Asymptotics of Laurent Polynomials of Even Degree Orthogonal with Respect to Varying Exponential Weights

Abstract

Let $\Lambda^{\mathbb{R}}$ denote the linear space over $\mathbb{R}$ spanned by $z^{k}$, $k \! \in \! \mathbb{Z}$. Define the real inner product (with varying exponential weights) $\langle \boldsymbol{\cdot},\boldsymbol{\cdot} \rangle_{\mathscr{L}} \colon \Lambda^{\mathbb{R}} \times \Lambda^{\mathbb{R}} \! \to \! \mathbb{R}$, $(f,g) \! \mapsto \! \int_{\mathbb{R}}f(s)g(s) \exp (-\mathscr{N} \, V(s)) \, ds$, $\mathscr{N} \! \in \! \mathbb{N}$, where the external field $V$ satisfies: (i) $V$ is real analytic on $\mathbb{R} \setminus \{0\}$; (ii) $\lim_{\vert x \vert \to \infty}(V(x)/\ln (x^{2} \! + \! 1)) \! = \! +\infty$; and (iii) $\lim_{\vert x \vert \to 0}(V(x)/\ln (x^{-2} \! + \! 1)) \! = \! +\infty$. Orthogonalisation of the (ordered) base $\lbrace 1,z^{-1},z,z^{-2},z^{2},\dotsc,z^{-k},z^{k},\dotsc \rbrace$ with respect to $\langle \boldsymbol{\cdot},\boldsymbol{\cdot} \rangle_{\mathscr{L}}$ yields the even degree and odd degree orthonormal Laurent polynomials $\lbrace \phi_{m}(z) \rbrace_{m=0}^{\infty}$: $\phi_{2n}(z) \! = \! \xi^{(2n)}_{-n} z^{-n} \! + \! \dotsb \! + \! \xi^{(2n)}_{n}z^{n}$, $\xi^{(2n)}_{n} \! > \! 0$, and $\phi_{2n+1}(z) \! = \! \xi^{(2n+1)}_{-n-1}z^{-n-1} \! + \! \dotsb \! + \! \xi^{(2n+1)}_{n}z^{n}$, $\xi^{(2n+1)}_{-n-1} \! > \! 0$. Asymptotics in the double-scaling limit as $\mathscr{N},n \! \to \! \infty$ such that $\mathscr{N}/n \! = \! 1 \! + \! o(1)$ of $\xi^{(2n)}_{n}$ and $\phi_{2n} (z)$ (in the entire complex plane) are obtained by formulating the even degree orthonormal Laurent polynomial problem as a matrix Riemann-Hilbert problem on $\mathbb{R}$, and then extracting the large-$n$ behaviour by applying the Deift-Zhou non-linear steepest-descent method in conjunction with the extension of Deift-Venakides-Zhou.