Triangular numbers with internal digits 4.

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) =741a(15) =946a(16) =6441a(17) =544446