Orthogonally Additive Polynomials and Applications Polinomios Ortogonalmente Aditivos Y Aplicaciones

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Orthogonally Additive Polynomials and Applications Polinomios Ortogonalmente Aditivos Y Aplicaciones Orthogonally additive polynomials and applications Polinomios ortogonalmente aditivos y aplicaciones Pablo Linares Briones Directores de Tesis: Alberto Ibort Latre Jos´eLuis Gonz´alez Llavona Memoria para optar al grado de Doctor en Matem´aticas. Diciembre de 2009 Departamento de An´alisisMatem´atico. Universidad Complutense de Madrid. ii Contents 1 Introduction. 1 1.1 Notations and preliminaries. 10 2 Orthogonally Additive Polynomials 13 2.1 Introduction. 13 2.2 The Representation Theorem . 15 2.2.1 Introduction. 15 2.2.2 The Representation Theorem. 16 2.2.3 Orthogonally Additive Polynomials in order continu- ous K¨othefunction spaces. 17 2.3 The Representation Theorem in `p. ............... 19 2.4 Orthogonally Additive Polynomials in Hilbert lattices. 25 2.5 Spectral Theorem . 29 3 O. A. Polynomials on Riesz espaces. 33 3.1 Riesz spaces. 34 3.2 Orthosymmetric Applications . 37 3.3 Powers of Riesz spaces. 45 3.4 Representation of polynomials . 47 4 Orthogonal and P-orthogonal 51 4.1 Introduction. 51 4.2 P-orthogonal Polynomials. 53 4.2.1 P-orthogonal Polynomials in C[0, 1]. 54 4.2.2 P-orthogonal Polynomials in Lp.............. 57 4.2.3 The problem of existence . 61 4.3 Orthogonal Polynomials respect to a bilinear form. 66 iii 5 Multilinear Hausdorff Problem 69 5.1 Introduction. 69 5.2 Weak Hausdorff Problem . 73 5.2.1 Polymeasures and a multilinear Riesz representation theorem. 73 5.2.2 The weak Hausdorff multilinear moment problem. 75 5.3 Classical Hausdorff problem . 82 5.4 Positivity and the Hausdorff problem. 84 5.5 The strong Hausdorff multilinear moment problem. 88 6 Multilinear Trigonometric Problem 91 6.1 The result. 93 6.2 Examples. 96 iv Resumen. En toda teor´ıade operadores, tanto lineales como no lineales, es fundamental disponer de teoremas de representaci´onque nos permitan el manejo de los mismos. Un buen ejemplo de este hecho lo encontramos en el marco del An´alisisFuncional lineal con el Teorema de Riesz para la representaci´on integral de los funcionales lineales y continuos sobre un espacio C(K). En los ´ultimos a˜noshan sido muchos los matem´aticos interesados en el estudio de operadores no lineales entre espacios de Banach, en particular de polinomios. De nuevo, siempre es deseable disponer de teoremas de rep- resentaci´on que nos permitan estudiar su comportamiento y propiedades. Recientemente, Benyamini, Lassalle y Llavona obtienen en [BLLl] un teo- rema de representaci´onpara la clase de polinomios ortogonalmente aditivos sobre ret´ıculos de Banach. La potencia de este teorema radica en que todo polinomio n-homog´eneode esta clase se representa a trav´esde un funcional lineal y continuo sobre la n-concavificaci´ondel espacio, es decir, su estudio se reduce a un problema lineal. Este es el punto de partida de esta tesis doctoral. Siguiendo el enfoque mencionado, hemos obtenido numerosas aplicaciones a campos tan diversos como la teor´ıaespectral de operadores en espacios de Hilbert, resultados de estructura de ret´ıculos de Hilbert, desarrollo de una teor´ıasimilar a la de polinomios ortogonales pero aplicada a una clase m´asamplia de funciones poniendo especial atenci´onen espacios C[0, 1] y Lp. Finalmente, hemos desarrollado la teor´ıacl´asicade momentos de Hausdorff, tanto en el caso real del intervalo [0, 1] como en el caso trigonom´etrico del c´ırculounidad T, estudiando el problema multilineal. El cap´ıtulo1 se dedica a notaciones y conceptos preliminares. En el cap´ıtulo2 introducimos los polinomios ortogonalmente aditivos so- bre ret´ıculos de Banach. Sundaresan fue el primer matem´atico que estudi´o la representaci´on de polinomios ortogonalmente aditivos en [S2]. Sus resul- v p tados se centran en los espacios `p y L de funciones. Recientemente, como ya hemos indicado, Benyamini, Lassalle y Llavona prueban el caso general en [BLLl]. Han aparecido otros trabajos para el caso C(K) (ver [PGV] y [CLZ1]) y para C∗-´algebras [PPV]. En este cap´ıtulopresentamos una nueva prueba del teorema de repre- sentaci´on para el caso `p que aparece en [ILLl3]: Teorema 1. Sea 1 ≤ p < ∞. El espacio de polinomios ortogonalmente n aditivos, n-homog´eneos Po( `p) es isom´etricamente isomorfo a `∞ para 1 ≤ p ≤ n y a `p/p−n para n < p < ∞. Nuestra prueba es totalmente independiente de las anteriores y usa la teor´ıade productos tensoriales. Exploramos adem´aslas aplicaciones del teorema de representaci´onen ret´ıculosde Hilbert probando el siguiente resultado: Teorema 2. Sea H un espacio de Hilbert real y separable. Si H es un ret´ıculo de Banach con orden ≤. Entonces, H es orden isom´etrico a L2(µ) para cierta medida µ. Este