Editorial Comments: These are the notes for a lecture given at the Institute for Advanced Study on March 30, 2000. It is a preliminary version of a much more carefully written account of essentially the same material published in the volume dedicated to Joseph Shalika, which will also appear on this site eventually.

Author's comments: This paper has appeared in the Shalika volume, published by the Johns Hopkins University Press. The published paper is not exactly the same as that here because the text was modified by the Press, apparently for reasons of economy. Among other things footnotes were removed and this required somewhat brutal measures. The version in this collection is the preferred version.

Author's comments: This paper is quite informal and I could not immediately reflect on the suggestions it contains. I am grateful to Freydoon Shahidi for suggesting that as an interim measure I write the paper for submission to the Canadian Mathematical Bulletin, where it appeared in volume 50 (2007).

Author's comments:The most important point for the innocent or inexperienced reader of this paper to understand is that it is the stable trace formula that is here invoked. The stable trace formula, introduced many years ago in the reference [L2], developed and applied in the references [K1], [K2], [K3] and, more recently, in a very systematic way and to extremely good effect in [A2], is what allows the introduction of the Steinberg-Hitchin base and of the Poisson summation formula. I, myself, hope to develop some of the consequences of this, some certain, some as yet only possible, in subsequent papers. The present paper was written together with Edward Frenkel and Ngô Bảo Châu. The paper is expected to appear in Annales des sciences mathématiques du Québec.

Author's comments (2021-06-22): The paper as presented here is not the published paper. That was unfortunately modified, namely slightly abridged, by the editors without consulting the author and without his approval. The present paper, the original paper, is the preferred form.

Author's comments: This text is provisional from a mathematical point of view, but it may be some time before the obstacles described in the concluding sections are overcome. Serious progress has been made by Ali Altuğ.

It has been easy to misconstrue the principal purpose of this paper and of the previous paper, at least my principal purpose. It was to introduce the use of the Poisson formula in combination with the stable transfer as a central tool in the development of the stable trace formula and its applications to global functoriality. Unfortunately the review in Math. Reviews was inadequate, simply reproducing the abtract, written not by me but by the editors, ``A transfer similar to that for endoscopy is introduced in the context of stably invariant harmonic analysis on reductive groups. For the group \(\mathrm{SL}(2)\), the existence of the transfer is verified and some aspects of the passage from the trace formula to the Poisson formula are examined.'' This transfer is for me a central issue for harmonic analysis on reductive groups over local fields. The problems it raises have, so far as I know, not been solved even over \(\mathbf R\) and \(\mathbf C\). Its construction for \(\mathrm{SL}(2)\) over \(p\)-adic fields, \(p\) odd, was, and remains, for me an interesting application of the explicit formulas of Sally-Shalika for the characters of that group.

Author's comments: There is as yet no text with the title Functoriality and Reciprocity. Begun as preparations for a lecture, the text appearing here was, and remains, a first attempt to come to terms with the two topics of the title, an attempt that is perhaps doomed by its nature and by my years to remain provisional. The attempt demands not only a great deal of reflection and a good number of novel ideas, mine or those of someone else, but also a mastery of several mathematical domains --- algebraic number theory, algebraic geometry, spectral analysis, representation theory, some differential geometry, some mathematical physics --- of all of which I have acquired a smattering of knowledge over the past decades, although without mastering any. As it stands here it is only a part of the Prologue. The present text, A prologue to ``Functoriality and Reciprocity'', Part I appears, in spite of its failings, in a volume dedicated to the memory of Jonathan Rogawski, Pacific Journal of Mathematics, vol. 260, No. 2, Dec. 2012.

Editorial comments: This essay was first posted here on December 20, 2013. The latest version dates to May 16, 2014.

Author's comments. Although these notes were written as a foreword to, or an appreciation of, a book by Qing Zou that is to appear soon, the primary purpose for me was to describe some possibilities in the theory of automorphic representations upon which I believe it is important for specialists to reflect. I was grateful to Qing Zou for the somewhat unexpected request to write an appreciation. The book itself will appear in Chinese; the translated title is ``From Kummer to Langlands---The history of the Langlands Program''

Author's comments: These terse comments were intended as a suggestion not to read the Mostow lecture or listen to it with preconceptions about the nature of a geometric theory.

Author's comments (Apr. 6, 2014): The concept ``Langlands program'' appears in the title of an article by Stephen Gelbart in the BAMS of April, 1984, but Gelbart himself assured me that it was already current, at least orally, before then. He also drew my attention to a phrase of Armand Borel in his Bourbaki seminar of June, 1975, ``plutôt un vaste programme, élaboré par R. P. Langlands depuis environ 1967.'' I do not recall that I was uneasy with the phrase ``Langlands program'' in 1984, but it then referred principally to matters on which I myself had long reflected. This has since changed. It has come to refer to a domain much larger than the analytic theory of automorphic forms and its arithmetic applications.

