a(n) = diagonal sum of right justified array T given by A027082.

Terms

a(0) =1a(1) =1a(2) =3a(3) =9a(4) =18a(5) =49a(6) =127a(7) =331a(8) =883a(9) =2376a(10) =6439a(11) =17591a(12) =48347a(13) =133589a(14) =370883a(15) =1033997a(16) =2893498a(17) =8124303a(18) =22880569a(19) =64616728a(20) =182943204a(21) =519146325a(22) =1476337528a(23) =4206621803a(24) =12008000771a(25) =34335363519a(26) =98332343337a(27) =282026580873