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%I #7 Mar 31 2012 14:47:12
%S 1,2,3,6,42,78,546,1806,23478
%N Numbers n such that n^4 divides 19^(n^2)-1.
%C a(n)^2 are the perfect squares in A128399.
%C a(10) > 10^13. [From Max Alekseyev]
%t Do[g=PowerMod[19,n^2,n^4]-1;If[IntegerQ[g/n^4],Print[n]],{n,1,10000}]
%Y Cf. A128399 = numbers n such that n^2 divides 19^n-1.
%K hard,more,nonn
%O 1,2
%A _Alexander Adamchuk_, Mar 28 2007
%E a(9) from _D. S. McNeil_, Mar 02 2009
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