OFFSET
1,1
LINKS
Robert Israel, Table of n, a(n) for n = 1..10000
EXAMPLE
10 is in the sequence because 10 = 2*5 and 2*10+1 = 21 = 3*7.
MAPLE
isA006881:= proc(n) local F;
F:= ifactors(n)[2];
nops(F)=2 and F[1, 2]+F[2, 2]=2
end proc:
filter:= n -> andmap(isA006881, [n, 2*n+1]); select(filter, [$1..1000]); # Robert Israel, Nov 09 2017
MATHEMATICA
f[n_]:=Last/@FactorInteger[n]=={1, 1}; lst={}; Do[If[f[n]&&f[2*n+1], AppendTo[lst, n]], {n, 0, 3*6!}]; lst
Select[Range[500], PrimeNu[#]==PrimeOmega[#]==PrimeNu[2#+1] == PrimeOmega[ 2#+1] == 2&] (* Harvey P. Dale, Feb 22 2018 *)
CROSSREFS
KEYWORD
nonn
AUTHOR
Vladimir Joseph Stephan Orlovsky, May 05 2010
STATUS
approved