Let $H_1, H_2$ be two Hilbert schemes parametrizing subschemes in $\mathbb{P}^{n_1}, \mathbb{P}^{n_2}$ with Hilbert polynomials $P_1, P_2$, respectively. Given a pair $(Z_1, Z_2)$ of subschemes in $\mathbb{P}^{n_1}, \mathbb{P}^{n_2}$, respectively, we say that $Z_1 \subset Z_2$ if there exists a closed immersion $i$ of $\mathbb{P}^{n_1}$ into $\mathbb{P}^{n_2}$ such that $i(Z_1)$ is a closed subscheme of $Z_2$. Denote by $\mathcal{D}$ the subet of $H_1 \times H_2$ consisting of all pairs $(Z_1, Z_2)$ such that $Z_i \in H_i$ and $Z_1 \subset Z_2$. Can we look at it as a scheme?

2$\begingroup$ For this construction to deserve the name "relative Hilbert scheme" I would think you'd want to fix $i$. $\endgroup$– Qiaochu YuanDec 19, 2013 at 2:41

$\begingroup$ @Yuan: Could you elaborate a bit? $\endgroup$– JanaDec 19, 2013 at 2:54

$\begingroup$ I mean, when I hear the word "relative" in this context I think that means that we typically have in mind a particular morphism. It seems strange to talk about a condition that refers only to the existence of some morphism instead. $\endgroup$– Qiaochu YuanDec 19, 2013 at 3:11

$\begingroup$ @Yuan: Please feel free to edit the title with your preferred terminology. $\endgroup$– JanaDec 19, 2013 at 3:21

$\begingroup$ The question is edited. $\endgroup$– JanaDec 19, 2013 at 22:43
1 Answer
I guess you are looking for a construction that gives an object over $H_2$, that parametrizes closed immersions $Z_1 \to Z_2$ for each $Z_2 \in H_2$. If I'm not mistaken, it is the closed subscheme of $Quot(\mathscr{O}_{\mathbb{P}^{n_2}}) \times Quot(\mathscr{O}_{\mathbb{P}^{n_2}})$ (which parametrizes pairs of ideals) defined by containment of one ideal in the other. Let's call this subscheme $H_3$. It is a special case of a generalized Hilbert scheme construction whose name I don't know, for any commutative diagram of closed immersions in a projective scheme.
Let $H_4 = \underline{\operatorname{Imm}}(\mathbb{P}^{n_1}/\mathbb{P}^{n_2})$ denote the scheme of closed immersions $\mathbb{P}^{n_1} \to \mathbb{P}^{n_2}$ (see page 268 of Grothendieck's Bourbaki 221  this is an open subscheme of the Hom scheme). Then we can make a subscheme $X$ of $H_1 \times H_4 \times H_3$ defined by commutativity of the compositions $Z_1 \to \mathbb{P}^{n_1} \to \mathbb{P}^{n_2}$ and $Z_1 \to Z_2 \to \mathbb{P}^{n_2}$.
It seems that the object you want is the image of $X$ under the natural map $H_1 \times H_4 \times H_3 \to H_1 \times H_2$ taking $(i_1, i_4, Z_1 \subset Z_2 \subset \mathbb{P}^{n_2})$ to $(i_1, Z_2 \subset \mathbb{P}^{n_2})$.

$\begingroup$ @Carnahan: Thank you. This is what I was looking for. Could you please elaborate/give reference of the first paragraph of your answer. This is what I am most interested in. Why is $H_3$ a scheme? $\endgroup$– JanaDec 20, 2013 at 3:16

$\begingroup$ @Jana I see you have an answer to your question, but I might as well say what I can. I think this scheme can be constructed by following the Quot construction in Grothendieck's Bourbaki 221 and making suitable changes. However, I have not worked out the details myself. $\endgroup$– S. Carnahan ♦Jan 2, 2014 at 19:37