Let \( M \) be the set of all \( 2 \times 2 \) matrices with entries from the set \( R \) of real numbers. Then the function \( f : M \to R \) defined by \( f(A) = |A| \) for every \( A \in M \) is
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Determinant maps matrices to real numbers, but different matrices can have the same determinant, so injectivity usually fails.