A366979 - OEIS

A366979

Number of divisors of n less than or equal to d(n).

1, 2, 1, 2, 1, 3, 1, 3, 2, 2, 1, 5, 1, 2, 2, 3, 1, 4, 1, 4, 2, 2, 1, 6, 1, 2, 2, 3, 1, 5, 1, 3, 2, 2, 1, 6, 1, 2, 2, 5, 1, 5, 1, 3, 3, 2, 1, 6, 1, 3, 2, 3, 1, 4, 1, 5, 2, 2, 1, 8, 1, 2, 2, 3, 1, 4, 1, 3, 2, 4, 1, 8, 1, 2, 3, 3, 1, 4, 1, 6, 2, 2, 1, 7, 1, 2, 2, 4, 1, 7

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OFFSET

1,2

COMMENTS

First differs from A126131 at a(25) = 1.

LINKS

Table of n, a(n) for n=1..90.

FORMULA

a(n) = Sum_{d|n, d <= d(n)} 1.

a(n) = 1 + Sum_{d|n} (Sum_{i=2..d(n)} ( sign(floor(i/d)) - sign(floor((i-1)/d)) )), where d(n) is the number of divisors of n (A000005).

EXAMPLE

a(8) = 3; There are 3 divisors of 8 that are <= d(8) = 4. They are: {1,2,4}.