Triangular numbers composed of digits {1,5,7}.

Terms

a(0) =1a(1) =15a(2) =55a(3) =171a(4) =1711a(5) =5151a(6) =11175a(7) =551775a(8) =1777555a(9) =17555775a(10) =51151555a(11) =115117551a(12) =1115171151a(13) =151577151715a(14) =171115717515a(15) =551171777775