters. Originally designed for teletype operations, it has found wide application in computers. ] has a locally solution. By applying this condition on the operator P = i[[partial derivative].sub.t] + [[partial derivative].sub.y]  2iy[[partial derivative].sub.z]  2x[[partial derivative].sub.z] (1.1) the hypotheses of this condition are not fulfilled for every open set [OMEGA] of [R.sup.4]. So the operatorP is not locally solvable. In dealing with the non existence of solutions of partial differential operators it was customary during the last fifty years and it still is to day in larger applications, to appeal to the necessary Hormander condition which guarantees the non existence of solutions on [R.sup.4]. The goal of this paper is to construct a solvable algebra of partial differential equations on [R.sup.2n+1] for which the equation P is among of its elements. Thanks to the magic (and obvious) relation between the 2n + 1dimensional Heisenberg group In mathematics, the term Heisenberg group, named after Werner Heisenberg, refers to the group of 3×3 upper triangular matrices of the form 2. A Solvable Algebra A linear partial differential operator differential operator In mathematics, any combination of derivatives applied to a function. It takes the form of a polynomial of derivatives, such as D^{2}_{xx} − D^{2}_{xy} ∙ D in 2n +1 independent variables z,[x.sub.1],[x.sub.2],...,[x.sub.n],[y.sub.1], [y.sub.2],...,[y.sub.n], with constant coefficients defined on [R.sup.2n+1] is polynomial polynomial, mathematical expression which is a finite sum, each term being a constant times a product of one or more variables raised to powers. With only one variable the general form of a polynomial is a_{0}x^{n}+a_{} in partial differentiations and has the form [MATHEMATICAL EXPRESSION NOT REPRODUCIBLE IN ASCII] (2.1) here [beta] and [gamma] are multiindex, that is, an ntuple of integers [[beta].sub.j] [greater than or equal to] 0 (reesp. [[gamma].sub.j] [greater than or equal to] 0) and a [member of] N; [absolute value of [alpha] + [beta] + [gamma]] denotes their length [alpha] + [[beta].sub.1] + [[beta].sub.2] + ... + [[beta].sub.n] + [[gamma].sub.1] + [[gamma].sub.2] + ... + [[gamma].sub.n] = m. Also [MATHEMATICAL EXPRESSION NOT REPRODUCIBLE IN ASCII] The integer m is usually the order of the operator; this assumes that for some multiindex [alpha], [beta], [gamma] with length [absolute value of [alpha] + [beta] + [gamma]] = m the coefficient [C.sub.[alpha],[beta],[gamma]] [member of] C is not identically equal to zero. Denote by [D.sub.c] the algebra of all linear partial differential operator in 2n + 1 independent variables z,[x.sub.1],[x.sub.2],...,[x.sub.n],[y.sub.1],[y.sub.2],...,[y.sub.n], with constant coefficients defined on [R.sup.2n+1]. Definition 2.1. For every f [member of] [C.sup.[infinity]]([R.sup.2n+1]), one can define a function [??](f) [member of] [C.sup.[infinity]]([R.sup.2n+1]), by the following manner [??](f)(z,y,x) = f(z  2<x,y>,y  x) (2.2) we see immediately that the mapping [??]: [C.sup.[infinity]]([R.sup.2n+1]) [right arrow] [C.sup.[infinity]]([R.sup.2n+1]) is topological isomorphism isomorphism (ī'səmôr`fĭzəm), of minerals, similarity of crystal structure between two or more distinct substances. Sodium nitrate and calcium sulfate are isomorphous, as are the sulfates of barium, strontium, and lead. from [C.sup.[infinity]]([R.sup.2n+1]) onto [C.sup.[infinity]]([R.sup.2n+1]) and [[??].sup.2] = I, where I is the identity operator of [C.sup.