Triangular numbers with internal digits 6.

Terms

a(0) =0a(1) =1a(2) =3a(3) =6a(4) =10a(5) =15a(6) =21a(7) =28a(8) =36a(9) =45a(10) =55a(11) =66a(12) =78a(13) =91a(14) =465a(15) =561a(16) =666a(17) =861a(18) =46665a(19) =5666661