a(1)=1; a(n+1) = Sum_{k=1..n} a(k) a(floor(n/k)).

Terms

a(0) =1a(1) =1a(2) =2a(3) =5a(4) =13a(5) =34a(6) =90a(7) =236a(8) =621a(9) =1629a(10) =4274a(11) =11193a(12) =29337a(13) =76818a(14) =201173a(15) =526730a(16) =1379178a(17) =3610804a(18) =9453695a(19) =24750281a(20) =64798235a(21) =169644626a(22) =444138288a(23) =1162770238a(24) =3044180080a(25) =7969770106a(26) =20865148382a(27) =54625676431a(28) =143011928942