How to keep u>0 when solving ODEs

General Usage
1

I’m using Rodas3() to solve a stiff ODEs in Julia. du/dt = F(u). The Analytical expression of u should be positive. However, when I use julia to solve the ODE, u may be less equal 0. I suspect that the time step was too large.

I wonder if there is parameters. When solving ODE, if the u of a certain step size is less than 0, the step size will be recalculated by rolling it back.

Thany you!

2

isoutofdomain (Under the Miscellaneous heading) can do that, however it is often helpful to transform your problem using something like u⁺ = ln(u) and then whatever the value of u⁺, exp(u⁺) will be positive.

3

I’d recommend reading through Frequently Asked Questions · DifferentialEquations.jl?