Let $X$ be an algebraic variety over an algebraically closed field. Consider the two subsets $X\_0\subseteq X\_1 \subseteq X$:
$$X\_0 = \{a\in X| a \mbox{ is a scheme-theoretic complete intersection in }X\},$$
$$X\_1 = \{a\in X| a \mbox{ is a set-theoretic complete intersection in }X\}.$$


**Question: what do we know about these sets?**


I am looking for any positive information, possibly for some restricted classes of varieties. Here are some precise questions:


(1) Is $X\_k$ open, closed, locally closed, constructible, etc.?


(2) What are the varieties with $X\_k=\emptyset$?


(3) What are the varieties with $X\_k=X$?



The Locus of Complete Intersection Points
This should be difficult in general. However there are some easy remarks to get going:


First, if $p$ is a CI point, then $X\_p$ is regular. That is because when you localize, the number of generators can only drops, and it is still have to be at least $n=\dim X$. So they are equal. 


Now let's try to answer number 3), when $X\_0=X$? The above remark says that $X$ is non-singular. But it is more, it says that the Chow group of points is trivial. 


Consider $X$ projective. Clearly the property $X\_0=X$ depends on the embedding. For example, with $P^1= \text{Proj} \ k[x,y,z]/(xy-z^2)$, the point $(x,z)$ is not CI. So we just consider $X= \text{Proj}\ S/I$ with $S=k[x\_0,...,x\_d]$. Then a point $p$ in $\text{Proj} S$ is defined by $d$ linear forms. Modulo $I$, the number of generators drops to $n$, so $I$ must contains $d-n$ forms, and by dimension reasons, $I$ is generated by those. So $X$ must be $P^n$. (indeed, here we only needs to assume that $X\_0$ is non-empty, so this also answers Question (2)). 


It is harder when $X$ is affine. Except in dimension one, then we are looking for a smooth affine curve with trivial Picard group, so it must be rational curve. 


I don't know much about the sCI case, or when $X\_1=X$. If $X$ is smooth, we are forcing the Chow group of points to have rank one, and this should be restrictive. 


As for number (1), there is a [paper by Weibel](https://www.tandfonline.com/doi/abs/10.1080/00927878408823142?journalCode=lagb20), where he conjectured that for affine $X$, the set $X\_0$ is a countable union of closed subsets, and solved it for dimension at most $3$. 


