a(n) = diagonal sum of right-justified array T given by A027052.

Terms

a(0) =1a(1) =1a(2) =1a(3) =4a(4) =6a(5) =12a(6) =31a(7) =73a(8) =183a(9) =476a(10) =1248a(11) =3322a(12) =8943a(13) =24271a(14) =66355a(15) =182538a(16) =504824a(17) =1402682a(18) =3913585a(19) =10959499a(20) =30792445a(21) =86775340a(22) =245204312a(23) =694603032a(24) =1972115945a(25) =5610955925a(26) =15994866669a(27) =45677496204a(28) =130661330526a(29) =374339736820