A008953 - OEIS

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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

(list; graph; refs; listen; history; text; internal format)

OFFSET

1,2

LINKS

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

FORMULA

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

MATHEMATICA

Array[IntegerDigits[# (# + 1) / 2][[1]]&, 100] (* Vincenzo Librandi, Apr 12 2019 *)

IntegerDigits[#][[1]]&/@Accumulate[Range[100]] (* Harvey P. Dale, Oct 15 2022 *)

PROG

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