A112751 - OEIS

A112751

Number of numbers of the form 3^i*5^j that are less than or equal to n.

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, 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, 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

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OFFSET

1,3

LINKS

Table of n, a(n) for n=1..97.

FORMULA

From Ridouane Oudra, Jul 29 2019: (Start)

a(n) = Card_{ k | A003593(k) <= n }.

a(n) = Sum_{k=1..n} mu(15*k)*floor(n/k), where mu is the Möbius function (A008683).

a(n) = Sum_{k=1..n} (floor(15^k/k)-floor((15^k-1)/k)). (End)

From Ridouane Oudra, Jul 17 2020: (Start)

a(n) = Sum_{i=0..floor(log_5(n))} (floor(log_3(n/5^i)) + 1).