# 16

> ode1 := diff( x(t), t ) = y(t) * ( x(t)^2 + y(t)^2 - 1 ) :

> ode2 := diff( y(t), t ) = -x(t) * ( x(t)^2 + y(t)^2 - 1 ) :

> MODEL := [ ode1, ode2 ] ; # define the differential equation model

> VAR := { x(t), y(t) } ; # identify the variables in the model in the form h(t), u(t)

>

(a) -- Equilibrium Solutions

(b) -- Direction Field

(c) -- Phase Portrait