Triangular numbers whose external digits as well as internal digits form triangular numbers.

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) =105a(15) =231a(16) =435a(17) =465a(18) =666a(19) =1035a(20) =2211a(21) =3916a(22) =4005a(23) =6216a(24) =6786a(25) =19900a(26) =41905a(27) =46665a(28) =48205a(29) =162165