teorema puede verse como un caso particular del cl´asico teorema de representaci´on de Kakutani para espacios Lp abstractos. Finalmente encontramos otra aplicaci´ondel teorema de polinomios orto- gonalmente aditivos que nos permite obtener una nueva prueba del teorema espectral de operadores autoadjuntos en espacios de Hilbert. Este estudio aparece en [ILL14]. El cap´ıtulo3 se centra en el estudio de polinomios ortogonalmente aditivos en espacios de Riesz. Siguiendo las ideas del estudio de la ortogonalidad aditiva mediante los productos tensoriales, R. Ryan nos indic´ola direcci´ondel estudio de la or- togonalidad en espacios de Riesz. Dedicamos un cap´ıtulo a la extensi´on de la ortogonalidad aditiva en este tipo de espacios. Relacionaremos nuestro con- cepto con el de ortosimetr´ıa, ya existente en la literatura, y probaremos el siguiente teorema de representaci´ondonde la clave deja de ser la continuidad: Teorema 3. Sean E, F espacios de Riesz Arquimedianos con F uniforme- J + n mente completo y ( n E, n) la n-potencia de E. El espacio Po ( E, F ) de polinomios n-homog´eneos, positivos y ortogonalmente aditivos en E es Riesz isomorfo al espacio L+(J E, F ) de aplicaciones lineales y positivas J n de n E a F . vi En el cap´ıtulo 4 extendemos la teor´ıacl´asica de polinomios ortogonales a una nueva clase de funciones, que contiene a los polinomios, que denom- inamos polinomios P -ortogonales. En la teor´ıacl´asica de polinomios orto- gonales, la ortogonalidad se define con respecto al funcional lineal llamado de momentos. La identificaci´onentre los polinomios ortogonalmente aditivos n-homog´eneos P y las formas lineales TP sobre la n-concavificaci´on,se˜nalada anteriormente, nos permite definir una nueva ortogonalidad con respecto al polinomio P , usando la ortogonalidad cl´asicarespecto a TP . Esta idea simple se manifiesta especialmente interesante en el caso de los espacios C[0, 1] y Lp. Extendemos a este contexto todos los resultados cl´asicosde la teor´ıacomo son el Teorema de Favard, los resultados sobre relaciones de recurrencia a tres t´erminos,teoremas de existencia, etc. Recientemente, entre los especialistas de la teor´ıade polinomios ortogo- nales, se le est´aconcediendo especial relevancia a ortogonalidad con respecto a una forma bilineal. En particular, se ha estudiado los polinomios ortogo- nales de Sobolev, polinomios ortogonales con respecto al producto escalar de Sobolev: k Z X (j) (j) (f, g)S = f g dµj j=0 donde µj son medidas de Borel positivas y las derivadas se consideran en sentido d´ebil (v´easepor ejemplo [MAR], [MMB], [MF1], [MF2], [M] y sus referencias). Usando un potente teorema de Boas [B], Dur´anprueba en [Dur2] un teo- rema que tiene aplicaci´on a la teor´ıade polinomios ortogonales y muestra cuando la ortogonalidad con respecto a una forma bilineal se puede reducir a la ortogonalidad cl´asica. Para terminar el cap´ıtulo 4 extendemos este re- sultado a productos escalares definidos en polinomios algebraicos sobre R obteniendo el siguiente teorema: Teorema 4. Sea B un producto escalar definido en el espacio de polinomios algebraicos reales sobre R. Son equivalentes: • El operador multiplicaci´onpor t es sim´etrico y acotado con respecto de B. • Existe una medida de Borel regular en un compacto de R tal que Z B(p, q) = p(t)q(t)dµ(t). vii Por ´ultimo, dedicamos dos cap´ıtulosal problema multilineal de momen- tos. Cada cap´ıtuloextiende el problema de momentos cl´asico al caso mul- tilineal. Estudiamos dichos problemas para el caso de Hausdorff, es decir cuando los polinomios est´andefinidos en I = [0, 1] y tambi´enpara el caso de polinomios trigonom´etricos definidos en el c´ırculo unidad T. El funcional de momentos L asociado a una sucesi´on µk, k ∈ N de n´umeros reales, es el elemento del dual (algebraico) del espacio de polinomios R[t] definido por: X L(p) = pkµk, k≥0 P k donde p(t) = k≥0 pkt ∈ R[t] es un polinomio arbitrario. Dado un intervalo I ⊂ R el problema cl´asicode momentos se traduce en encontrar la integra- lidad de dicho operador, es decir, bajo que condiciones, existe una medida k R k regular µ en I ⊂ R tal que L(t ) = I t dµ(t), k = 0, 1 ... Si I = [0, 1] el problema de momentos se conoce como problema de momentos (cl´asico) de Hausdorff. La soluci´on del problema cl´asicode Hausdorff (ver por ejemplo [ST] y sus referencias) establece que la medida µ existe si para cierta constante C: k X |λ(k,n)| < C, m=0 para todo k = 0, 1,..., y 0 ≤ m ≤ k, donde k λ = (−1)k−m∆k−mµ , 0 ≤ m ≤ k, (k,m) m m y ∆ denota el operador: ∆µn = µn − µn−1, que cumple: k X k ∆kµ = (−1)m µ . n m n+k−m m=0 La soluci´on original de este problema, basada en propiedades de compaci- dad del conjunto de funciones de variaci´on acotada, fue utilizada para obtener varias versiones del teorema de Riesz (ver por ejemplo [Hil2]).
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