A080215 Binomial(greatest prime factor of n, smallest odd prime factor of n).

1, 2, 1, 2, 1, 1, 1, 2, 1, 1, 1, 1, 1, 1, 10, 2, 1, 1, 1, 1, 35, 1, 1, 1, 1, 1, 1, 1, 1, 10, 1, 2, 165, 1, 21, 1, 1, 1, 286, 1, 1, 35, 1, 1, 10, 1, 1, 1, 1, 1, 680, 1, 1, 1, 462, 1, 969, 1, 1, 10, 1, 1, 35, 2, 1287, 165, 1, 1, 1771, 21, 1, 1, 1, 1, 10, 1, 330, 286, 1, 1, 1, 1, 1, 35

Offset: 1

Author

Reinhard Zumkeller, Feb 06 2003

Keywords

• nonn

Links

• Antti Karttunen, Table of n, a(n) for n = 1..16384

Crossrefs

Cf. A006530, A078701, A080214, A080212.

Programs

Mathematica

a[n_] := With[{f = Select[FactorInteger[n], #[[1]] != 2&]}, If[f == {}, 2, Binomial[f[[-1, 1]], f[[1, 1]]]]];
Array[a, 100] (* Jean-François Alcover, Dec 02 2021 *)

PARI

A080215(n) = if(1==n,n,my(f = factor(n)[, 1], v = select(x->((x%2)==1), f)); binomial(f[#f], if(#v, vecmin(v), 1))); \\ Antti Karttunen, Dec 23 2018

Formula

a(n) = binomial(A006530(n), A078701(n)). - Antti Karttunen, Dec 23 2018