a(n) = 4*a(n-2) - a(n-4).

Terms

a(0) =3a(1) =3a(2) =9a(3) =12a(4) =33a(5) =45a(6) =123a(7) =168a(8) =459a(9) =627a(10) =1713a(11) =2340a(12) =6393a(13) =8733a(14) =23859a(15) =32592a(16) =89043a(17) =121635a(18) =332313a(19) =453948a(20) =1240209a(21) =1694157a(22) =4628523a(23) =6322680a(24) =17273883a(25) =23596563a(26) =64467009a(27) =88063572a(28) =240594153a(29) =328657725