Numbers k such that the k-th triangular number contains only digits {1,7,8}.

Terms

a(0) =1a(1) =12a(2) =18a(3) =58a(4) =132a(5) =153a(6) =1332a(7) =1541a(8) =6018a(9) =11981a(10) =13332a(11) =40278a(12) =124753a(13) =133332a(14) =603121a(15) =1332507a(16) =1333332a(17) =13332507a(18) =13333332a(19) =19072906a(20) =133332507a(21) =133333332a(22) =485569333a(23) =486040907a(24) =612880278a(25) =1247972658a(26) =1333332507