OFFSET
1,1
LINKS
Robert Israel, Table of n, a(n) for n = 1..10000
EXAMPLE
247 is a term because 247 = 13*19, 2*247 - 1 = 493 = 17*29, 4*247-3 = 985 = 5*197, and 8*247 - 1 = 1969 = 11*179.
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, 4*n-3, 8*n-7]);
select(filter, [$1..10000]); # Robert Israel, Jul 11 2017
MATHEMATICA
f[n_]:=Last/@FactorInteger[n]=={1, 1}; lst={}; Do[If[f[n]&&f[2*n-1]&&f[4*n-3]&&f[8*n-7], AppendTo[lst, n]], {n, 0, 3*7!}]; lst
p2dpQ[n_]:=Transpose[FactorInteger[n]][[2]]=={1, 1}; With[{s=Select[Range[ 3500], p2dpQ]}, Select[s, AllTrue[{2#-1, 4#-3, 8#-7}, p2dpQ]&]] (* The program uses the AllTrue function from Mathematica version 10 *) (* Harvey P. Dale, Mar 27 2015 *)
CROSSREFS
KEYWORD
nonn
AUTHOR
Vladimir Joseph Stephan Orlovsky, May 04 2010
STATUS
approved