a(n) = Sum_{j=0..n} T(n, j), where T is given by A026552.

Terms

a(0) =1a(1) =2a(2) =6a(3) =11a(4) =33a(5) =64a(6) =191a(7) =376a(8) =1122a(9) =2222a(10) =6636a(11) =13180a(12) =39395a(13) =78373a(14) =234414a(15) =466840a(16) =1397034a(17) =2784266a(18) =8335242a(19) =16620976a(20) =49773018a(21) =99291358a(22) =297406884a(23) =593484440a(24) =1777995535a(25) =3548969075a(26) =10633840743a(27) =21230215328a(28) =63620551947