a(n) = Sum{T(n,k)}, k = 0,1,...,n, where T is the array in A026148.

Terms

a(0) =1a(1) =2a(2) =4a(3) =11a(4) =33a(5) =97a(6) =287a(7) =848a(8) =2507a(9) =7411a(10) =21910a(11) =64780a(12) =191555a(13) =566515a(14) =1675740a(15) =4957788a(16) =14671083a(17) =43424427a(18) =128560308a(19) =380699619a(20) =1127616675a(21) =3340750035a(22) =9899863295a(23) =29343770140a(24) =86996647805a(25) =257980647221a(26) =765187113746