A259649 Smallest prime factor of the n-th pentagonal number (A000326).

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

Colin Barker, Table of n, a(n) for n = 2..1000

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

Cf. A000326, A020639, A259650, A259651, A259652.

Sequence in context: A318332 A177925 A161462 * A217868 A153842 A125136

Adjacent sequences: A259646 A259647 A259648 * A259650 A259651 A259652

Keyword

nonn

Author

Colin Barker, Jul 02 2015

Status

approved