a(n) = 1 + ceiling((n-1)*phi^2), phi = (1+sqrt(5))/2.

Terms

a(0) =1a(1) =4a(2) =7a(3) =9a(4) =12a(5) =15a(6) =17a(7) =20a(8) =22a(9) =25a(10) =28a(11) =30a(12) =33a(13) =36a(14) =38a(15) =41a(16) =43a(17) =46a(18) =49a(19) =51a(20) =54a(21) =56a(22) =59a(23) =62a(24) =64a(25) =67a(26) =70a(27) =72a(28) =75a(29) =77