a(n) is the leading digit of the n-th triangular number, n*(n+1)/2.

Terms

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