Little explorations with HP calculators (no Prime)

[pier4r]

new day, new problem (trivial for all of you but me).



Now I did solve this iteratively (and I checked the f(3) with the two combinations that I could build, the result is a "small" prime number). Is there a way to solve it in a more general way, without computing every single step?

Also what happens if I ask for f(1.5) ? I do not have ways to compute this, like information is missing.

Is there a way to solve it on the 50g (or other non-prime calculators?)? I could not identify any useful built in procedure.