In Vakil’s FOAG, exercise 15.4.M, there is a remark:
15.4.M. EXERCISE: A TORSION PICARD GROUP. Suppose that $Y$ is a hypersurface in $\Bbb{P}^n_k$ corresponding to an irreducible degree d polynomial. Show that $Pic(\Bbb{P}^n_k-Y) = Z/(d)$. (For differential geometers: this is related to the fact that $π_1(\Bbb{P}^n_k - Y)= Z/(d)$.)
Is there any reference for the related differential geometry fact? and what is the full statement for it? Is the topology still using the Zariski topology? If not, should the field $k$ be the $\Bbb{C}$?
Thank you in advance.