a(n) = 6*a(n-2), starting with 1, 3, 9.

Terms

a(0) =1a(1) =3a(2) =9a(3) =18a(4) =54a(5) =108a(6) =324a(7) =648a(8) =1944a(9) =3888a(10) =11664a(11) =23328a(12) =69984a(13) =139968a(14) =419904a(15) =839808a(16) =2519424a(17) =5038848a(18) =15116544a(19) =30233088a(20) =90699264a(21) =181398528a(22) =544195584a(23) =1088391168a(24) =3265173504a(25) =6530347008a(26) =19591041024a(27) =39182082048