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%I #18 Jul 12 2021 03:01:18
%S 2,3,11,42,147,13818,21450,27594,41370,55011,126291,265722,417123,
%T 1315635,3994571,5704611,6860490,9298842,13941354,14349027,17658578,
%U 20382810,26557874,27841338,69831363,86550090,113272170,130457571,163013235,192688650,211142538,333792522
%N Numbers k such that 9^k == -1 (mod k-1).
%t Do[If[PowerMod[9, n, n-1]==n-2, Print[n]], {n, 2, 10^8}]
%o (PARI) isok(k) = Mod(9, k-1)^k == -1; \\ _Michel Marcus_, Jul 12 2021
%Y Cf. A001019.
%K nonn
%O 1,1
%A _Robert G. Wilson v_, Jun 09 2000
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