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A259649
Smallest prime factor of the n-th pentagonal number (A000326).
4
5, 2, 2, 5, 3, 2, 2, 3, 5, 2, 2, 13, 7, 2, 2, 5, 3, 2, 2, 3, 5, 2, 2, 5, 7, 2, 2, 29, 3, 2, 2, 3, 17, 2, 2, 5, 19, 2, 2, 41, 3, 2, 2, 3, 23, 2, 2, 7, 5, 2, 2, 53, 3, 2, 2, 3, 29, 2, 2, 7, 5, 2, 2, 5, 3, 2, 2, 3, 5, 2, 2, 73, 13, 2, 2, 5, 3, 2, 2, 3, 5, 2, 2
OFFSET
2,1
LINKS
FORMULA
a(n) = A020639(A000326(n)).
EXAMPLE
a(3) = 2 because A000326(3) = 12 = 2^2 * 3.
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(5, n))
CROSSREFS
KEYWORD
nonn
AUTHOR
Colin Barker, Jul 02 2015
STATUS
approved