a(n) = diagonal sum of right justified array T given by A027113.

Terms

a(0) =1a(1) =2a(2) =4a(3) =12a(4) =27a(5) =70a(6) =182a(7) =477a(8) =1269a(9) =3413a(10) =9249a(11) =25255a(12) =69389a(13) =191680a(14) =532026a(15) =1482926a(16) =4148949a(17) =11647260a(18) =32797042a(19) =92608175a(20) =262157690a(21) =743845685a(22) =2115093984a(23) =6026041793a(24) =17199956143a(25) =49176678286a(26) =140824260498a(27) =403865784020