a(n) = 3a(n-1) + 3a(n-2). a(0) = 1, a(1) = 4.

Terms

a(0) =1a(1) =4a(2) =15a(3) =57a(4) =216a(5) =819a(6) =3105a(7) =11772a(8) =44631a(9) =169209a(10) =641520a(11) =2432187a(12) =9221121a(13) =34959924a(14) =132543135a(15) =502509177a(16) =1905156936a(17) =7222998339a(18) =27384465825a(19) =103822392492a(20) =393620574951