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%I #7 Mar 25 2016 07:49:22
%S 9,15,21,25,27,28,33,35,39,45,49,51,55,57,63,65,69,75,77,81,85,87,91,
%T 93,95,99,105,111,112,115,117,119,121,123,124,125,129,133,135,141,143,
%U 145,147,148,153,155,159,161,165,169,171,175,177,183,185,187,189
%N Composites c where at least one composite d with d < c exists such that c^(d-1) == 1 (mod d) and d^(c-1) == 1 (mod c).
%C A002808(i) is in the sequence iff A270573(i) > 0.
%H Felix Fröhlich, <a href="/A270574/b270574.txt">Table of n, a(n) for n = 1..10000</a>
%o (PARI) forcomposite(c=1, 500, forcomposite(d=1, c-1, if(Mod(c, d)^(d-1)==1 && Mod(d, c)^(c-1)==1, print1(c, ", "); break({1}))))
%Y Cf. A270573, A270575.
%K nonn
%O 1,1
%A _Felix Fröhlich_, Mar 19 2016
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