f−1{0}={i,−i}\mathrm{f}^{-1}\{0\}=\{\mathrm{i},-\mathrm{i}\}f−1{0}={i,−i}
f−1(0)={∅}f^{-1}(0)=\{\emptyset\}f−1(0)={∅}
f−1(0)=∅\mathrm{f}^{-1}(0)=\emptysetf−1(0)=∅
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