a(n) = (-2*I)^n + (2*I)^n + (1/2 + 1/2*I*sqrt(3))^n + (1/2 - 1/2*I*sqrt(3))^n.

Terms

a(0) =4a(1) =1a(2) =-9a(3) =-2a(4) =31a(5) =1a(6) =-126a(7) =1a(8) =511a(9) =-2a(10) =-2049a(11) =1a(12) =8194a(13) =1a(14) =-32769a(15) =-2a(16) =131071a(17) =1a(18) =-524286a(19) =1a(20) =2097151a(21) =-2a(22) =-8388609a(23) =1a(24) =33554434a(25) =1a(26) =-134217729a(27) =-2a(28) =536870911a(29) =1