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These notes are devoted to the theory of exponential sums over finite fields. The first chapter recalls some of the number-theoretic interest of such sums. The second chapter discusses the $L$-functions attached to such sums, the "Weil conjectures" for these $L$-functions as established by Deligne, andthe consequences for the exponential sums themselves. The third chapter is devoted to the cohomological interpretation of exponential sums and of their associated $L$-functions. These first three chapters are largely of an expository nature. The main results are found in chapters four and five. Chapter four is devoted to theorems of uniformity "for almost all $p$" for the cohomological structure of quite general exponential sums. Chapter five is devoted to a precise analysis of the cohomological structure of certain specific classes of exponential sums for which the associated algebro-geometric situation is especially attractive., Comment: arXiv admin comment: This version has been removed by arXiv administrators as the submitter did not have the rights to agree to the license at the time of submission

We prove some distribution results for the $k$-fold divisor function in arithmetic progressions to moduli that exceed the square-root of length $X$ of the sum, with appropriate constrains and averaging on the moduli, saving a power of $X$ from the trivial bound. On assuming the Generalized Riemann Hypothesis, we obtain uniform power saving error terms that are independent of $k$. We follow and specialize Y.T. Zhang's method on bounded gaps between primes to our setting. Our arguments are essentially self-contained, with the exception on the use of Deligne's work on the Riemann Hypothesis for varieties over finite fields. In particular, we avoid the reliance on Siegel's theorem, leading to some effective estimates., Comment: in: J. Number Theory 227 (2021), pp. 30-93

We heuristically study the shifted convolution $\sum_{n\le X} \tau_k(n) \tau_\ell(n+h)$ using a normalized version of Ramanujan-Fourier expansions for $\tau_k(n)$ and verify they produce the expected answer., Comment: six pages

Let $\zeta^k(s) = \sum_{n=1}^\infty \tau_k(n) n^{-s}, \Re s > 1$. We present three conditional results on the ternary additive correlation sum $$\sum_{n\le X} \tau_3(n) \tau_3(n+h),\quad (h\ge 1),$$ and give numerical verifications of our method. The first is a conditional proof for the full main term of the above correlation sum for any composite shift $1 \le h \le X^{2/3}$, on assuming an averaged level of distribution for the three-fold divisor function $\tau_3(n)$ in arithmetic progressions to level two-thirds. The second is a conditional derivation for the leading order main term asymptotics of this correlation sum, also valid for any composite shift $1 \le h \le X^{2/3}$. The third result gives a complete expansion of the polynomial for the full main term for the special case $h=1$ from both our method and from the delta-method, showing that our answers match. Our method is essentially elementary, especially for the $h=1$ case, uses congruences, and, as alluded to earlier, gives the same answer as in prior prediction of Conrey and Gonek [Duke Math. J. 107 (3) 2002], previously computed by Ng and Thom [Funct. Approx. Comment. Math. 60(1) 2019], and unpublished heuristic probabilistic arguments of Tao. Our procedure is general and works to give the full main term with a power-saving error term for any correlations of the form $\sum_{n\le X} \tau_k(n) f(n+h)$, to any composite shift $h$, and for a wide class of arithmetic function $f(n)$., Comment: details on contours shifting added; presentation improved. 46 pages, 2 tables, 4 figures

In this paper, we confirm a smoothed version of a recent conjecture on the variance of the k-fold divisor function in arithmetic progressions to individual composite moduli, in a restricted range. In contrast to a previous result of Rodgers and Soundararajan, we do not require averaging over the moduli. Our proof adapts a technique of S. Lester who treated in the same range the variance of the k-fold divisor function in the short intervals setting, and is based on a smoothed Voronoi summation formula but twisted by multiplicative characters. The use of Dirichlet characters allows us to extend to a wider range from previous result of Kowalski and Ricotta who used additive characters. Smoothing also permits us to treat all k unconditionally. This result is closely related to moments of Dirichlet L-functions., Comment: 29 pages, added 1 figure, incorporated referee's suggestions--in particular, fixed a flaw in the original choice of the parameter N and a notational issue, exposition heavily revised

In pathology protocols, each tissue block can generate a large number of sections making it impractical to analyze every section. X-ray microscopy that provides a rapid survey of intact tissue blocks can help pinpoint the relevant structures in 3D space for subsequent analysis, and thus reduce workload and enable further automation downstream. Unlike dedicated virtual histology studies by traditional micro computed tomography (CT), routine scout imaging is constrained by a time window of minutes and minimal sample handling to avoid interfering with the pathology protocols. Traditional micro CT was not able to meet the requirements due to lengthy study times or sample alteration by the introduction of x-ray contrast agents. A form of x-ray tomosynthesis used in security screening was found to be efficient for rapid microscopy of unstained samples. When compared to a commercial micro CT scanner, it provided a 10-fold increase in imaging speed and a 4.8-fold increase in contrast-to-noise ratio. We report the results from a variety of human and animal tissue samples, where it served as an integral step of pathology protocols. In cases of vascular disease, it provided quantitative measurements of calcification in intact samples, which were difficult to obtain by standard pathology procedures. The prospect of continuous and automated screening of many samples in an assembly-line approach is discussed., Comment: 27 pages, 7 figures and 1 supplemental figure