a(n) = (5 + (-2)^n)/3.

Terms

a(0) =2a(1) =1a(2) =3a(3) =-1a(4) =7a(5) =-9a(6) =23a(7) =-41a(8) =87a(9) =-169a(10) =343a(11) =-681a(12) =1367a(13) =-2729a(14) =5463a(15) =-10921a(16) =21847a(17) =-43689a(18) =87383a(19) =-174761a(20) =349527a(21) =-699049a(22) =1398103a(23) =-2796201a(24) =5592407a(25) =-11184809a(26) =22369623a(27) =-44739241a(28) =89478487a(29) =-178956969