From bug-request at octave dot org Fri Apr 14 11:22:34 2006
Subject: Ode with Negative Delta t
From: Brad Bell <bradbell at apl dot washington dot edu>
To: bug at octave dot org
Date: Fri, 14 Apr 2006 11:16:20 -0500


I am having some trouble with Odes in Octave when tspan(2) < tspan(1);
i.e., when integrating back in time (this is often useful when reversing a
process). The Ode corresponding to the code below is:
	y'(t) = 1
	y(0)  = 0
This function has final value y(-1) = -1 (requested by tspan = [0, -1]).
Unfortunately, Octave returns "ans = 0", on the other hand, Matlab returns
ans = -1.


ode_negative_dt.m:
----------------------------------------------------
tspan = [0, -1];
y0    = 0;
[t, y] = ode45( at ode_negative_dt_fun, tspan, y0);
y(end)
----------------------------------------------------

ode_negative_dt_fun.m
----------------------------------------------------
function dydt  = ode_negative_dt_fun(t, y)
dydt = 1;
return
----------------------------------------------------

octave results:
---------------------------------------------------
octave:1> version
ans = 2.1.73
octave:2> ode_negative_dt
ans = 0
octave:3>
----------------------------------------------------



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