Dubrovin-Natanzon divisors on MM-curves

Abstract

${\mathtt{MM}}$-curves are rational degenerations of ${\mathtt{M}}$-curves, i.e. they are maximal Mumford in the sense that they posses $g$ tropical cycles and exactly $g+1$ real ovals, where $g$ is the arithmetic genus. For rational curves the ``naive'' definition of divisors as formal sums of points requires a refinement. In the finite-gap theory of KP II equation the real regular solutions correspond to the Dubrovin-Natanzon (DN) divisors on ${\mathtt{M}}$-curves. In the case of real regular multiline KP II solitons, it was shown by the authors that for any given solution there exists a normalization time such that the spectral data are smooth DN divisor on ${\mathtt{MM}}$-curve. However, to show that DN divisors parameterize the; full positroid cell, it is necessary to fix the normalization time and consider both smooth and non-smooth divisors. In this paper we start such an investigation, and show that on ${\mathtt{MM}}$-curves whose dual graphs are trivalent Le-graphs of totally positive Schubert cells, the construction of non-smooth DN divisors requires combinations of just two basic types of blow-ups.