Reference request: monodromy and isomorphic projections I am looking for a reference to the following fact: monodromy group acting on the cohomology of smooth hyperplane sections of a smooth projective variety $X$ over $\mathbb C$ is preserved under isomorphic projection.
To be precise, suppose that $X\subset \mathbb P^N$ is a smooth projective variety of dimension $n+1$ and $Y$ is its smooth hyperplane section. As $Y$ varies in the class of smooth hyperplane sections of $X$, a monodromy action on $H^n(Y,\mathbb Z)$ arises; by monodromy group of $X$ I will mean the corresponding subgroup of $\mathrm{Aut}(H^n(Y,\mathbb Z))$.
Suppose now that there exists a pont $p\in\mathbb P^N\setminus Y$ such that the projection $\pi_p\colon X\to X'\subset\mathbb P^{N-1}$ is an isomorphism. Do you know a reference to the effect that this monodromy group of $X'$ is the same as that of $X$?
Thank you in advance,
Serge
 A: The comments above are too long, so I am replacing them by an answer.  The reference for most such results in algebraic geometry is SGA 7_2.  I am sure that the following is in there somewhere.
Your assertion follows from a more general statement.  Let $X\subset \mathbb{P}^N$ be a smooth projective variety.  Let $\widehat{\mathbb{P}}^N$ denote the dual projective space parameterizing hyperplanes in $\mathbb{P}^N$.  Let $\widehat{X}\subset \mathbb{P}^N$ be the dual variety of $X$, i.e., the image of the natural map to $\widehat{\mathbb{P}}^N$ from the projectivized normal bundle of $X$ in $\mathbb{P}^N$, $\mathbb{P}_X \mathcal{N}_{X/\mathbb{P}^N}$.  Since the projectivized normal bundle has dimension $N-1$, the image $\widehat{X}$ is a proper subvariety; typically it is a hypersurface.  Denote by $B\subset \widehat{X}$ the closed locus over which the morphism $\mathbb{P}_X\mathcal{N}_{X/\mathbb{P}^N} \to \widehat{X}$ is not an isomorphism.  In characteristic $0$, always $B$ has codimension $\geq 2$ in $\widehat{\mathbb{P}}^N$.  In characteristic $p$, there are sufficient conditions to assure this, but usually people just re-embed $X$ in a bigger projective space by the $d$-uple Veronese map for $d\geq 2$; then $B$ will have codimension $\geq 2$.
A Lefschetz pencil is a line $\ell \in \widehat{\mathbb{P}}^N$ that is disjoint from the codimension $2$ subset $B$ and that intersects $\widehat{X}$ (tangentially)  transversally at every intersection point.  When $B$ has codimension $2$, Lefschetz pencils exist (by the same sort of parameter count that establishes Bertini's smoothness theorem).  All of the above is in SGA 7_2.
The assertion is that for every Lefschetz pencil $\ell$, the induced homomorphism $$f_{X,\ell}:\pi_1(\ell\setminus \ell\cap \widehat{X}) \to \pi_1(\widehat{\mathbb{P}}^N\setminus \widehat{X})$$ is surjective.  This proves your assertion as follows.  For a point $p$ such that the linear projection from $p$ restricts to an isomorphism from $X$ to its image $Y$ in $\mathbb{P}^{N-1}$, the corresponding hyperplane $H_p\subset \widehat{\mathbb{P}}^N$ equals $\widehat{\mathbb{P}}^{N-1}$ and $H_p\cap \widehat{X}$ equals $\widehat{Y}$. In characteristic $0$ at least, existence of a Lefschetz pencil implies that there exists $\ell\in H_p$ that is a Lefschetz pencil.  Thus, we have a composition of group homomorphisms, $$\pi_1(\ell \setminus \ell \cap \widehat{X}) \xrightarrow{f_{Y,\ell}} \pi_1(H_p\setminus H_p\cap \widehat{X}) \xrightarrow{f_{X,Y}} \pi_1(\widehat{\mathbb{P}}^N\setminus \widehat{X}).$$  Since the composition $f_{X,Y}\circ f_{Y,\ell}$ is surjective, also $f_{X,Y}$ is surjective.  Thus, for every group homomorphism $\phi:\pi_1(\widehat{\mathbb{P}}^N\setminus \widehat{X}) \to G$, the image of $\phi$ equals the image of $\phi\circ f_{X,Y}$.  
Thus it suffices to prove that for every Lefschetz pencil $\ell$, $f_{X,\ell}$ is surjective.  Fix a point $p$ of $\ell$.  Denote by $U\subset \widehat{\mathbb{P}}^N\setminus (\widehat{X}\cup \{p\})$ the set of points $q$ such that line $\ell_q$ spanned by $p$ and $q$ is a Lefschetz pencil.  The transversality conditions in the definition of a Lefschetz pencil are open conditions.  Thus, $U$ is an open subset.  Since for every $q\in \ell\setminus\{p\}$, $q$ is in $U$, $U$ is a nonempty open subset.  Thus, $U$ is a dense open subset of $\widehat{\mathbb{P}}^N\setminus (\widehat{X}\cup \{p\})$.  In particular, $U$ is connected.  Thus, the images of the homomorphisms $f_{X,\ell_q}$ for $q\in U$ are all equal to a common subgroup $\Gamma$ of $\pi_1(\widehat{\mathbb{P}}^N\setminus \widehat{X},p)$.  
Let $\rho:Y\to \widehat{\mathbb{P}}^N\setminus \widehat{X}$ be the covering space associated to the standard left action of $\pi_1(\widehat{\mathbb{P}}^N\setminus \widehat{X},p)$ on the set of right $\Gamma$-cosets.  The restriction of this left action to the subgroup $\Gamma$ has a fixed point: the neutral coset $\Gamma$.  Thus, the restriction of $\rho$ over every line $\ell_q\setminus \ell_q\cap \widehat{X}$ has a cross section.  The union of all of these cross sections defines a cross section of the restriction of $\rho$ over the open dense subset $U$.  For every point $r\in \widehat{\mathbb{P}}^N\setminus \widehat{X}$, there exists a neighborhood $B_\epsilon(r)$ of $r$ in this open over which $\rho$ is trivialized.  Since the complement of $U$ is an analytic subvariety of complex codimension $1$, real codimension $2$, $B_\epsilon(r)\cap U$ is nonempty and connected.  Thus, the cross section of $\rho$ over $U$ extends uniquely to a cross section over all of $B_\epsilon(r)$.  Since this holds for every $r$, the cross section extends over all of $\widehat{\mathbb{P}}^N\setminus \widehat{X}$.  It follows that the neutral coset $\Gamma$ is a fixed point for all of $\pi_1(\widehat{\mathbb{P}}^N\setminus \widehat{X},p)$.  Thus, $\Gamma$ is the entire group.
