Asymptotic Behavior of Underlying NT Paths in Interior Point Methods for Monotone Semidefinite Linear Complementarity Problems

An interior point method (IPM) defines a search direction at each interior point of the feasible region. These search directions form a direction field, which in turn gives rise to a system of ordinary differential equations (ODEs). Thus, it is natural to define the underlying paths of the IPM as solutions of the system of ODEs. In Sim and Zhao (2007), these off‐central paths are shown to be well‐defined analytic curves and any of their accumulation points is a solution to the given monotone semidefinite linear complementarity problem (SDLCP). In Sim and Zhao (2008) and Sim (2009), the asymptotic behavior of off‐central paths corresponding to the HKM direction is studied. In particular, in Sim and Zhao (2007), the authors study the asymptotic behavior of these paths for a simple example, while, in Sim and Zhao (2008) and Sim (2009), the asymptotic behavior of these paths for a general SDLCP is studied. In this paper, we study off‐central paths corresponding to another well‐known direction, the Nesterov‐Todd (NT) direction. Again, we give necessary and sufficient conditions for these off‐central paths to be analytic w.r.t. $\sqrt{\mu }$ and then w.r.t. μ, at solutions of a general SDLCP. Also, as in Sim and Zhao (2007), we present off‐central path examples using the same SDP, whose first derivatives are likely to be unbounded as they approach the solution of the SDP. We work under the assumption that the given SDLCP satisfies a strict complementarity condition.