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A069902
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Largest prime factor of n(n+1)/2, the n-th triangular number.
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8
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1, 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
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OFFSET
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1,2
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COMMENTS
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Essentially the same as A074399, which has many comments, references and links.
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LINKS
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FORMULA
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EXAMPLE
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A000217(9) = 9*(9+1)/2 = 45 = 3*3*5, therefore a(9) = 5.
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MATHEMATICA
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PrimeFactors[n_]:=Flatten[Table[ #[[1]], {1}]&/@FactorInteger[n]]; Table[PrimeFactors[n*(n-1)/2][[ -1]], {n, 2, 6!}] (* Vladimir Joseph Stephan Orlovsky, Aug 12 2009 *)
(* Second program: *)
Array[FactorInteger[PolygonalNumber[#]][[-1, 1]] &, 66] (* Michael De Vlieger, Sep 14 2023 *)
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PROG
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(PARI) \\ written for a(n), n >= 2
a(n)=vecmax(factor(n*(n+1)/2)[, 1]) \\ M. F. Hasler, May 02 2015
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CROSSREFS
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KEYWORD
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nonn,easy
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AUTHOR
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EXTENSIONS
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STATUS
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approved
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