OFFSET
1,1
COMMENTS
An integer k is in this sequence if and only if there is no solution to the congruence x^2 == 2^(2^k) + 1 (mod p), where p is a prime of the form 5*2^k + 1.
a(7) > 9*10^6.
LINKS
Alexander Aigner, Über Primzahlen, nach denen (fast) alle Fermatzahlen quadratische Nichtreste sind, Monatsh. Math., Vol. 101 (1986), pp. 85-93; alternative link.
PROG
(PARI) isok(k)=my(p=5*2^k+1); k>2 && Mod(k, 2)==1 && Mod(3, p)^((p-1)/2)+1==0 && kronecker(lift(Mod(2, p)^2^k)+1, p)==-1;
CROSSREFS
KEYWORD
nonn,hard,more
AUTHOR
Arkadiusz Wesolowski, Sep 16 2021
STATUS
approved