Triangular numbers repeated.

Terms

a(0) =1a(1) =1a(2) =3a(3) =3a(4) =6a(5) =6a(6) =10a(7) =10a(8) =15a(9) =15a(10) =21a(11) =21a(12) =28a(13) =28a(14) =36a(15) =36a(16) =45a(17) =45a(18) =55a(19) =55a(20) =66a(21) =66a(22) =78a(23) =78a(24) =91a(25) =91a(26) =105a(27) =105a(28) =120a(29) =120