a(n) = 3*a(n-1)-3*a(n-2)+3*a(n-3) with a(0)=1, a(1)=3, a(2)=7.

Terms

a(0) =1a(1) =3a(2) =7a(3) =15a(4) =33a(5) =75a(6) =171a(7) =387a(8) =873a(9) =1971a(10) =4455a(11) =10071a(12) =22761a(13) =51435a(14) =116235a(15) =262683a(16) =593649a(17) =1341603a(18) =3031911a(19) =6851871a(20) =15484689a(21) =34994187a(22) =79084107a(23) =178723827a(24) =403901721a(25) =912786003a(26) =2062824327a(27) =4661820135a(28) =10535345433a(29) =23809048875