Problem with NonNegative option of ODE45

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Luca Vergani 2022-9-15

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评论： James Tursa 2022-9-15

Hi everyone!

I'm tryng to solve an implicit system of ordinary differential equation of the type:

where

are some parameters. The problem is that if the resulting y is

the ode solution ends up to be complex. In order to avoid this I''ve tried to add the option NonNegative to the ode solver. I've a sistem of 5 differential equation

so i've written:

options = odeset('NonNegative',[1:5]);

But it does not work because I still find complex solution. Wat's wrong?

Thanks in advance!

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Torsten 2022-9-15

编辑：Torsten 2022-9-15

Maybe your differential equations are such that the solution variables become negative.

E.g. The differential equation dy/dt = -1 with y(0) = 1 has solution y(t) = -t+1 and it will become negative if t>1. The NonNegative option won't prevent this from happening.

The NonNegative option only might help if the variables become negative because of inaccuracies of integration.