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Let $f:{\Bbb C}^{n+1}\to {\Bbb C}^n$ be a map defined by homogeneous polynomials. There is a point $p\in f^{-1}(0)$ and a neighbourhood $U$ of $p$ in $f^{-1}(0)$, such that $d(f)$ has rank $n$ at every point in $U-\{p\}$. Then, by the implicit function theorem $f^{-1}(0)$ is a graph in some neighbourhood of every point in $U-\{p\}$.

Can we conclude that, $f^{-1}(0)$ is also a graph in some neighbourhood of $p$ (even though $d(f)$ may have rank $< n$ at $p$)?

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No. A counterexample is given by $n=1$, $f(x,y)=x^2-y^2$, and $p=(0,0)$.

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  • $\begingroup$ Thanks! I am reading a note and a similar fact is used. So it is wrong! $\endgroup$
    – RKS
    Commented Jan 16 at 5:32

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