[infinity]]([R.sup.2n+1]). Theorem theorem, in mathematics and logic, statement in words or symbols that can be established by means of deductive logic; it differs from an axiom in that a proof is required for its acceptance. 2.2. Let Q([partial derivative]/[[partial derivative].sub.z], [partial derivative]/[partial derivative]y, [partial derivative]/[[partial derivative].sub.x]) be a linear partial differential operator with constant coefficients on [R.sup.2n+1], then there is a differential operator P([partial derivative]/[[partial derivative].sub.z], [partial derivative]/[partial derivative]y, [partial derivative]/[[partial derivative].sub.x]) with variable coefficients, such that [??]Pf = Q[??]f (2.3) where X = (z,y,x) and [MATHEMATICAL EXPRESSION NOT REPRODUCIBLE IN ASCII] (2.4) Proof. Let f [member of] [C.sup.[infinity]]([R.sup.2n+1]), and [MATHEMATICAL EXPRESSION NOT REPRODUCIBLE IN ASCII] the partial differentiation the variable [x.sub.j], then we have for any 1 [less than or equal to] j [less than or equal to] n [MATHEMATICAL EXPRESSION NOT REPRODUCIBLE IN ASCII] (2.5) where x + [t.sub.j] = ([x.sub.1],[x.sub.2],...,[x.sub.j] + [t.sub.j],...,[x.sub.n]). A similar for the partial differentiations [MATHEMATICAL EXPRESSION NOT REPRODUCIBLE IN ASCII](1 [less than or equal to] j [less than or equal to] n) and [partial [derivative].sub.z], we get [MATHEMATICAL EXPRESSION NOT REPRODUCIBLE IN ASCII] (2.6) for every f [member of] [C.sup.[infinity]]([R.sup.2n+1]). Which is the theorem. Corollary 2.3. Let D be the algebra of partial differential operators with variable coefficients on [R.sup.2n+1], then the mapping T: [D.sub.c] [right arrow] D (2.7) defined by T(Q) = [??]Q[??] is onetoone algebra homomorphism A homomorphism between two algebras over a field K, A and B, is a map such that for all k in K and x,y in A,
2.1. Application to the Local Solvability solv·a·ble adj. Possible to solve: solvable problems; a solvable riddle. solv of the Equation (1.1) Definition 2.4. For every f [member of] [C.sup.[infinity].sub.0]([R.sup.4]), one can define a function [??]f [member of] [C.sup.[infinity]]([R.sup.4]) as follows: [??]f(z,y,x,t) = f(z  2xy,y,0,t + x) (2.8) for any (z,y,x,t) [member of] L. Note that the function [??]f is invariant (programming) invariant  A rule, such as the ordering of an ordered list or heap, that applies throughout the life of a data structure or procedure. Each change to the data structure must maintain the correctness of the invariant. in the following sense: [??]f(k(z,y),x  k, t + k) = [??]f(z,y,x,t) (2.9) for any z [member of] R, y [member of] R, x [member of] R, t [member of] R and k [member of] R. Let D'([R.sup.4]) be the space of distributions on [R.sup.4] and let S(y,x) be a distribution on [R.sup.2], such that QS(y,x) = [delta](y,x) where Q = i[[partial derivative].sub.x] + [[partial derivative].sub.y] and [delta](y,x) is the Dirac measure In mathematics, a Dirac measure is a measure δ_{x} on a set X (with any σalgebra of subsets of X) that gives the singleton set the measure 1, for a chosen element x ∈ X: <T(z,y,x,t),f(z,y,x,t)> = <[delta](z)S(y,x)[delta](t), f(z,y,x,t)> (2.10) for all f [member of] [C.sup.[infinity]]([R.sup.4]) and (z,y,x,t) [member of] [R.sup.4], where [delta](z) (resp.[delta](t)) is the Dirac measure on R at 0 Theorem 2.5. For every f [member of] [C.sup.