3*n - 2*floor(sqrt(4*n+5)) + 5.

Terms

a(0) =1a(1) =2a(2) =5a(3) =6a(4) =9a(5) =10a(6) =13a(7) =16a(8) =17a(9) =20a(10) =23a(11) =24a(12) =27a(13) =30a(14) =33a(15) =34a(16) =37a(17) =40a(18) =43a(19) =44a(20) =47a(21) =50a(22) =53a(23) =56a(24) =57a(25) =60a(26) =63a(27) =66a(28) =69a(29) =70