> I do wonder if fully implementing, and thoroughly testing a good, ML compatible version of ode15s isn't a fairly substantial project on it's own, but you'll have a much better idea of the work involved in this.
For reference, this is the paper describing the ODE suite, and it has some details about ode15s:
The link above might not be accessible to some; there is also the preprint version on the mathworks website:
https://www.mathworks.com/help/pdf_doc/otherdocs/ode_suite.pdf