The focusing complex mKdV equation with nonzero background: Large N-order asymptotics of multi-rational solitons and related Painlevé-III hierarchy

In this paper, we investigate the large-order asymptotics of multi-rational solitons of the focusing complex modified Korteweg-de Vries (c-mKdV) equation with nonzero background via the Riemann-Hilbert problems. First, based on the Lax pair, inverse scattering transform, and a series of deformations, we construct a multi-rational soliton of the c-mKdV equation via a solvable Riemann-Hilbert problem (RHP). Then, through a scale transformation, we construct a RHP corresponding to the limit function which is a new solution of the c-mKdV equation in the rescaled variables $X,T$, and prove the existence and uniqueness of the RHP's solution. Moreover, we also find that the limit function satisfies the ordinary differential equations (ODEs) with respect to space X and time T, respectively. The ODEs with respect to space X are identified with certain members of the Painlevé-III hierarchy. We study the large X and transitional asymptotic behaviors of near-field limit solutions, and we provide some part results for the case of large T. These results will be useful to understand and apply the large-order rational solitons in the nonlinear wave equations.