The first few Fermat numbers are $F_{0}=2^{2^0}+1=3$, $F_{1}=2^{2^1}+1=5$, $F_{2}=2^{2^2}+1=17$.

I read that $F_5$ through $F_{32}$ have been shown to be composite. And right now $F_{33}$ is the smallest Fermat number whose primality is unknown.

My question is: When was $F_{32}$ shown to be composite?


A short Google search yields the page http://www.prothsearch.net/fermat.html, which says a factor of the 32nd Fermat number was found in 1963 by Wrathall and that only one prime factor is known.

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  • $\begingroup$ And the Wrathall paper is here. $\endgroup$ – Kenny LJ Jul 1 '16 at 2:00

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