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Number Theory

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A Fermat Number which is Divisible by 641

NUMTH-FH8V1W

Fermat number is a positive integer of the form:

$$ F_n = 2^{2^n}+1 $$

...for a nonnegative integer $n$.

Which of the following Fermat numbers are divisible by $641$? Select ALL that apply.

A

$F_3$.

B

$F_5$.

C

$F_7$.

D

$F_9$.

E

$F_{11}$.