This number is a composite.
Chinese mathematicians (circa 500 B.C.) erroneously believed that 2(2^(n-1)-1) is not divisible by n if n is not prime. This was disproved by Sarrus in 1819 with n = 341, i.e., the smallest pseudoprime to base two.
(There are 3 curios for this number that have not yet been approved by an editor.)
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