%I #34 Mar 27 2023 18:31:17
%S 1,1,2,2,3,3,3,3,4,4,4,4,4,4,5,5,5,5,5,5,5,5,5,5,6,6,7,7,7,7,7,7,7,7,
%T 7,7,7,7,7,7,7,7,7,7,8,8,8,8,8,8,8,8,8,8,8,8,8,8,8,8,8,8,8,8,8,8,8,8,
%U 8,8,8,8,8,8,9,9,9,9,9,9,10,10,10,10,10,10,10,10,10,10,10,10,10,10,10,10,10
%N Number of numbers of the form 3^i*5^j that are less than or equal to n.
%F From _Ridouane Oudra_, Jul 29 2019: (Start)
%F a(n) = Card_{ k | A003593(k) <= n }.
%F a(n) = Sum_{k=1..n} mu(15*k)*floor(n/k), where mu is the Möbius function (A008683).
%F a(n) = Sum_{k=1..n} (floor(15^k/k)-floor((15^k-1)/k)). (End)
%F From _Ridouane Oudra_, Jul 17 2020: (Start)
%F a(n) = Sum_{i=0..floor(log_5(n))} (floor(log_3(n/5^i)) + 1).
%F a(n) = Sum_{i=0..floor(log_3(n))} (floor(log_5(n/3^i)) + 1). (End)
%p with(numtheory): seq(add(mobius(15*k)*floor(n/k), k=1..n), n=1..90); # _Ridouane Oudra_, Jul 29 2019
%o (Magma) [&+[MoebiusMu(15*k)*Floor(n/k):k in [1..n]]: n in [1..97]]; // _Marius A. Burtea_, Jul 30 2019
%Y Cf. A003593, A071520.
%K nonn
%O 1,3
%A _Reinhard Zumkeller_, Sep 18 2005