A112755 - OEIS

%I #6 Feb 07 2020 09:03:47

%S 1,3,5,3,3,5,3,3,3,3,5,3,3,3,3,3,5,3,3,3,3,3,3,3,5,3,3,3,3,3,3,3,3,5,

%T 3,3,3,3,3,3,3,3,3,3,5,3,3,3,3,3,3,3,3,3,3,3,5,3,3,3,3,3,3,3,3,3,3,3,

%U 3,3,5,3,3,3,3,3,3,3,3,3,3,3,3,3,3,5,3,3,3,3,3,3,3,3,3,3,3,3,3,3,3,3,5,3,3

%N Smallest prime factor of n-th number of the form 3^i*5^j.

%H Amiram Eldar, <a href="/A112755/b112755.txt">Table of n, a(n) for n = 1..10000</a>

%F a(n) = A020639(A003593(n));

%F a(n) <= A112756(n) <= 5.

%t s = {}; m = 12; Do[n = 5^k; While[n <= 5^m, AppendTo[s, n]; n *= 3], {k, 0, m}]; FactorInteger[#][[1, 1]] & /@ Union[s] (* _Amiram Eldar_, Feb 07 2020 *)

%Y Cf. A003593, A020639, A086410, A112756, A112763.

%K nonn

%O 1,2

%A _Reinhard Zumkeller_, Sep 18 2005