Triangular numbers with palindromic indices.

Terms

a(0) =0a(1) =1a(2) =3a(3) =6a(4) =10a(5) =15a(6) =21a(7) =28a(8) =36a(9) =45a(10) =66a(11) =253a(12) =561a(13) =990a(14) =1540a(15) =2211a(16) =3003a(17) =3916a(18) =4950a(19) =5151a(20) =6216a(21) =7381a(22) =8646a(23) =10011a(24) =11476a(25) =13041a(26) =14706a(27) =16471a(28) =18336a(29) =20503