This is a front-end for the Online Encyclopedia of Integer Sequences, made by Christian Perfect. The idea is to provide OEIS entries in non-ancient HTML, and then to think about how they're presented visually. The source code is on GitHub.
%I A112751 #40 May 04 2025 01:57:37
%S A112751 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 A112751 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 A112751 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 A112751 Number of numbers of the form 3^i*5^j that are less than or equal to n.
%H A112751 Amiram Eldar, <a href="/A112751/b112751.txt">Table of n, a(n) for n = 1..10000</a>
%F A112751 From _Ridouane Oudra_, Jul 29 2019: (Start)
%F A112751 a(n) = Card_{ k | A003593(k) <= n }.
%F A112751 a(n) = Sum_{k=1..n} mu(15*k)*floor(n/k), where mu is the Möbius function (A008683).
%F A112751 a(n) = Sum_{k=1..n} (floor(15^k/k)-floor((15^k-1)/k)). (End)
%F A112751 From _Ridouane Oudra_, Jul 17 2020: (Start)
%F A112751 a(n) = Sum_{i=0..floor(log_5(n))} (floor(log_3(n/5^i)) + 1).
%F A112751 a(n) = Sum_{i=0..floor(log_3(n))} (floor(log_5(n/3^i)) + 1). (End)
%p A112751 with(numtheory): seq(add(mobius(15*k)*floor(n/k), k=1..n), n=1..90); # _Ridouane Oudra_, Jul 29 2019
%t A112751 Accumulate[Table[Boole[n == Times @@ ({3, 5}^IntegerExponent[n, {3, 5}])], {n, 1, 100}]] (* _Amiram Eldar_, May 04 2025 *)
%o A112751 (Magma) [&+[MoebiusMu(15*k)*Floor(n/k):k in [1..n]]: n in [1..97]]; // _Marius A. Burtea_, Jul 30 2019
%Y A112751 Cf. A003593, A008683, A071520.
%K A112751 nonn
%O A112751 1,3
%A A112751 _Reinhard Zumkeller_, Sep 18 2005