[infinity]]([R.sup.4)], let [??]T be the distribution on [R.sup.4] defined by <[??]T(z,y,x,t), f(z,y,x,t)> = <T(z,y,x,t), [??]f(z,y,x,t)> (2.11) Then [??]T is a fundamental solution of the equation P = i[[partial derivative].sub.t] + [[partial derivative].sub.y]  2iy[[partial derivative].sub.z]  2x[[partial derivative].sub.z] P = i[[partial derivative].sub.t] + [[partial derivative].sub.y]  2iy[[partial derivative].sub.z]  2x[[partial derivative].sub.z] (2.12) Proof. For any f [member of] [C.sup.[infinity]]([R.sup.4]), we get [MATHEMATICAL EXPRESSION NOT REPRODUCIBLE IN ASCII] (2.13) Definition 2.6. [6] One can define a topological isomorphism H of [C.sup.[infinity]]([R.sup.3] x [R.sup.*.sub.+]),as follows H(f)(z,y,x,t) = f(z  [lambda]xy, ty, [t.sup.1] x, [t.sup.1]) (2.14) Corollary 2.7. Let Q([partial derivative]/[[partial derivative].sub.z], [partial derivative]/[partial derivative]y, [partial derivative]/[[partial derivative].sub.x],[partial derivative]/[partial derivative]t) be a linear partial differential operator with constant coefficients on [R.sup.3] x [R.sup.*.sub.+], then there is a differential operator P{X, [partial derivative]/[[partial derivative].sub.z], [partial derivative]/[partial derivative]y, [partial derivative]/[[partial derivative].sub.x],[partial derivative]/[partial derivative]t), and X = (z,y,x,t). HPf = QHf (2.15) where [MATHEMATICAL EXPRESSION NOT REPRODUCIBLE IN ASCII] The proof of this corollary is similar to the proof of theorem 2.1. 3. Lewy Operator and Heisenberg Group Lewy in 1957 [9] had proved that if the equation LT = ([[partial derivative].sub.x]  i[[partial derivative].sub.y] + 2ix[partial derivative]z  2y[partial derivative]z)T = f (3.1) for a real function f [member of] [C.sup.1](R) have any solution function T, with T in [C.sup.1]([R.sup.3]). Then f is analytic. In dealing with the non existence of solutions of partial differential operators it was customary during the last fifty years and it still is to day in larger applications, to appeal to the example of the Lewy operator which guarantees the non existence of solutions for any f [member of] [C.sup.[infinity]]([R.sup.3]), where [C.sup.[infinity]]([R.sup.3]) is the space of [C.sup.[infinity]]functions on [R.sup.3]. Understanding the nature of these kind of partial differential operators and their invariance in·var·i·ant adj. 1. Not varying; constant. 2. Mathematics Unaffected by a designated operation, as a transformation of coordinates. n. An invariant quantity, function, configuration, or system. on the Heisenberg group requires the admission of solutions. It was therefor a matter of considerable surprise to the author, to discover that this inference is returned erroneous. More precisely, the Lewy operator L is solvable. Theorem 3.1. The Lewy operator L is globally solvable. Proof. Let [??] be the transformation of [C.sup.[infinity]]([R.sup.3]) defined by [??](f)(z,y,x) = f(z  2xy, y, x) (3.2) and let Q = [[partial derivative].sub.x]  i[[partial derivative].sub.y] be the CauchyRiemann operator, then we have [MATHEMATICAL EXPRESSION NOT REPRODUCIBLE IN ASCII] (3.3) where [sigma]f(z,y,x) = f(z,y,x). Since the operator R = [[partial derivative].sub.x]  2y[[partial derivative].sub.z]  i[[partial derivative].sub.y] + 2ix[[partial derivative].sub.z] is solvable because [sigma]([??]([[partial derivative].sub.x]  i[[partial derivative].sub.y])[??]f)(z,y,x) = R([sigma]f)(z,y,x), i.e for any function g [member of] [C.sup.[infinity]]([R.sup.3]), there is a function [psi] [member of] [C.sup.