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A008953 a(n) is the leading digit of the n-th triangular number, n*(n+1)/2. 3
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
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
(Magma) [Intseq((n*(n+1) div 2))[#Intseq((n*(n+1) div 2))]: n in [1..100]]; // Vincenzo Librandi, Apr 12 2019
CROSSREFS
Sequence in context: A142464 A177022 A325634 * A104611 A076790 A095693
KEYWORD
nonn,base
AUTHOR
STATUS
approved

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Last modified March 28 18:04 EDT 2024. Contains 371254 sequences. (Running on oeis4.)