Lucky numbers with size of gaps equal to 4 (lower terms).

Terms

a(0) =3a(1) =9a(2) =21a(3) =33a(4) =63a(5) =69a(6) =75a(7) =111a(8) =129a(9) =159a(10) =189a(11) =201a(12) =219a(13) =231a(14) =237a(15) =285a(16) =303a(17) =327a(18) =357a(19) =429a(20) =483a(21) =537a(22) =579a(23) =615a(24) =639a(25) =651a(26) =723a(27) =735a(28) =741a(29) =777