a(n) = ceiling( 4^n/20 - (n^2 + 1)*2^(n-1) ).

Terms

a(0) =0a(1) =-1a(2) =-9a(3) =-36a(4) =-123a(5) =-364a(6) =-979a(7) =-2380a(8) =-5043a(9) =-7884a(10) =717a(11) =84788a(12) =541901a(13) =2659124a(14) =11807949a(15) =49984308a(16) =206326989a(17) =839988020a(18) =3393375437a(19) =13648999220a(20) =54765341901a(21) =219438854964a(22) =878592183501