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!)
 A252659 Numbers m such that 6^m - m is a semiprime. 2
 2, 3, 5, 10, 15, 23, 34, 37, 47, 70 (list; graph; refs; listen; history; text; internal format)
 OFFSET 1,1 COMMENTS From Robert Israel, Sep 06 2016: (Start) Even n is in this sequence iff (6^n-n)/2 is prime. 3*k is in this sequence iff (2*6^(3*k-1)-k is prime. Also contains 275, 278 and 683. The only other possible member less than 275 is 259. (End) LINKS EXAMPLE 2 is in this sequence because 6^2-2 = 2*17 is semiprime. 10 is in this sequence because 6^10-10 = 2*30233083 and these two factors are prime. MAPLE Res:= NULL: for n from 1 to 100 do     F:= ifactors(6^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(6^n-n)=2)     fi;   if flag then  Res:= Res, n fi od: Res; # Robert Israel, Sep 06 2016 MATHEMATICA Select[Range[90], PrimeOmega[6^# - #]== 2&] PROG (MAGMA) IsSemiprime:=func; [m: m in [1..90] | IsSemiprime(s) where s is 6^m-m]; CROSSREFS Cf. similar sequences listed in A252656. Sequence in context: A043859 A043868 A048329 * A004691 A133335 A062925 Adjacent sequences:  A252656 A252657 A252658 * A252660 A252661 A252662 KEYWORD nonn,more AUTHOR Vincenzo Librandi, Dec 21 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.

Last modified April 17 12:40 EDT 2021. Contains 343063 sequences. (Running on oeis4.)