Number of ways to place 3n nonattacking kings on a vertical cylinder 6 X 2n.

Terms

a(0) =16a(1) =90a(2) =344a(3) =1082a(4) =3036a(5) =7918a(6) =19648a(7) =47058a(8) =109796a(9) =251126a(10) =565512a(11) =1257754a(12) =2769196a(13) =6046014a(14) =13107536a(15) =28246370a(16) =60555636a(17) =129237382a(18) =274727320a(19) =581960106a(20) =1228931516a(21) =2587886030a(22) =5435818464a(23) =11391730162a(24) =23823647236a(25) =49727668758a(26) =103616086568