|First digit-last digit| for triangular numbers.

Terms

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