a(n) = 2^(n+1)*cos(n*arctan(sqrt(15))).

Terms

a(0) =2a(1) =1a(2) =-7a(3) =-11a(4) =17a(5) =61a(6) =-7a(7) =-251a(8) =-223a(9) =781a(10) =1673a(11) =-1451a(12) =-8143a(13) =-2339a(14) =30233a(15) =39589a(16) =-81343a(17) =-239699a(18) =85673a(19) =1044469a(20) =701777a(21) =-3476099a(22) =-6283207a(23) =7621189a(24) =32754017a(25) =2269261a(26) =-128746807a(27) =-137823851a(28) =377163377a(29) =928458781