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Numbers k that divide 3^k - 1.
28

%I #24 Nov 01 2024 11:26:06

%S 1,2,4,8,16,20,32,40,64,80,100,128,160,200,220,256,272,320,328,400,

%T 440,500,512,544,640,656,800,880,1000,1024,1088,1100,1220,1280,1312,

%U 1360,1600,1640,1760,2000,2048,2176,2200,2420,2440,2500,2560,2624,2720,3088

%N Numbers k that divide 3^k - 1.

%C Closed under multiplication: if x and y are terms then so is x*y.

%C The number (3^k-1)/2 is a term of this sequence if and only if k is a term. - _Thomas Ordowski_, Jul 25 2024

%H Charles R Greathouse IV, <a href="/A067945/b067945.txt">Table of n, a(n) for n = 1..10000</a>

%t Join[{1},Select[Range[3100],PowerMod[3,#,#]==1&]] (* _Harvey P. Dale_, Nov 01 2024 *)

%o (PARI) is(n)=Mod(3,n)^n==1 \\ _Charles R Greathouse IV_, Aug 01 2016

%Y Cf. A024023.

%K easy,nonn,changed

%O 1,2

%A _Benoit Cloitre_, Mar 05 2002