SpringerBriefs in MathematicsThis work is subject to copyright. All rights are reserved by the Publisher, whether the whole or part of the material is concerned, specifically the rights of translation, reprinting, reuse of illustrations, recitation, broadcasting, reproduction on microfilms or in any other physical way, and transmission or information storage and retrieval, electronic adaptation, computer software, or by similar or dissimilar methodology now known or hereafter developed. Exempted from this legal reservation are brief excerpts in connection with reviews or scholarly analysis or material supplied specifically for the purpose of being entered and executed on a computer system, for exclusive use by the purchaser of the work. Duplication of this publication or parts thereof is permitted only under the provisions of the Copyright Law of the Publisher's location, in its current version, and permission for use must always be obtained from Springer. Permissions for use may be obtained through RightsLink at the Copyright Clearance Center. Violations are liable to prosecution under the respective Copyright Law. The use of general descriptive names, registered names, trademarks, service marks, etc. in this publication does not imply, even in the absence of a specific statement, that such names are exempt from the relevant protective laws and regulations and therefore free for general use. While the advice and information in this book are believed to be true and accurate at the date of publication, neither the authors nor the editors nor the publisher can accept any legal responsibility for any errors or omissions that may be made. The publisher makes no warranty, express or implied, with respect to the material contained herein.Printed on acid-free paper Springer is part of Springer Science+Business Media (www.springer.com) To Ceci, Fede, and Selva Preface I used to think that the Sturm-Liouville theory of second-order ordinary differential equations was one of the most beautiful areas of mathematics. Its simplicity, together with the power of the comparison and oscillation theorems, shed a different light on second-order ordinary differential equations. However, while reading a transcription of a talk of G.C. Rota, I realized something: there are many interesting problems, both of theoretical and applied origin, that cannot be analyzed with the Sturmian tools.Take the unit ball in R N : just the simple reduction to polar coordinates introduces the coefficient r N−1 , which vanishes at the origin and is bounded above by 1, for all N. Moreover, Bessel, Hermite, Legendre, . . . , almost all the special families of functions that appear as eigenfunctions of second-order ordinary differential operators, are indeed eigenfunctions of singular or degenerate operators, and the Sturmian arguments fail. What can we do now?If we write the Sturmian bounds in modern notation, we are using the L ∞ norm of the weight, and what happens if we change it to another norm, say L 1 ? Indeed, the answer is known, and it is rela...