a(1)=0; thereafter a(n+1) = a(n) - n if a(n) >= n otherwise a(n+1) = a(n) + n.

Terms

a(0) =0a(1) =1a(2) =3a(3) =0a(4) =4a(5) =9a(6) =3a(7) =10a(8) =2a(9) =11a(10) =1a(11) =12a(12) =0a(13) =13a(14) =27a(15) =12a(16) =28a(17) =11a(18) =29a(19) =10a(20) =30a(21) =9a(22) =31a(23) =8a(24) =32a(25) =7a(26) =33a(27) =6a(28) =34a(29) =5