On properties of a deformed Freud weight

Abstract

We study the recurrence coefficients of the monic polynomials $P_n(z)$ orthogonal with respect to the deformed (also called semi-classical) Freud weight \begin{equation*} w_α(x;s,N)=|x|^α{\rm e}^{-N\left[x^{2}+s\left(x^{4}-x^{2}\right)\right]}, ~~x\in\mathbb{R}, \end{equation*} with parameters $α>-1,~N>0,~s\in[0,1]$. We show that the recurrence coefficients $β_{n}(s)$ satisfy the first discrete Painlevé equation (denoted by d${\rm P_{I}}$), a differential-difference equation and a second order nolinear ordinary differential equation (ODE) in $s$. Here $n$ is the order of the Hankel matrix generated by $w_α(x;s,N)$. We describe the asymptotic behavior of the recurrence coefficients in three situations, (i) $s\rightarrow0$, $n,N$ finite, (ii) $n\rightarrow\infty$, $N$ finite, (iii) $n, N\rightarrow\infty$, such that the radio $r:=\frac{n}{N}$ is bounded away from $0$ and closed to $1$. We also investigate the existence and uniqueness for the positive solutions of the d${\rm P_{I}}$. Further more, we derive, using the ladder approach, a second order linear ODE satisfied by the polynomials $P_n(z)$. It is found as $n\rightarrow\infty$, the linear ODE turns to be a biconfluent Heun equation. This paper concludes with the study of the Hankel determinant, $D_{n}(s)$, associated with $w_α(x;s,N)$ when $n$ tends to infinity.

33 pages, 25 figures