Solution
= Solution
A value $y$ is regular when $df_x:T_xX\to T_yY$ is surjective for every $x\in f^{-1}(y)$. The <preimage theorem> says $f^{-1}(y)$ is then a submanifold of dimension $\dim X-\dim Y$.
= Solution
A value $y$ is regular when $df_x:T_xX\to T_yY$ is surjective for every $x\in f^{-1}(y)$. The <preimage theorem> says $f^{-1}(y)$ is then a submanifold of dimension $\dim X-\dim Y$.