Multilevel sieve: at k-th step, accept k numbers, reject k, accept k, ...

Terms

a(0) =1a(1) =3a(2) =9a(3) =25a(4) =57a(5) =145a(6) =337a(7) =793a(8) =1921a(9) =3849a(10) =8835a(11) =18889a(12) =41473a(13) =92305a(14) =203211a(15) =432699a(16) =944313a(17) =2027529a(18) =4077769a(19) =8745153a(20) =18133305a(21) =37898113a(22) =80713737a(23) =169730259a(24) =358760457a(25) =750591867a(26) =1575313473a(27) =3255787851a(28) =6751959507a(29) =14108682265