Hankel determinant and orthogonal polynomials arising from the matrix model in 2D quantum gravity

We study the Hankel determinant and orthogonal polynomials with respect to the two-parameter weight function $$w(x)=w(x;t_1, t_2):=\exp(-x^6-t_2 x^4-t_1 x^2),\qquad x\in\mathbb{R},$$ with $t_1,\; t_2 \in \mathbb{R}$. This problem arises from the matrix model in 2D quantum gravity investigated by Fokas, Its and Kitaev [Commun. Math. Phys. \textbf{142} (1991) 313--344]. By making use of the ladder operator approach, we find that the recurrence coefficient $\beta_{n}(t_1,t_2)$ for the monic orthogonal polynomials satisfies a nonlinear fourth-order difference equation, which is within the discrete Painlev\'{e} I hierarchy. We show that the orthogonal polynomials satisfy a second-order linear differential equation whose coefficients are all expressed in terms of $\beta_{n}(t_1,t_2)$. The relations between the logarithmic partial derivative of the Hankel determinant, the nontrivial leading coefficient of the monic orthogonal polynomials, and the recurrence coefficient are established. By using Dyson's Coulomb fluid approach, we obtain the large $n$ asymptotic expansions of the recurrence coefficient $\beta_{n}(t_1,t_2)$, the nontrivial leading coefficient $\mathrm{p}(n,t_1,t_2)$, the normalized constant $h_n(t_1,t_2)$ and the Hankel determinant $D_{n}(t_1,t_2)$.