Ode23 doesn't finish executing

Question

I'm new to Matlab, and I'm trying to solve the differential equation y'=-y/n for a constant n. I define the function in a script like this:

function dv = lc(v1)
dv = -v1/(0.0000047*0.000001);

And then try and solve it like this:

[t,v] = ode23('lc',[0 5],1)

But the operation never finishes executing. It just eats up my RAM and says "Busy" in the corner, until I press ctrl+c to terminate it. What am I doing wrong here?

Try with `n=0.2` and see if it does solve. It's possible that you are getting underflow. — Eli Sadoff, Nov 15 '16 at 15:45
There aren't an infinite number of floating point numbers and sometimes, especially when using small floats, you'll get a number so small the computer will represent it as 0. — Eli Sadoff, Nov 15 '16 at 15:46
The process used by ode23 I believe is finite differencing and with underflow this can lead to a division by 0 error — Eli Sadoff, Nov 15 '16 at 15:48
@EliSadoff Thanks very much. Put that in an answer and I'll accept it. — imulsion, Nov 15 '16 at 15:48

score 1 · Accepted Answer · edited May 23 '17 at 11:53

You're a victim of underflow. What happens is that there is not an infinite number of floating point numbers (see Is floating point math broken?), so the results are not fully precise. When the numbers are small enough there is the possibility of the number being treated as 0 by the computer. The process that matlab uses for ode23 is based on finite differencing, which involves division. Considering the underflow error, there either will be a division by 0 issue, or possible overflow with an incredibly small divisor trending the result toward infinity and not fulfilling the conditions for solution.

Ode23 doesn't finish executing

1 Answers1