a(n) = 2*a(n-1) + a(n-2) - a(n-3) for n >= 3, starting with a(0) = 1, a(1) = 3, and a(2) = 6.

Terms

a(0) =1a(1) =3a(2) =6a(3) =14a(4) =31a(5) =70a(6) =157a(7) =353a(8) =793a(9) =1782a(10) =4004a(11) =8997a(12) =20216a(13) =45425a(14) =102069a(15) =229347a(16) =515338a(17) =1157954a(18) =2601899a(19) =5846414a(20) =13136773a(21) =29518061a(22) =66326481a(23) =149034250a(24) =334876920a(25) =752461609a(26) =1690765888a(27) =3799116465a(28) =8536537209a(29) =19181424995