### Abstract

Solving an integrable nonlinear differential equation reduces to solving the matrix Riemann-Hilbert problem (RHP) that arises from the invese scattering procedure. Typically, this means finding a matrix m(z) that is analytic in the closed complex plane except on a given oriented contour on which m(z) has a jump discontinuity: the value of m(z) left of the contour equals its value on the right multiplied by a given "jump matrix'' V(z). The space-time variables appear as parameters in the jump matrix; the complex variable z is the spectral variable of the linear differential operator that effects the linearization of the nonlinear system through the Lax pair associated with the system.

We derive an explicit formula for the leading asymptotic (oscillatory) solution of the semiclassical focusing nonlinear Schroedinger equation for a class of initial data and prove an error estimate. We extend the analysis when time is large, the asymptotic formula is then expressed in terms of elementary functions.

We first outline the notion of a Lax pair and how the RHP arises naturally.We then describe the steepest descent procedure that leads to an explicitly solvable asymptotic RHP through jump matrix factorizations and contour deformations. We also outline the g-funciton mechanism and describe how it determines the successful deformations.