[infinity]]([R.sup.3]) such that R[psi](z,y,x) = g(z,y,x), and for any [psi] [member of] [C.sup.[infinity]]([R.sup.3]), there is f [member of] [C.sup.[infniity]]([R.sup.3]), such that [sigma]f(z,y,x) = [psi](z,y,x). Thus we get ([[partial derivative].sub.x]  i[[partial derivative].sub.y] + 2x[partial derivative]z  2iy[[partial derivative].sub.z])f(z,y,x) = R([sigma]f)(z,y,x) = g(z,y,x) Hence the Theorem. Theorem 3.2. The following conditions are verified: (i) The extended Lewy operator is globally solvable. (ii) The Lewy operator L has a fundamental solution. Proof. (i) results immediately from equations (16). To prove (ii), we use the fact that the operator P = [[partial derivative].sub.x]  2y[[partial derivative].sub.z]  i[[partial derivative].sub.y]  2ix[[partial derivative].sub.z], is locally solvable. Let T [member of] D'(G) and let [??] be the distribution defined by [MATHEMATICAL EXPRESSION NOT REPRODUCIBLE IN ASCII] (3.4) for any [phi] [member of] [C.sup.[infinity].sub.0]([R.sup.3]). Then we have [MATHEMATICAL EXPRESSION NOT REPRODUCIBLE IN ASCII] (3.5) So if T is a fundamental solution of P then [??] is a fundamental solution of L. 3.1. Two Examples In the followings will give two examples of different type of the Heisenberg group on which the Lewy operator L is invariant on each one. For each type we find a relationship between L and the CauchyReimann operator which gives the solvability of L. Example 3.3. G. B. Folland, In his book [4, P.56] had considered the following Lewy operator L = [[partial derivative].sub.x] + i[[partial derivative].sub.y]  [1/2]y[[partial derivative].sub.z]  [i/2]x[[partial derivative].sub.z] (3.6) as a left invariant on the 3dimensional Heisenberg group [R.sup.3] with law (z,y x)(z',y',x') = (z + z' + [1/2](xy'  x'y), y + y', x + x') (3.7) To solve this operator, we consider the mapping [LAMBDA]: [C.sup.[infinity]]([R.sup.3]) [right arrow] [C.sup.[infinity]]([R.sup.3]), which is defined by [LAMBDA](f)(z,y,x) = f(z  [1/2]xy, y, x) (3.8) and the CauchyReimann operator Q = [[partial derivative].sub.x] + i[[partial derivative].sub.y] (3.9) The mapping [LAMBDA] is topological isomorphism of [C.sup.[infinity]]([R.sup.3]), So we obtain the following theorem. Theorem 3.4. The Lewy operator L verifies the following properties L[C.sup.[infinity]]([R.sup.3]) = [C.sup.[infinity]]([R.sup.3]) (3.10) Proof. In fact if f [member of] [C.sup.[infinity]]([R.sup.3]), then we have [LAMBDA]([[partial derivative].sub.x]+i[[partial derivative].sub.y])[LAMBDA]f(z,y,x) = ([[partial derivative].sub.x] [1/2]y[[partial derivative].sub.z] + i[[partial derivative].sub.y][i/2]x[[partial derivative].sub.z]) f(z,y,x) (3.11) So the solvability of L. Example 3.5. F. Rouviere had proved in his paper [3] the following Lewy operator P = [[partial derivative].sub.x]  i[[partial derivative].sub.y] + 2y[[partial derivative].sub.z]  2ix[[partial derivative].sub.z] (3.12) is left invariant on the 3dimensional Heisenberg group [MATHEMATICAL EXPRESSION NOT REPRODUCIBLE IN ASCII] (3.13) where x [member of] R, y [member of] R, and z [member of] R. Let R = [[partial derivative].sub.x]  i[[partial derivative].sub.y] and define an operator T on [C.sup.[infinity]]([R.sup.3]), carrying a function f(x) into the function defined by Tf(z,y,x) = f ([[xy]/2] + [z/4],y, x) (3.14) It is clear that the mapping f [right arrow] Tf is a topological isomorphism of [C.sup.