Product_{i=0..3} (2^floor((n+i)/4)-1).

Terms

a(0) =0a(1) =0a(2) =0a(3) =0a(4) =1a(5) =3a(6) =9a(7) =27a(8) =81a(9) =189a(10) =441a(11) =1029a(12) =2401a(13) =5145a(14) =11025a(15) =23625a(16) =50625a(17) =104625a(18) =216225a(19) =446865a(20) =923521a(21) =1876833a(22) =3814209a(23) =7751457a(24) =15752961a(25) =31755969a(26) =64016001a(27) =129048129a(28) =260144641a(29) =522337665