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%I #13 Jul 23 2021 05:25:24
%S 2,3,5,6,9,26,45,66,87,345,765,906,3926,8405,11766,23805,35145,42966,
%T 59685,95289,317250,413325,416757,722745,770066,890825,938457,1325826,
%U 1921986,3315378,3675555,5299250,6791445,6899685,9371826,10892313
%N Numbers k such that 7^k == -1 (mod k-1).
%H Amiram Eldar, <a href="/A055690/b055690.txt">Table of n, a(n) for n = 1..285</a>
%t Do[If[PowerMod[7, n, n-1]==n-2, Print[n]], {n, 2, 2*10^7}]
%Y Cf. A055685, A055686, A055687, A055688, A055689, A055691, A055692, A055693, A055694, A055695.
%K nonn
%O 1,1
%A _Robert G. Wilson v_, Jun 09 2000