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

 Hints (Greetings from The On-Line Encyclopedia of Integer Sequences!)
 A252660 Numbers k such that 7^k - k is a semiprime. 2
 1, 20, 26, 32, 54, 162, 204 (list; graph; refs; listen; history; text; internal format)
 OFFSET 1,2 COMMENTS From Robert Israel, Sep 02 2016: (Start) Odd k is in the sequence iff (7^k-k)/2 is prime. If k == 1 (mod 3) then k is in the sequence iff (7^k-k)/3 is prime. 708 is in the sequence but is not necessarily a(7). (End) a(8) >= 384. - Daniel Suteu, Aug 05 2019 LINKS EXAMPLE 1 is in this sequence because 7^1-1 = 2*3 is semiprime. 20 is in this sequence because 7^20-20 = 1511201*52800564781 and these two factors are prime. MAPLE Res:= NULL: for n from 1 to 100 do       F:= ifactors(7^n-n, easy)[2];     if add(t[2], t=F) >= 3 or (hastype(F, symbol) and add(t[2], t=F) >= 2)        then flag:= false     elif add(t[2], t=F) = 2 and not hastype(F, symbol) then flag:= true     else       flag:= evalb(numtheory:-bigomega(7^n-n)=2)     fi;   if flag then  Res:= Res, n fi od: Res; # Robert Israel, Sep 02 2016 MATHEMATICA Select[Range[80], PrimeOmega[7^# - #]==2 &] PROG (MAGMA) IsSemiprime:=func; [m: m in [1..80] | IsSemiprime(s) where s is 7^m-m]; CROSSREFS Cf. similar sequences listed in A252656. Sequence in context: A269308 A167306 A061840 * A265401 A276503 A067065 Adjacent sequences:  A252657 A252658 A252659 * A252661 A252662 A252663 KEYWORD nonn,more AUTHOR Vincenzo Librandi, Dec 21 2014 EXTENSIONS a(6) from Robert Israel, Sep 02 2016 a(7) from Daniel Suteu, Aug 05 2019 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 May 18 23:41 EDT 2021. Contains 344009 sequences. (Running on oeis4.)