OFFSET
1,1
LINKS
Reinhard Zumkeller, Table of n, a(n) for n = 1..1000
FORMULA
2 <= A001221(a(n)) <= 3.
5 <= A006530(a(n)) <= 7.
Sum_{n>=1} 1/a(n) = 11/16. - Amiram Eldar, Oct 25 2024
EXAMPLE
a(10) = 60 = 2^2 * 3 * 5.
a(11) = 63 = 3^2 * 7.
a(12) = 70 = 2 * 5 * 7.
MATHEMATICA
pfsQ[n_]:=Module[{fs=Transpose[FactorInteger[n]][[1]]}, Max[fs]<8 && 1<Length[fs]<4]; upto=3000; With[{max7=Floor[upto/7], max15= Floor[ upto/15]}, Union[Select[Join[7Range[max7], 15Range[max15]], pfsQ]]] (* Harvey P. Dale, Aug 21 2011 *)
PROG
(Haskell)
a195238 n = a195238_list !! (n-1)
a195238_list = filter (\x -> a001221 x `elem` [2, 3] &&
a006530 x `elem` [5, 7] &&
(mod x 7 == 0 || mod x 15 == 0)) [1..]
-- Reinhard Zumkeller, Sep 13 2011
(PARI) is(n)=my(v=apply(p->valuation(n, p), [2, 3, 5, 7])); n==2^v[1]*3^v[2]*5^v[3]*7^v[4] && (v[4] || v[2]*v[3]) && factorback(v)==0 && !!v[1]+!!v[2]+!!v[3]+!!v[4]>1 \\ Charles R Greathouse IV, Sep 14 2015
CROSSREFS
KEYWORD
nonn,easy,changed
AUTHOR
Harvey P. Dale, Sep 13 2011
STATUS
approved