Numbers of form 21k+1 or 21k+2 that use only digits 3 and 7.

Terms

a(0) =337a(1) =737a(2) =7373a(3) =73333a(4) =73733a(5) =333377a(6) =333733a(7) =737333a(8) =737773a(9) =777337a(10) =777737a(11) =3333373a(12) =3333773a(13) =3373337a(14) =3373777a(15) =3777377a(16) =3777733a(17) =7337737a(18) =7373333a(19) =7373773a(20) =7733377a(21) =7733777a(22) =7777373a(23) =33337333a(24) =33337733a(25) =33377737a(26) =33733373