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A260233
Smallest prime factor of the n-th hexagonal number (A000384).
4
2, 3, 2, 3, 2, 7, 2, 3, 2, 3, 2, 5, 2, 3, 2, 3, 2, 19, 2, 3, 2, 3, 2, 5, 2, 3, 2, 3, 2, 31, 2, 3, 2, 3, 2, 37, 2, 3, 2, 3, 2, 5, 2, 3, 2, 3, 2, 7, 2, 3, 2, 3, 2, 5, 2, 3, 2, 3, 2, 11, 2, 3, 2, 3, 2, 7, 2, 3, 2, 3, 2, 5, 2, 3, 2, 3, 2, 79, 2, 3, 2, 3, 2, 5, 2
OFFSET
2,1
LINKS
FORMULA
a(n) = A020639(A000384(n)).
EXAMPLE
a(3) = 3 because A000384(3) = 15 = 3 * 5.
MATHEMATICA
Table[FactorInteger[n(2n-1)][[1, 1]], {n, 2, 100}] (* Harvey P. Dale, May 05 2016 *)
PROG
(PARI)
pg(m, n) = (n^2*(m-2)-n*(m-4))/2 \\ n-th m-gonal number
spf(m) = factorint(m)[1, 1] \\ Smallest prime factor
a(n) = spf(pg(6, n))
CROSSREFS
KEYWORD
nonn
AUTHOR
Colin Barker, Jul 20 2015
STATUS
approved