# Irreducible of finite Krull dimension implies quasi-compact?

Let $$X$$ be the underlying space of a scheme.

• If $$X$$ is irreducible of finite Krull dimension, is it necessarily quasi-compact?
• Is it necessarily Noetherian?
• What if we assume not only that Krull dimension is finite but also that it is 1?
• a separated 1d example: Enumerate the primes: $p_1=2,p_2,...$. Let $R_n$ be the ring of rational numbers whose denominator is not divisible by the first $n$ primes $p_1,...,p_n$. This has spectrum with 1 generic point and $n$ closed points with local rings $\mathbb Z_{(p_i)}$. Each ring is a localization of the next: $R_n=R_{n+1}[p_{n+1}^{-1}]$, so its spectrum is open in the next. Then $X=\bigcup_n \mathrm{Spec}R_n$ is a noncompact scheme with global sections $\mathbb Z$. Clearly not $\mathrm{Spec}\mathbb Z$! – Ben Wieland Apr 18 at 0:11
• A 2d example closer to practical geometry: Let $X_0=\mathbb P^2$ and $p_0$ some point on it. Let $X_{n+1}$ be the blow-up of $X_n$ at $p_n$ and let $p_{n+1}\in D\subset X_{n+1}$ be some point of the exceptional divisor. Let $U_n=X_n-\{p_n\}$. Each is open in the next. So $U=\bigcup U_n$ is a noncompact scheme... The inverse limit of the $X_n$ is probably a compact non-noetherian scheme with open set $U$... The sequence of $p_n$ specifies a 2d valuation ring, necessarily non-noetherian. It has compact spectrum, but the complement of the unique closed point is related to $U$, maybe even 1d. – Ben Wieland Apr 18 at 0:16
• @BenWieland I think that should be an answer, it's a better example than mine and it's a pity to leave it in the comments. – Denis Nardin Apr 18 at 8:59

The answer is no for all these questions. Take the line with infinite origins: the scheme obtained by gluing an infinite amount of copies of $$\mathbb{A}^1$$ along the open subsets $$\mathbb{G}_m$$. This has Krull dimension 1 (there are only closed points and the unique generic point) and it is irreducible (the only proper nonempty closed subsets are the closed points) but it is not quasi-compact (it contains an infinite discrete set), and so in particular not Noetherian.