[infinity]]([R.sup.3]), and by a similar calculation as in Example 3.3 we obtain the following equation and the solvability of the operator P Pf(z,y,x) = JRJf(z,y,x) (3.15) Acknowledgements Author would like to thank Abdaleziz Majed AlEnad, for his support. References [1] Atiyah M.F., 1970, Resolution of Singularities In algebraic geometry, the problem of resolution of singularities asks whether any algebraic variety has a nonsingular model (a nonsingular variety birational to it). For varieties over fields of characteristic 0 this was proved in , while for varieties over fields of and Division of Distributions, Comm See comms. . on Pure and App. Math., 23:145150. [2] Agmon S., 1965, Lectures on Elliptic Boundary Value Problems, Van Nostrand Mathenmatical Studies 2, Princeton, NJ. [3] Cerezo et A., Rouviere F., 1971, Resolubilite Local d'um operateur differentiel invariant du prenuier ordre, Ann. Scient. E c. Norm. sup [4.sup.e] serie, t.4, 2130. [4] Folland G.B., 1995, Introduction to Partial Differential Equations, Princeton University Princeton University, at Princeton, N.J.; coeducational; chartered 1746, opened 1747, rechartered 1748, called the College of New Jersey until 1896. Schools and Research Facilities Press. [5] ElHussein K., 2006, On the Existence Theorem In mathematics, an existence theorem is a theorem with a statement beginning 'there exist(s) ..', or more generally 'for all x, y, ... there exist(s) ...'. for Invariant Differential Operators Invariant differential operators appear often in mathematics and theoretical physics. There is no universal definition for them and the meaning of invariance may depend on the context. Usually, an invariant differential operator on the Heisenberg Group, Global Journal of Pure and Applied Mathematics, ISSN ISSN abbr. International Standard Serial Number 09731768, 2(2):103109. [6] ElHussein K., 2009, A Fundamental Solution of an Invariant Differential Operator on the Heisenberg Group, International Mathematical Forum, 4(12):601612. [7] ElHussein K., 2009, Research Announcements. Unsolved Problems A list of unsolved problems may refer to several conjectures or open problems in various fields. The problems are listed below:
[8] Hormander L., 1963, Linear Partial Differential Operators, SpringerVerlag, Berlin. [9] Lewy H., 1957, An Example of a Smooth Linear Partial Differential Equation differential equation Mathematical statement that contains one or more derivatives. It states a relationship involving the rates of change of continuously changing quantities modeled by functions. Without Solution, Ann. Math., 66(2):155158. [10] Lions J.L. and Magenes E., 1972, Nonhomogeneous Boundary Value Problems and Applications, SpringerVerlag, Berlin. [11] Malgrange B., 1955, Existence and Approximation des Solutions des Equations aux Derivees Partielles et des Equations de Convolutions, Ann. Inst. Fourier Grenoble, Vol. 6, pp. 271. [12] Nerenberg L., 1959, On Elliptic Partial Differential Equations, Ann. Scuola Norm. Sup. Pisa, 13(3):115162. [13] Treves F., 1966, Linear Partial Differential Equations with Constant Coefficients, Gordon and Breach. Kahar ElHussein Department of Mathematics, Faculty of Science, AlJouf University, KSA KSA Kingdom of Saudi Arabia KSA Korean Student Association (student organization providing cultural awareness and community empowerment) KSA Knowledge, Skills & Abilities KSA Knowledge, Skills and Attitudes KSA Korean Standards Association Email: kahar_h990@yahoo.com, kumath@hotmail.com 

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