I've been working with the Fermat numbers recently but this problem has really tripped me up. If the Fermat theorem is set as $f_a=2^{2^a}+1$, then how can we say that for an integer $b$ less than $a$ that $\gcd(f_b,f_a)=1$?
Fermat numbers and GCD
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elementary-number-theory
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2By "Fermat theorem", do you mean "Fermat number"? – 2011-09-30