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How many equivalent classes for language $L_k$

given a constant k we will define the language:

$L_k$={x#y s.t x$\in${0,1}$^k$, y$\in${0,1}$^*$ $\wedge$ x$!=y$}

How many equivalent class does $L_k$ have? I need to show the most tight bounds I can find.

Can anyone give me a clue?

Thanks