There exists the following result in the literature: There exists a polarized $K3$ surface $(X, H)$ of genus $3$ and a smooth irreducible curve $C$ on $X$ satisfying $C^2 =4$, $C.H=6$ such that $\text{Pic}(X) \cong \mathbb Z[H] \oplus Z[C]$. The theorem follows from [https://arxiv.org/pdf/math/9805140.pdf] theorem $1.1(iv)$.

Now Let's consider $X$ to be a smooth quintic hypersurface in $\mathbb P^3$ with Picard number $2$. Then I think it can be shown that $\text{Pic}(X) \cong \mathbb Z[H] \oplus Z[H']$, where $H$ is the hyperplane class and $H'$ is some divisor. Now in order to locate the ample line bundles in $\text{Pic}(X)$ using the Nakai-Moishezon criterion, we must know the intersection numbers $H.H'$ and $H'^2$.

In this context my question is the following: Does there exist in the literature an analogous existence result as the first-mentioned theorem for smooth quintic hypersurface with Picard number $2$?

To be more precise: Does there exist a polarized smooth quintic hypersurface $(X, H)$ in $\mathbb P^3$ and a smooth irreducible curve $C$ for which the intersection numbers $C^2$ and $C.H$ are known and $\text{Pic}(X) \cong \mathbb Z[H] \oplus Z[C]$?

Can someone give me any reference which could be even remotely useful in the context of finding out such $(X,H)$ and $C$

Any help from anyone is welcome.


A natural source of surfaces in $\mathbb{P}^{3}$ with Picard number $> 1$ is given by linear determinantal surfaces, i.e. zero sets of square matrices of linear forms. In what follows, let $X \subseteq \mathbb{P}^{3}$ be a smooth linear determinantal surface of degree $d \geq 2$ (smoothness can arranged by taking the $d \times d$ matrix to be sufficiently general) and let $H \in {\rm Pic}(X)$ be the class of a hyperplane section.

It can be checked that $X$ contains a curve $C$ which is the degeneracy locus of a $d \times (d-1)$ matrix of linear forms. For degree/genus reasons, this $C$ cannot be a complete intersection of $X$ and another surface in $\mathbb{P}^{3}$.

There is a Noether-Lefschetz-type result which says that if $X$ is a generic linear determinantal surface of degree $d,$ the Picard group ${\rm Pic}(X)$ is isomorphic to $\mathbb{Z}H \oplus \mathbb{Z}C.$ Here is a reference for this result, as well as others which go further than determinantal surfaces:

Lopez, Angelo Felice; Noether-Lefschetz theory and the Picard group of projective surfaces, Mem. Amer. Math. Soc. 89 (1991), no. 438, x+100 pp.

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    $\begingroup$ thanks for the answer. Is the curve $C$ mentioned in the answer irreducible? Are the intersection numbers $C.C$ and $C.H$ immediate from the discussion? (apology in advance, if this is trivial to compute). Finally, Is theorem $(II)3.1$ mentioned there relevant in this context? $\endgroup$ – UPOKROMONIKA May 26 at 11:49
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    $\begingroup$ also as the general surface of degree $d \geq4$ is not linear determinantal, but linear pfaffian if $d \leq 15$, so in this case are we talking about smooth quintic surfaces which is not general? $\endgroup$ – UPOKROMONIKA May 26 at 14:26
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    $\begingroup$ The curve $C$ is cut out by the maximal minors of a $d \times (d-1)$-submatrix of the matrix of linear forms cutting out $X,$ so by general results on determinantal varieties we can ensure $C$ is smooth and connected, and therefore irreducible. The degree $C \cdot H$ and genus can of course be read off the Hilbert polynomial, which in turn can be read off the Eagon-Northcott resolution of the associated determinantal ideal. You can then determine $C \cdot C$ from the adjunction formula on $X.$ I can't access the reference at the moment, so I can't speak to the relevance of Theorem (II)3. $\endgroup$ – Yusuf Mustopa May 26 at 17:12
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    $\begingroup$ As to your second comment, you are correct that the smooth surfaces in under discussion are not general when $d \geq 4.$ $\endgroup$ – Yusuf Mustopa May 26 at 17:14

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