Numbers k such that k concatenated with k+4 is a lucky number.

Terms

a(0) =1a(1) =3a(2) =15a(3) =19a(4) =21a(5) =39a(6) =55a(7) =63a(8) =73a(9) =85a(10) =99a(11) =111a(12) =117a(13) =135a(14) =159a(15) =165a(16) =201a(17) =219a(18) =261a(19) =267a(20) =273a(21) =327a(22) =333a(23) =345a(24) =357a(25) =369a(26) =381a(27) =399a(28) =423a(29) =471