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 A069902 Largest prime factor of n-th triangular number n(n+1)/2. 8
 3, 3, 5, 5, 7, 7, 3, 5, 11, 11, 13, 13, 7, 5, 17, 17, 19, 19, 7, 11, 23, 23, 5, 13, 13, 7, 29, 29, 31, 31, 11, 17, 17, 7, 37, 37, 19, 13, 41, 41, 43, 43, 11, 23, 47, 47, 7, 7, 17, 17, 53, 53, 11, 11, 19, 29, 59, 59, 61, 61, 31, 7, 13, 13, 67 (list; graph; refs; listen; history; text; internal format)
 OFFSET 2,1 COMMENTS a(n) = A006530(A000217(n)). LINKS EXAMPLE A000217(9) = 9*(9+1)/2 = 45 = 3*3*5, therefore a(9) = 5. MATHEMATICA PrimeFactors[n_]:=Flatten[Table[ #[[1]], {1}]&/@FactorInteger[n]]; Table[PrimeFactors[n*(n-1)/2][[ -1]], {n, 3, 6!}] (* Vladimir Joseph Stephan Orlovsky, Aug 12 2009 *) PROG (PARI) a(n)=vecmax(factor(n*(n+1)/2)[, 1]) \\ M. F. Hasler, May 02 2015 CROSSREFS Cf. A069901, A069903, A069904. Essentially the same as A074399. Sequence in context: A172170 A233808 A141424 * A085779 A078936 A293702 Adjacent sequences:  A069899 A069900 A069901 * A069903 A069904 A069905 KEYWORD nonn AUTHOR Reinhard Zumkeller, Apr 10 2002 STATUS approved

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Last modified October 17 10:07 EDT 2019. Contains 328108 sequences. (Running on oeis4.)