Say $k$ is an algebraically closed field and define the equivalence relation on $k^{n+1}$ given by $x \sim y \iff x=\lambda y $ for some $\lambda \in \mathbb{k}^{\times}$. Clearly $\mathbb{P}^{n} = k^{n+1}/\sim$. Let the map $q:k^{n+1} \setminus 0 \longrightarrow \mathbb{P}^{n}$ be the quotient map that sends $x$ to its equivalence class. I have to show that it is an open map (for the Zariski topology). Any hint is appreciated.

  • $\begingroup$ What is your definition for the topology on $\mathbb{P^n}$? $\endgroup$ – lulu Jan 25 '19 at 16:07
  • 1
    $\begingroup$ Quotient topology $\endgroup$ – Dalamar Jan 25 '19 at 16:12


  1. $q^{-1}(q(U))=\bigcup_{\lambda\in k^{\times}}\lambda U$

  2. Multiplication by $\lambda\neq 0$ is a homeomorphism as for basic opens $\lambda D(f)=D(f(\frac{1}{\lambda}(\cdot))) $

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