a(n)=a(a(n-1)-1)+a(n+1-a(n-1)).

Terms

a(0) =1a(1) =1a(2) =2a(3) =3a(4) =3a(5) =4a(6) =5a(7) =6a(8) =6a(9) =6a(10) =7a(11) =8a(12) =9a(13) =10a(14) =10a(15) =11a(16) =11a(17) =12a(18) =13a(19) =14a(20) =15a(21) =16a(22) =16a(23) =16a(24) =16a(25) =17a(26) =18a(27) =18a(28) =19a(29) =20