A core concept in mathematics, engineering, and physics is the Fourier Transform (FT) and its many variants (Generalized Fourier Series, Green's Function, Pontryagin duality).

The basic algorithm is to find dual sets of eigenvectors/eigenfunctions parametrized by a continuous (e.g., $\omega$ below) or discrete index (e.g., $n$ below), that satisfy completeness and orthogonality relations encapsulated in Dirac delta function resolutions such as that for the FT

$$\delta(x-y)= \int_{-\infty}^{\infty}\exp(i2\pi \omega x)\exp(-i2\pi \omega y)d\omega$$

giving

$$\int_{-\infty}^{\infty}f(y)\delta(x-y)dy=f(x)=\int_{-\infty}^{\infty}\exp(i2\pi \omega x)\int_{-\infty}^{\infty}f(y)\exp(-i2\pi \omega y) dy d\omega$$

$$=\int_{-\infty}^{\infty}\exp(i2\pi \omega x)\hat{f}(\omega) d\omega,$$

or that for the eigenvectors of Sturm-Liouville differential operators over finite domains

$$\delta(x-y)=\sum_{n=0}^{\infty }\Psi_n(x)\Psi_n^*(y)$$

giving

$$f(x)=\sum_{n=0}^{\infty }\Psi_n(x)\int_{a}^{b}f(y)\Psi_n^*(y) dy,$$

or Kronecker delta resolutions such as that for the associated Laguerre functions

$$\frac{(n+\alpha)!}{n!}\delta_{mn}=\int_{0}^{\infty}x^{\alpha}e^{-x}L_{n}^{\alpha}(x)L_{m}^{\alpha}(x)dx$$

giving

$$f(x)=\sum_{n=0}^{\infty }\frac{n!L_{n}^{\alpha}(x)}{(n+\alpha)!}\hat{f}_n$$

with

$$\hat{f}_n=\int_{0}^{\infty}x^{\alpha}e^{-x}L_{n}^{\alpha}(x)f(x)\,dx.$$

The basic "physical" operation (BPO) at work here can be regarded as destructive/constructive interference; the product at a point of the value of the function (to be resolved) with the corresponding value of an eigenfunction has a negative or positive value (or phase factor) that may sum constructively or destructively with products at other points (seen as a matched filtering or correlation by replacing $y$ with $x-z$ above). Alternatively, the BPO may be viewed as projection of vectors onto a set of orthonormal axes. In addition, if the function and operations are discretized and/or the domains restricted (in one space or its dual or both, as for the DFT) aliasing (which seems analogous to the introduction of equivalence classes) is introduced and periodicity imposed.

**Can you explain the machinery behind the Mukai-Fourier transform in terms of these BPOs or close analogies?**

(Edit 1/16/212) Further to Carnahan's answer below:

Kapustin and Witten in "Electric-magnetic duality and the geometric Langlands Program" state, ". . . it must be possible to understand the geometric Langlands program using four-dimensional electric-magnetic duality (which leads to this particular T-duality) and branes (the natural quantum field theory setting for interpreting T-duality as a Fourier-Mukai transform). This hint was the starting point for the present paper." The T-duality is analogous to the modular/automorphic function symmetry involving reciprocals of a parameter/variable. They refer to the paper "Lectures on the Langlands Program and conformal field theory" by Frenkel as a good intro.

Frenkel notes, "It has long been suspected that the Langlands duality should somehow be related to various dualities observed in quantum field theory and string theory. Indeed, both the Langlands correspondence and the dualities in physics have emerged as some sort of non-abelian Fourier transforms. Moreover, the so-called Langlands dual group introduced by R. Langlands that is essential in the formulation of the Langlands correspondence also plays a prominent role in the study of S-dualities in physics and was in fact also introduced by the physicists P. Goddard, J. Nuyts, and D. Olive in the framework of four-dimensional gauge theory. ... The goal of these notes is two-fold: first, it is to give a motivated introduction to the Langlands Program, including its geometric reformulation, addressed primarily to physicists. I have tried to make it as self-contained as possible, requiring very little (lol) mathematical background. The second goal is to describe the connections between the Langlands Program and two-dimensional conformal field theory that have been found in the last few years. These connections give us important insights into the physical implications of the Langlands duality."

On pages 53 and 54, Frenkel presents an elaboration on the parallels, as noted by Carnahan, between the Dirac delta function-complex exponential duality central to the Fourier transform and the skyscraper sheaf-line bundle duality associated with the Fourier-Mukai transform.

Extrapolating, one would expect to find analogies to Poisson (or Dirac comb) summation, zeta function functional symmetry equations, and Euler product factorizations, as listed in Table I of "A Correspondence Principle" by Hughes and Ninham (subsumed, I suppose, by the analytic Langlands Program), and to log/exp, trace/determinant, moment/cumulant, and other dualities found in symmetric function theory, as well as 'gauge/conjugation' transformations. References to surveys sketching such associations, briefer than Frenkel's, would be appreciated, especially if linked to Fourier(Mellin)-Mukai transforms. (This is related to another MO_Q.)

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