MATLAB help: Problem 1: Stability of xed points of linear systems of ODEs. 2.050J/12.006J/18.353J Nonlinear Dynamics I: Chaos, Fall 2012 Problem Set 2. vector elds of a dynamical system on a plane. Main concepts: phase portraits for linear systems, xed points of nonlinear systems, their stability, phase portraits in the vicinity of a xed point. Polar Coordinates . In a rectangular coordinate system, we were plotting points based on an ordered pair of (x, y). In the polar coordinate system, the ordered pair will now be (r, θ). The ordered pair specifies a point's location based on the value of r and the angle, θ, from the polar axis. The value of r can be positive, negative, or zero.