Won't some of the Runge-Kutta solvers be really unstable with a small negative Delta t? What happens when you use a fixed time step method: does it run and diverge?
On Friday, June 24, 2016 at 8:50:42 PM UTC+1, Mauro wrote: > > This is a bug, could you file an issue? Thanks! Note that some solvers, > even some of the Runge-Kutta family work, for instance ode78, ode45_fe, > ode21 or ode23s. > > On Fri, 2016-06-24 at 20:48, Chris <[email protected] <javascript:>> > wrote: > > Hello, > > > > I came across this issue (recreated here using example code): > > > > julia> using ODE > > > > julia> function f(t, y) > > # Extract the components of the y vector > > (x, v) = y > > > > # Our system of differential equations > > x_prime = v > > v_prime = -x > > > > # Return the derivatives as a vector > > [x_prime; v_prime] > > end; > > > > julia> # Initial condtions -- x(0) and x'(0) > > const start = [0.0; 0.1] > > WARNING: redefining constant start > > 2-element Array{Float64,1}: > > 0.0 > > 0.1 > > > > julia> # Time vector going from 0 to 2*PI in 0.01 increments > > time = 0.:-.1:-4pi; > > > > julia> t, y = ode45(f, start, time); > > > > This hangs for a long time (I've waited up to a minute before killing > it). > > Is this a bug, or something I'm doing wrong? I do notice that reversing > the > > time vector (-4pi:.1:0.0) yields a result. > > > > Thanks, > > Chris >
