n, a(n)
2, a2=3
3, a3=5
4, a4=9
5, a5=13
so,
n>=4, a(n)=2*a(n-2)+3 ,
it can be proved by induction
so,
n=2k,k>=1, a(2k)=2^(k-1)*a2 + 2^(k-2)*3+2^(k-3)*3+......+2*3+3
=3*(2^k -1)=3*2^k-3
n=2k+1,k>=1, a(2k+1)=2^(k-1)*a3 + 2^(k-2)*3+2^(k-3)*3+......+2*3+3
=5*2^(k-1)+3*(2^(k-1)-1)=8*2^(k-1)-3=2^(k+2)-3
a30=3*2^15 -3
[em01]