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

 Please make a donation to keep the OEIS running. We are now in our 56th year. In the past year we added 10000 new sequences and reached almost 9000 citations (which often say "discovered thanks to the OEIS"). Other ways to donate

 Hints (Greetings from The On-Line Encyclopedia of Integer Sequences!)
 A067780 Numbers n such that sigma(n)/phi(n) is prime. 1
 2, 3, 12, 15, 35, 56, 78, 140, 190, 357, 630, 812, 910, 1045, 1672, 2214, 2580, 3339, 4180, 4522, 4674, 5049, 5278, 5544, 8008, 10659, 11160, 12441, 16065, 16120, 16770, 18630, 18810, 24244, 24871, 24969, 25070, 25758, 27000, 27170, 28896 (list; graph; refs; listen; history; text; internal format)
 OFFSET 1,1 LINKS Vincenzo Librandi, Table of n, a(n) for n = 1..600 EXAMPLE sigma(12)/phi(12) = 28/4 = 7, a prime, so 12 is a term of the sequence. MATHEMATICA Select[Range[10^5], PrimeQ[DivisorSigma[1, # ]/EulerPhi[ # ]] &] CROSSREFS Sequence in context: A090512 A076175 A289870 * A290168 A124486 A260908 Adjacent sequences:  A067777 A067778 A067779 * A067781 A067782 A067783 KEYWORD nonn AUTHOR Joseph L. Pe, Feb 06 2002 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.

Last modified November 26 09:38 EST 2020. Contains 338639 sequences. (Running on oeis4.)