login
The OEIS Foundation is supported by donations from users of the OEIS and by a grant from the Simons Foundation.

 

Logo


Hints
(Greetings from The On-Line Encyclopedia of Integer Sequences!)
A243128 Squarefree numbers k such that 4k <= sum of squarefree divisors of 4k. 1
3, 15, 21, 33, 35, 39, 51, 57, 69, 87, 93, 105, 111, 123, 129, 141, 159, 165, 177, 183, 195, 201, 213, 219, 231, 237, 249, 255, 267, 273, 285, 291, 303, 309, 321, 327, 339, 345, 357, 381, 385, 393, 399, 411, 417, 429, 435, 447, 453 (list; graph; refs; listen; history; text; internal format)
OFFSET

1,1

COMMENTS

Odd squarefree numbers n such that sigma(n)/n >= 4/3. - Charles R Greathouse IV, May 30 2014

Includes all odd squarefree multiples of its terms. The primitive members are 3, 35, 715, 935, 1001, 1045, 1105, 1235, 1265, .... - Charles R Greathouse IV, May 30 2014

LINKS

Charles R Greathouse IV, Table of n, a(n) for n = 1..10000

EXAMPLE

3 is in this sequence because 3 is squarefree and 4*3 = A048250(4*3) = 12;

21 is in this sequence because 21 is squarefree and 4*21 = 84 < A048250(4*21) = 96.

MATHEMATICA

Select[2Range[250] + 1, MoebiusMu[#] != 0 && DivisorSigma[1, #]/# >= 4/3 &] (* Alonso del Arte, May 31 2014 *)

PROG

(PARI) isok(n) = issquarefree(n) && (sumdiv(4*n, d, issquarefree(d)*d) >= 4*n); \\ Michel Marcus, May 30 2014

(PARI) is(n)=my(f=factor(n)); n%2 && n>1 && vecmax(f[, 2])==1 && sigma(f, -1) >= 4/3 \\ Charles R Greathouse IV, May 30 2014

CROSSREFS

Cf. A005117, A048250.

Sequence in context: A074214 A036897 A129966 * A306771 A216521 A110172

Adjacent sequences:  A243125 A243126 A243127 * A243129 A243130 A243131

KEYWORD

nonn

AUTHOR

Juri-Stepan Gerasimov, May 29 2014

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 July 12 22:46 EDT 2020. Contains 335669 sequences. (Running on oeis4.)