cp's OEIS Frontend

A271842 Positive numbers m such that m^2 - 1 divides 4^m - 1.

%I A271842 #42 Jun 29 2024 16:40:16
%S A271842 2,4,6,16,36,52,66,256,378,456,1296,1470,1548,1800,2002,2556,4356,
%T A271842 6480,8008,11952,23580,26320,33930,36636,37170,43290,44100,47520,
%U A271842 47880,49680,57240,65536,74448,84420,97812,101920,127050,134946,139860,141156,157080,164880,165600,209220,225456
%N A271842 Positive numbers m such that m^2 - 1 divides 4^m - 1.
%H A271842 Charles R Greathouse IV, <a href="/A271842/b271842.txt">Table of n, a(n) for n = 1..1000</a>
%e A271842 2 is in this sequence because (4^2 - 1)/(2^2 - 1) = 5.
%p A271842 A271842:=n->`if`((4^n-1) mod (n^2-1) = 0, n, NULL): seq(A271842(n), n=2..10^4); # _Wesley Ivan Hurt_, Apr 18 2016
%t A271842 Select[Range[1, 100], IntegerQ[(4^# - 1)/(#^2 - 1)] &] (* _G. C. Greubel_, Apr 15 2016 *)
%o A271842 (Magma) [0] cat [n: n in [2..240000] | Denominator((4^n-1)/(n^2-1)) eq 1];
%o A271842 (PARI) is(n)=Mod(4,n^2-1)^n==1 \\ _Charles R Greathouse IV_, Apr 15 2016
%Y A271842 Cf. positive numbers m such that m^2 - 1 divides (2^k)^m - 1:
%Y A271842 A247219 (k=1), this sequence (k=2), A242062 (k=3).
%K A271842 nonn
%O A271842 1,1
%A A271842 _Juri-Stepan Gerasimov_, Apr 15 2016