Answer: a(n)=f(n+2); f for Fibonacci number Solution a(n) must cover all the qualifying subsets of n1 integers i.e, a(n1) plus all the qualifying subsets of n2 elements, where to each of them integer n was added (the integer n1 is absent!).
a(n)= a(n1) + a(n2)
a(1)=2=f(3) so a(n)=f(n+2)
