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