Start with a(0) = 1; thereafter a(n) is obtained from 5*a(n-1) by removing all 7's.

Terms

a(0) =1a(1) =5a(2) =25a(3) =125a(4) =625a(5) =3125a(6) =15625a(7) =8125a(8) =40625a(9) =203125a(10) =1015625a(11) =508125a(12) =2540625a(13) =1203125a(14) =6015625a(15) =3008125a(16) =15040625a(17) =5203125a(18) =26015625a(19) =13008125a(20) =65040625a(21) =325203125a(22) =1626015625a(23) =813008125a(24) =4065040625a(25) =20325203125a(26) =101626015625a(27) =50813008125a(28) =254065040625