a(n) = 4*a(n-1) - 3*a(n-2) + 2*a(n-3) - 1*a(n-4).

Terms

a(0) =1a(1) =1a(2) =1a(3) =2a(4) =6a(5) =19a(6) =61a(7) =197a(8) =637a(9) =2060a(10) =6662a(11) =21545a(12) =69677a(13) =225337a(14) =728745a(15) =2356778a(16) =7621874a(17) =24649315a(18) =79716449a(19) =257804821a(20) =833746693a(21) =2696355892a(22) =8720076682a(23) =28200927617a(24) =91202445513a(25) =294950796673a(26) =953877628705