a(n) = 3 * prime(n).

Terms

a(0) =6a(1) =9a(2) =15a(3) =21a(4) =33a(5) =39a(6) =51a(7) =57a(8) =69a(9) =87a(10) =93a(11) =111a(12) =123a(13) =129a(14) =141a(15) =159a(16) =177a(17) =183a(18) =201a(19) =213a(20) =219a(21) =237a(22) =249a(23) =267a(24) =291a(25) =303a(26) =309a(27) =321a(28) =327a(29) =339