a(n) = 2*(-i)^n*(n*sin(c*(n+1)) - i*sin(-c*n))/sqrt(5) where c = arccos(i/2).

Terms

a(0) =0a(1) =2a(2) =5a(3) =11a(4) =23a(5) =45a(6) =86a(7) =160a(8) =293a(9) =529a(10) =945a(11) =1673a(12) =2940a(13) =5134a(14) =8917a(15) =15415a(16) =26539a(17) =45525a(18) =77842a(19) =132716a(20) =225685a(21) =382877a(22) =648165a(23) =1095121a(24) =1846968a(25) =3109850a(26) =5228261a(27) =8777315a(28) =14716223a(29) =24643389