ode15s Preparation

Hi all

I notice that when I use ode15s (or any other solver, really), it calls my derivative function ("odefun") multiple times at t=0 (given tspan=[0 T]). It seems the larger the problem, the more times odefun is called at t=0. What is it evaluating?

For more info, these t=0 calls are before the backwards steps taken (to estimate an appropriate initial step size I guess).

Calls at 0 step size also seem to occur throughout integration. Not sure if it's for the same purpose as the first set of t=0 (and so 0 step size) calls.

Thanks in advance.

MATLAB2021a