Numbers k such that the k-th triangular number contains only digits {0,1,9}.

Terms

a(0) =1a(1) =4a(2) =13a(3) =19a(4) =44a(5) =141a(6) =195a(7) =199a(8) =424a(9) =1955a(10) =1995a(11) =1999a(12) =14141a(13) =19955a(14) =19995a(15) =19999a(16) =42473a(17) =44741a(18) =47138a(19) =199955a(20) =199995a(21) =199999a(22) =1999955a(23) =1999995a(24) =1999999a(25) =4713620a(26) =19999955a(27) =19999995a(28) =19999999a(29) =199999955