A008953 a(n) is the leading digit of the n-th triangular number, n*(n+1)/2.

1, 3, 6, 1, 1, 2, 2, 3, 4, 5, 6, 7, 9, 1, 1, 1, 1, 1, 1, 2, 2, 2, 2, 3, 3, 3, 3, 4, 4, 4, 4, 5, 5, 5, 6, 6, 7, 7, 7, 8, 8, 9, 9, 9, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 2, 2, 2, 2, 2, 2, 2, 2, 2, 2, 2, 2, 2, 2, 3, 3, 3, 3, 3, 3, 3, 3, 3, 3, 3, 3, 4, 4, 4, 4, 4, 4, 4, 4, 4, 4, 4

Offset: 1

Author

N. J. A. Sloane

Keywords

• nonn
• base
• easy

Links

• Reinhard Zumkeller, Table of n, a(n) for n = 1..1000

Crossrefs

Cf. A000030, A000217, A008954. - Reinhard Zumkeller, May 12 2010

Programs

Magma

[Intseq((n*(n+1) div 2))[#Intseq((n*(n+1) div 2))]: n in [1..100]]; // Vincenzo Librandi, Apr 12 2019

Mathematica

Array[IntegerDigits[# (# + 1) / 2][[1]]&, 100] (* Vincenzo Librandi, Apr 12 2019 *)
IntegerDigits[#][[1]]&/@Accumulate[Range[100]] (* Harvey P. Dale, Oct 15 2022 *)

PARI

a(n) = digits(n*(n+1)/2)[1]; \\ Michel Marcus, Jun 20 2018

Formula

a(n) = A000030(A000217(n)). - Reinhard Zumkeller, May 12 2010