Sum{T(k,k-1)}, k = 1,2,...,n, where T is the array in A026148.

Terms

a(0) =1a(1) =2a(2) =6a(3) =16a(4) =45a(5) =126a(6) =356a(7) =1008a(8) =2862a(9) =8140a(10) =23188a(11) =66144a(12) =188916a(13) =540216a(14) =1546560a(15) =4432512a(16) =12717513a(17) =36526626a(18) =105016686a(19) =302228080a(20) =870613689a(21) =2510249302a(22) =7244285436a(23) =20924179920a(24) =60487084775a(25) =174994990326a(26) =506669921982