a(n) = Sum_{k=0..n} C(n,floor(k/2))*(-3)^(n-k).

Terms

a(0) =1a(1) =-2a(2) =8a(3) =-24a(4) =84a(5) =-272a(6) =920a(7) =-3040a(8) =10180a(9) =-33840a(10) =112968a(11) =-376224a(12) =1254696a(13) =-4181088a(14) =13939248a(15) =-46459584a(16) =154873860a(17) =-516229040a(18) =1720795880a(19) =-5735921440a(20) =19119861304a(21) =-63732624672a(22) =212442552528a(23) =-708140901184