a(n) = 6*a(n-2) - a(n-4).

Terms

a(0) =1a(1) =1a(2) =2a(3) =4a(4) =11a(5) =23a(6) =64a(7) =134a(8) =373a(9) =781a(10) =2174a(11) =4552a(12) =12671a(13) =26531a(14) =73852a(15) =154634a(16) =430441a(17) =901273a(18) =2508794a(19) =5253004a(20) =14622323a(21) =30616751a(22) =85225144a(23) =178447502a(24) =496728541a(25) =1040068261a(26) =2895146102a(27) =6061962064a(28) =16874148071a(29) =35331704123