login
This site is supported by donations to The OEIS Foundation.

 

Logo


Hints
(Greetings from The On-Line Encyclopedia of Integer Sequences!)
A209291 Sum of the refactorable numbers less than or equal to n. 0
1, 3, 3, 3, 3, 3, 3, 11, 20, 20, 20, 32, 32, 32, 32, 32, 32, 50, 50, 50, 50, 50, 50, 74, 74, 74, 74, 74, 74, 74, 74, 74, 74, 74, 74, 110, 110, 110, 110, 150, 150, 150, 150, 150, 150, 150, 150, 150, 150, 150, 150, 150, 150, 150, 150, 206, 206, 206, 206, 266 (list; graph; refs; listen; history; text; internal format)
OFFSET

1,2

COMMENTS

A number is refactorable if it is divisible by the number of its divisors.

The first 8 terms of a(n) are odd.  The next odd term after 11 is a(225) = 2395.

LINKS

Table of n, a(n) for n=1..60.

FORMULA

a(n) = sum( (i) * ( 1 + floor(i/d(i)) - ceil(i/d(i)) ), i = 1..n ), where d(n) is the number of divisors less than or equal to n.

MAPLE

with(numtheory); a:=n->sum( (i) * ( 1 + floor(i/tau(i)) - ceil(i/tau(i)) ), i = 1..n );

CROSSREFS

Cf. A033950, A000005, A141586, A057265, A036896, A036898, A114617.

Sequence in context: A210746 A283986 A105159 * A176873 A227727 A050499

Adjacent sequences:  A209288 A209289 A209290 * A209292 A209293 A209294

KEYWORD

nonn

AUTHOR

Wesley Ivan Hurt, Jan 16 2013

STATUS

approved

Lookup | Welcome | Wiki | Register | Music | Plot 2 | Demos | Index | Browse | More | WebCam
Contribute new seq. or comment | Format | Style Sheet | Transforms | Superseeker | Recent
The OEIS Community | Maintained by The OEIS Foundation Inc.

License Agreements, Terms of Use, Privacy Policy. .

Last modified June 25 10:10 EDT 2019. Contains 324351 sequences. (Running on oeis4.)