Grothendieck universe
Collection
| definiendum | ${\mathfrak G}$ in it |
| postulate | ${\mathfrak G}$ … set |
| forall | $X\in{\mathfrak G}$ |
| postulate | $\mathcal{P}(X) \subseteq{\mathfrak G}$ |
| forall | $x\in X$ |
| postulate | $x\in{\mathfrak G}$ |
| exists | $P\in{\mathfrak G}$ |
| postulate | $\mathcal{P}(X) \subseteq P$ |
| forall | $Y$ … set |
| postulate | $Y \subseteq {\mathfrak G} \implies Y\ {\approx}\ {\mathfrak G}\lor Y \in {\mathfrak G} $ |
Discussion
Formalities
The symbol ${\approx}$ in the last postulate is an abbreviation. For subsets $Y$ of ${\mathfrak G}$, equinumerosity can be defined as the existence of a set of pairs, $f=\{\{y,u\},\{y',u'\},\dots\}$, which puts elements $y\in Y$ uniquely in correspondence with $u\in{\mathfrak G}$:
I reverse-engineered this from the metamath page, so it needs to be checked
$Y\ {\approx}\ {\mathfrak G}\equiv\exists f. \forall x.\left((x \in Y) \implies \exists!u.\ \{x, u\} \in f\right) \land \left((x \in {\mathfrak G}\setminus Y) \implies \exists(y \in Y).\ \{y,x\} \in f\right)$
The first clause says $f$ corresponds to a function $F$ and moreover implies $F(b) \in Y \implies F(F(b))=b$. The second says $F$ is surjective into ${\mathfrak G}\setminus Y$.
strangely, to me, the metamath defintion doesn't include transitivity in an obvious way, and I can't see it… I added it here anyway, since it's part of any other definition I've seen.
Reference
Wikipedia: Grothendieck universe, Universe (Mathematics)
They have the corresponding axiom in pure first order logic language and 6 shorter version on metamath.
Metamath: grothprim (long), axgroth5 (the one above)
nLab: Grothendieck universe
Requirements
