a(n) = 3*a(n-1) - a(n-2) - 1 with a(0) = 1 and a(1) = 2.

Terms

a(0) =1a(1) =2a(2) =4a(3) =9a(4) =22a(5) =56a(6) =145a(7) =378a(8) =988a(9) =2585a(10) =6766a(11) =17712a(12) =46369a(13) =121394a(14) =317812a(15) =832041a(16) =2178310a(17) =5702888a(18) =14930353a(19) =39088170a(20) =102334156a(21) =267914297a(22) =701408734a(23) =1836311904a(24) =4807526977a(25) =12586269026a(26) =32951280100a(27) =86267571273