This is reflected in the message to Sarnak. Although this perhaps does not correspond exactly to the historical development, the enlargement can be described in two stages. I discuss them separately. The first is the extension of the theory---both the established and, in some regards, also the conjectural form---from a finite extension \(F\) of \(\mathbf Q\) as the base field to a different kind of base field, to an algebraic function field \(F\) over an algebraic curve associated to a field of constants that is either a finite field or the complex number field \(\mathbf C\). It is the second possibility that has the novel ramifications. My source of information on the first and on the second enlargement has been various articles of Edward Frenkel. These articles are impressive achievements but often freewheeling, so that, although I have studied them with considerable care and learned a great deal from them that I might never have learned from other sources, I find them in a number of respects incomplete or unsatisfactory.

As I attempted to explain in the Mostow lecture, I believe that the geometric theory as such should be separated from any kind of duality in physics and treated purely mathematically with less emphasis on sheaf theory than is usually met and with a larger dose of ``classical mathematics'': spectral theory, differential geometry, and algebraic geometry. The algebraic geometry and the spectral theory will, I believe, have to mesh, thus one will be obliged, despite the algebraic geometers, to work with the full classifying space \(\mathrm{Bun}_G\). I am hopeful that such a theory can be constructed in a satisfactory and natural fashion, but this demands a mastery of the pertinent mathematics.

It is the third item of the message to Sarnak whose complexity I did not adequately appreciate as I wrote it. Before explaining this, let me comment on the structures implicit in the classical theory and in the geometric theory. In the classical theory, there is a reductive group \(G\) over a number field and a reductive group \({}^LG\) over \(\mathbf C\). The second group may have several components whose source lies in Galois theory, but that is not the issue here. Their relation is expressed by functoriality. A second aspect of the theory is the relation between the group \({}^LG\) and the Galois group of \(F\) or, more conjecturally, the motivic group over \(F\). This I refer to as reciprocity, the first manifestation of the pertinent phenomena being the law of quadratic reciprocity. Neither aspect has yet been developed to the extent I believe possible, but both principles have led to strong results whose mathematical importance is undeniable and unlikely to be ephemeral.

In the geometric theory as such there is, so far as I can see, no reciprocity, just functoriality and this manifests itself as a parametrization of automorphic representations by connections, in the sense of differential geometry, with values in \({}^LG\), which is often taken to be connected. The possible construction of such a theory, envisioned in the third item of the message, is one of the problems considered in the Mostow lecture. The marked difference between the geometric theory over a closed nonsingular Riemann surface and the arithmetic theory is that in the geometric theory \(G\) and \({}^LG\) are groups of the same type, namely effectively groups over \(\mathbf C\). This leads to a third possibility in addition to functoriality and reciprocity and to a second stage, namely duality. Here we meet problems outside the domain of pure mathematics.

One popular introduction to the topic is Frenkel's Bourbaki lecture, Gauge theory and Langlands duality. On the first page, he describes electro-magnetic duality as an aspect of the Maxwell equations and their quantum-theoretical form or, more generally, as an aspect of four-dimensional gauge theory. This duality is quite different than the functoriality and reciprocity introduced in the arithmetic theory. It entails a supplementary system of differential equations. Moreover, it has to be judged by different criteria. One is whether it is physically relevant. There is, I believe, a good deal of scepticism, which, if I am to believe my informants, is experimentally well-founded. Although the notions of functoriality and reciprocity have, on the whole, been well received by mathematicians, they have had to surmount some entrenched resistance, perhaps still latent. So I, at least, am uneasy about associating them with vulnerable physical notions. On the other hand, as strictly mathematical notions this duality and various attendant constructions, such as the Hitchin fibration, appear to have proven value, especially for topologists and geometers. Whether it is equal to that of functoriality and reciprocity is open to discussion.

In contrast to what I foresaw when describing item (iii), the considerable reflection on duality contemplated there will demand a more sophisticated understanding of topology, geometry, and the relevant physics than I can ever hope to possess.

Recoverable fatal error: Argument 1 passed to date_granularity_precision() must be of the type array, null given, called in /var/www/publications/sites/all/modules/date/date.theme on line 79 and defined in date_granularity_precision() (line 1665 of /var/www/publications/sites/all/modules/date/date_api/date_api.module).