On the asymptotic behavior of Jacobi polynomials with first varying parameter

We investigate the large $n$ behavior of Jacobi polynomials with varying parameters $P_{n}^{(an+α,\,bn+β)}(1-2λ^{2})$ for $a,b >-1$ and $λ\in(0,\,1)$. This is a well-studied topic in the literature but some of the published results appear to be discordant. To address this issue we provide an in-depth investigation of the case $b = 0$, which is most relevant for our applications. Our approach is based on a new and surprisingly simple representation of $P_{n}^{(an+α,\,β)}(1-2λ^{2}),\:a>-1$ in terms of two integrals. The integrals' asymptotic behavior is studied using standard tools of asymptotic analysis: one is a Laplace integral and the other is treated via the method of stationary phase. As a consequence we prove that if $a\in(\frac{2λ}{1-λ},\infty)$ then $λ^{an}P_{n}^{(an+α,β)}(1-2λ^{2})$ shows exponential decay and we derive simple exponential upper bounds in this region. If $a\in(\frac{-2λ}{1+λ},\,\frac{2λ}{1-λ})$ then the decay of $λ^{an}P_{n}^{(an+α,β)}(1-2λ^{2})$ is $\mathcal{O}(n^{-1/2})$ and if $a\in\{\frac{-2λ}{1+λ},\,\frac{2λ}{1-λ}\}$ then $λ^{an}P_{n}^{(an+α,β)}(1-2λ^{2})$ decays as $\mathcal{O}(n^{-1/3})$. A new phenomenon occurs in the parameter range $a\in(-1,\frac{-2λ}{1+λ})$, where we find that the behavior depends on whether or not $an+α$ is an integer: If $a\in(-1,\frac{-2λ}{1+λ})$ and $an+α$ is an integer then $λ^{an}P_{n}^{(an+α,β)}(1-2λ^{2})$ decays exponentially. If $a\in(-1,\frac{-2λ}{1+λ})$ and $an+α$ is not an integer then $λ^{an}P_{n}^{(an+α,β)}(1-2λ^{2})$ may increase exponentially depending on the proximity of the sequence $(an + α)_n$ to integers.

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