Mixed Space and Time Derivatives

[dynamic-queries]

Consider

Any unspecified symbol is a real constant.

The time domain is not explicitly separable. I guess you could regard the time variable in the mixed derivatives as a stand-in auxiliary space variable... but not sure. @xtalax What do you think ?

Can we add this feature here in the future?

[xtalax]

Auxiliary variables should work here, substituting u_xx as an auxiliary variable and taking its time derivative for those mixed derivatives. Second order time is already implemented, but there is currently a bug upstream preventing solution retrieval. When this is fixed I expect you to be able to solve this though! What are your BCS? The pi suggests periodic, which is even better, though there is currently some weirdness with multiple non periodic BCs per boundary, which you'd need for the u_xxxx term if this is not the case.