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

 

Logo


Hints
(Greetings from The On-Line Encyclopedia of Integer Sequences!)
A247086 Integers n such that numerator and denominator of sigma(n)/n are both prime. 1
2, 12, 24, 90, 234, 528, 588, 1456, 4320, 4680, 9024, 11466, 26208, 35640, 55104, 55552, 90816, 114660, 121024, 123648, 185562, 199584, 297600, 510720, 674880, 2142720, 2190336, 4112640, 4316928, 5322240, 5659641, 7600320, 8714160, 10281600, 12999168, 15268368 (list; graph; refs; listen; history; text; internal format)
OFFSET

1,1

LINKS

Michel Marcus, Table of n, a(n) for n = 1..81

EXAMPLE

For n=2, sigma(n)/n = 3/2 with both numerator and denominator prime.

MATHEMATICA

a247086Q[n_] := Block[{f = DivisorSigma[1, n]/n}, And[PrimeQ@ Numerator@ f, PrimeQ@ Denominator@ f]]; a247086[n_] := Select[Range@ n, a247086Q@ # &]; a247086[10^6] (* Michael De Vlieger, Jan 11 2015 *)

PROG

(PARI) isok(n) = my(ab = sigma(n)/n); isprime(numerator(ab)) && isprime(denominator(ab));

CROSSREFS

Cf. A000203 (sigma(n)).

Cf. A017665 (numerator of sigma(n)/n), A017666 (denominator of sigma(n)/n).

Sequence in context: A176679 A278407 A224923 * A121119 A226899 A052565

Adjacent sequences:  A247083 A247084 A247085 * A247087 A247088 A247089

KEYWORD

nonn

AUTHOR

Michel Marcus, Jan 10 2015

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 18 18:52 EDT 2019. Contains 324215 sequences. (Running on oeis4.)