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

Terms

a(0) =1a(1) =1a(2) =3a(3) =6a(4) =14a(5) =29a(6) =61a(7) =124a(8) =252a(9) =507a(10) =1019a(11) =2042a(12) =4090a(13) =8185a(14) =16377a(15) =32760a(16) =65528a(17) =131063a(18) =262135a(19) =524278a(20) =1048566a(21) =2097141a(22) =4194293a(23) =8388596a(24) =16777204a(25) =33554419a(26) =67108851a(27) =134217714a(28) =268435442a(29) =536870897