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We study linear transformations $T \colon \mathbb{R}[x] \to \mathbb{R}[x]$ of the form $T[x^n]=P_n(x)$ where $\{P_n(x)\}$ is a real orthogonal polynomial system. Such transformations that preserve or shrink the location of the complex zeros of polynomials is a recent object of study, motivated by the Riemann Hypothesis. In particular, we are interested in linear transformations that map polynomials with all real zeros to polynomials with all real zeros. It is well known that any transformation $T \colon \mathbb{C}[x] \rightarrow \mathbb{C}[x]$ has a differential operator representation $T = \sum_{k = 0}^\infty \frac{Q_k(x)}{k!} D^k$. Motivated by the work of Chasse \cite{Chasse-PhD-2011}, Forg\'acs, and Piotrowski \cite{Forgacs-Piotrowski-Hermite-2015}, we seek to understand the behavior of the transformation $T$ by studying the roots of the $Q_k(x)$. We prove four main things. First, we show that the only case where the $Q_k(x)$ are constant and $\{P_n(x)\}$ are an orthogonal system is that when the $P_n$ form a shifted set of generalized probabilist Hermite polynomials. Second, we show that the coefficient polynomials $Q_k(x)$ have real roots when the $P_n(x)$ are the physicist Hermite polynomials or the Laguerre polynomials. Next, we show that in these cases, the roots of successive polynomials strictly interlace, a property that has not yet been studied for coefficient polynomials. We conclude by discussing the Chebyshev and Legendre polynomials, proving a conjecture of Chasse, and presenting several open problems., Comment: 16 pages. This version improves exposition and corrects several typographical errors

We prove a Jensen-disc type theorem for polynomials $p\in\mathbb{R}[z]$ having all their zeros in a sector of the complex plane. This result is then used to prove the existence of a collection of linear operators $T\colon\mathbb{R}[z]\to\mathbb{R}[z]$ which map polynomials with their zeros in a closed convex sector $|\arg z| \leq \theta<\pi/2$ to polynomials with zeros in a smaller sector $|\arg z| \leq \gamma<\theta$. We, therefore, provide the first example of a zero-sector reducing operator., Comment: 11 pages, 3 figures

In this paper, we study linear transformations of the form $T[x^n]=P_n(x)$ where $\{P_n(x)\}$ is an orthogonal polynomial system. Of particular interest is understanding when these operators preserve real-rootedness in polynomials. It is known that when the $P_n(x)$ are the Hermite polynomials or standard Laguerre polynomials, the transformation $T$ has this property. It is also known that the transformation $T[x^n]=H_n^{\alpha}(x)$, where $H_n^{\alpha}(x)$ is the $n$th generalized Hermite Polynomial with real parameter $\alpha$, has the differential operator representation $T[x^n]=e^{-\frac{\alpha}{2}D^2}x^n$. The main result of this paper is to prove that a differential operator of the form $\sum_{k=0}^\infty \frac{\gamma_k}{k!} D^k$ induces a system of monic orthogonal polynomials if and only if $\sum_{k=0}^\infty \frac{\gamma_k}{k!} D^k=\gamma_0e^{-\frac{ \alpha}{2}D^2-\beta D}$ where $\gamma_0,\alpha,\beta \in \mathbb{C}$ and $\alpha,\gamma_0 \neq 0$. This operator will produce a shifted set of generalized Hermite polynomials when $\alpha \in \mathbb{R}$. We also express the transformation from the standard basis to the standard Laguerre basis, $T[x^n]=L_n(x)$ as a differential operator of the form $\sum_{k=0}^\infty \frac{p_k(x)}{k!} D^k$ where the $p_k$ are polynomials, an identity that has not previously been shown.

In this paper we study the effect of linear differential operators coming from the Laguerre-Polya class that act on functions in the extended Laguerre-Polya class with zeros in a horizontal strip in the complex plane. These operator decrease the size of the strip containing the zeros., Comment: 17 pages, corrected several typos, added a citation. appears in Math. Anal. Appl. (2015)

Using the relationship between totally nonnegative matrices and directed acyclic weighted planar networks, we show that $2\times 2$ minors of minor matrices of totally nonnegative matrices are also nonnegative. We give a combinatorial interpretation for the minors of minor matrices in terms of the weights of families of paths in a network.

Let $f(z)=e^{-bz^2}f_1(z)$ where $b \geq 0$ and $f_1(z)$ is a real entire function of genus 0 or 1. We give a necessary and sufficient condition in terms of a sequence of inequalities for all of the zeros of $f(z)$ to be real. These inequalities are an extension of the classical Laguerre inequalities., Comment: The paper is based on a talk given at the 7th ISAAC Congress held at Imperial College in London in July 2009

We attach a certain $n \times n$ matrix $A_n$ to the Dirichlet series $L(s)=\sum_{k=1}^{\infty}a_k k^{-s}$. We study the determinant, characteristic polynomial, eigenvalues, and eigenvectors of these matrices. The determinant of $A_n$ can be understood as a weighted sum of the first $n$ coefficients of the Dirichlet series $L(s)^{-1}$. We give an interpretation of the partial sum of a Dirichlet series as a product of eigenvalues. In a special case, the determinant of $A_n$ is the sum of the M\"obius function. We disprove a conjecture of Barrett and Jarvis regarding the eigenvalues of $A_n$., Comment: 17 pages

We show that certain sums of products of Hermite-Biehler entire functions have only real zeros, extending results of Cardon. As applications of this theorem we construct sums of exponential functions having only real zeros, we construct polynomials having zeros only on the unit circle, and we obtain the three-term recurrence relation for an arbitrary family of real orthogonal polynomials. We discuss a similarity of this result with the Lee-Yang circle theorem from statistical mechanics. Also, we state several open problems., Comment: 10 pages, corrected minor typos