Left border of triangle A137680.

Terms

a(0) =1a(1) =1a(2) =3a(3) =7a(4) =17a(5) =40a(6) =96a(7) =228a(8) =544a(9) =1296a(10) =3089a(11) =7361a(12) =17544a(13) =41810a(14) =99643a(15) =237471a(16) =565946a(17) =1348773a(18) =3214424a(19) =7660679a(20) =18257085a(21) =43510652a(22) =103695461a(23) =247129108a(24) =588963062a(25) =1403628615a(26) =3345155947a(27) =7972242937a(28) =18999609718a(29) =45280252031