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%I #15 Aug 15 2016 16:54:37
%S 1,2,3,4,5,8,12,15,16,24,28,30,40,48,56,60,72,80,85,88,96,104,105,112,
%T 120,140,145,160,168,232,240,255,265,280,312,408,420,520,624,680,760,
%U 840,1040,1320,1360,1848,1904,4080
%N Numbers n such that all x^4 mod n are nonprimes.
%C A065428 is a subsequence.
%C No more such numbers up to 10^6.
%C Observe that many of these numbers are of the form k^2-1. Why?
%o (PARI) for(n=1,10^6,q=1;for(x=1,n-1,if(isprime(component(Mod(x,n)^4,2)),q=0;break()));if(q,print1(n,", ")));
%o (PARI) {a(n) = my(c, m); if(n<=1, n==1, c=1; m=1; while( c<n, m++; c += prod(x=1, m-1, !isprime( lift( Mod(x, m)^4 ) ) ) ); m)}; /* _Michael Somos_, Jan 07 2011 */
%Y A065428 (x^2 mod n).
%K nonn
%O 1,2
%A _Joerg Arndt_, Jan 07 2011
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