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%I #9 Dec 12 2016 09:18:15
%S 1,25,52606009,1088670025,1315150225
%N Odd numbers n such that 2^n == 1 (mod sigma(n)).
%e 25 is a term because it is an odd number and sigma(25) divides 2^25 - 1.
%o (PARI) is(n)=Mod(2, sigma(n))^n==1&&n%2==1;
%Y Cf. A000203, A000225, A278836.
%K nonn,more
%O 1,2
%A _Altug Alkan_, Dec 04 2016
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