a(n) = T(n,n), where T is the array in A026148.

Terms

a(0) =1a(1) =1a(2) =2a(3) =4a(4) =12a(5) =32a(6) =89a(7) =246a(8) =685a(9) =1912a(10) =5355a(11) =15038a(12) =42339a(13) =119484a(14) =337935a(15) =957738a(16) =2719539a(17) =7736208a(18) =22044444a(19) =62916696a(20) =179841270a(21) =514793944a(22) =1475586687a(23) =4234989242a(24) =12169352003a(25) =35009302296a(26) =100826681530a(27) =290683486132a(28) =838873595124