On the Lax pairs of the sixth Painleve' equation

Abstract

The dependence of the sixth equation of Painleve' on its four parameters $(2 \alpha,-2 \beta,2 \gamma,1-2 \delta) =(\theta_{\infty}^2,\theta_{0}^2,\theta_{1}^2,\theta_{x}^2)$ is holomorphic, therefore one expects all its Lax pairs to display such a dependence. This is indeed the case of the second order scalar ``Lax'' pair of Fuchs, but the second order matrix Lax pair of Jimbo and Miwa presents a meromorphic dependence on $\theta_\infty$ (and a holomorphic dependence on the three other $\theta_j$). We analyze the reason for this feature and make suggestions to suppress it.