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

Terms

a(0) =1a(1) =1a(2) =1a(3) =2a(4) =6a(5) =21a(6) =76a(7) =277a(8) =1011a(9) =3691a(10) =13476a(11) =49202a(12) =179641a(13) =655886a(14) =2394701a(15) =8743277a(16) =31922521a(17) =116552106a(18) =425542626a(19) =1553695877a(20) =5672688776a(21) =20711516601a(22) =75619681751a(23) =276094521627