Answer 1

Hey Äva( Ñinja Frozen) if you don't mind showing your attempt and stating precisely where you get stuck

Answer 2

Äva( Ñinja Frozen)
I think it is not a question because you know it is Kollatz Series... right?

Answer 3

try this:
i = n%2
k = n + i
Sum(F(n)) = (7*k*k + 6*k) / 8 - i*(n+1)/2
Is this right?