On meromorphic solutions of Malmquist type difference equations

Abstract

Recently, the present authors used Nevanlinna theory to provide a classification for the Malmquist type difference equations of the form $f(z+1)^n=R(z,f)$ $(\dag)$ that have transcendental meromorphic solutions, where $R(z,f)$ is rational in both arguments. In this paper, we first complete the classification for the case $\deg_{f}(R(z,f))=n$ of~$(\dag)$ by identifying a new equation that was left out in our previous work. We will actually derive all the equations in this case based on some new observations on~$(\dag)$. Then, we study the relations between $(\dag)$ and its differential counterpart $(f')^n=R(z,f)$. We show that most autonomous equations, singled out from~$(\dag)$ with $n=2$, have a natural continuum limit to either the differential Riccati equation $f'=a+f^2$ or the differential equation $(f')^2=a(f^2-\tau_1^2)(f^2-\tau_2^2)$, where $a\not=0$ and $\tau_i$ are constants such that $\tau_1^2\not=\tau_2^2$. The latter second degree differential equation and the symmetric QRT map are derived from each other using the bilinear method and the continuum limit method.