Question: differentiate something by unknown variable


I'm trying to creat u which is a function of ξ. but I do not have the full definition of u yet. how can i write it in a way where when i differentiate it, it'll give me something like u(ξ),ξ? And that i can differentiate it once more with ξ and it gives me u(ξ),ξξ and so on?

please advice. Thanks.

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