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

Terms

a(0) =1a(1) =1a(2) =6a(3) =11a(4) =26a(5) =51a(6) =106a(7) =211a(8) =426a(9) =851a(10) =1706a(11) =3411a(12) =6826a(13) =13651a(14) =27306a(15) =54611a(16) =109226a(17) =218451a(18) =436906a(19) =873811a(20) =1747626a(21) =3495251a(22) =6990506a(23) =13981011a(24) =27962026a(25) =55924051a(26) =111848106a(27) =223696211a(28) =447392426a(29) =894784851