Can't understand what I need to do...

There is a integer n.
Find primes that is < than n and they can be expressed in the form 2^k-1.
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An integer X can be expressed in the form (2^k)-1 if there is an integer value of k that means X=(2^k)-1

For example,

The prime number 3 can be expressed like this because (2^2)-1 = 4-1 = 3
The prime number 5 cannot.
The prime number 7 can be expressed like this because (2^3)-1 = 8-1 = 7
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