Number of directed aggregates of height <= 2 with n cells.

Terms

a(0) =1a(1) =2a(2) =5a(3) =11a(4) =26a(5) =62a(6) =153a(7) =385a(8) =988a(9) =2573a(10) =6786a(11) =18084a(12) =48621a(13) =131718a(14) =359193a(15) =985185a(16) =2715972a(17) =7521567a(18) =20915256a(19) =58373586a(20) =163462815a(21) =459136809a(22) =1293223230a(23) =3651864606a(24) =10336625731a(25) =29321683082a(26) =83344398533a(27) =237344961291