Numbers k such that the k-th triangular number contains only digits {1,5,7}.

Terms

a(0) =1a(1) =5a(2) =10a(3) =18a(4) =58a(5) =101a(6) =149a(7) =1050a(8) =1885a(9) =5925a(10) =10114a(11) =15173a(12) =47226a(13) =550594a(14) =585005a(15) =1049925a(16) =1533085a(17) =56174285a(18) =104987349a(19) =123053789a(20) =1017425330a(21) =1230586669a(22) =1245114914a(23) =38878044589a(24) =101111538130a(25) =177627538130