# Jensen hierarchy

In set theory, a mathematical discipline, the Jensen hierarchy or J-hierarchy is a modification of Gödel's constructible hierarchy, L, that circumvents certain technical difficulties that exist in the constructible hierarchy. The J-Hierarchy figures prominently in fine structure theory, a field pioneered by Ronald Jensen, for whom the Jensen hierarchy is named. Rudimentary functions describe a method for iterating through the Jensen hierarchy.

## Definition

As in the definition of L, let Def(X) be the collection of sets definable with parameters over X:
    ${\textrm {Def}}(X):=\{\{y\in X\mid \Phi (y,z_{1},...,z_{n}){\text{ is true in }}(X,\in )\}\mid \Phi {\text{ is a first order formula}},z_{1},...,z_{n}\in X\}$
The constructible hierarchy, $L$ is defined by transfinite recursion. In particular, at successor ordinals, $L_{\alpha +1}={\textrm {Def}}(L_{\alpha })$.
The difficulty with this construction is that each of the levels is not closed under the formation of unordered pairs; for a given $x,y\in L_{\alpha +1}\setminus L_{\alpha }$, the set $\{x,y\}$ will not be an element of $L_{\alpha +1}$, since it is not a subset of $L_{\alpha }$.
However, $L_{\alpha }$ does have the desirable property of being closed under Σ$_0$ separation.
Jensen's modification of the L hierarchy retains this property and the slightly weaker condition that $J_{\alpha +1}\cap {\mathcal {P}}(J_{\alpha })={\textrm {Def}}(J_{\alpha })$, but is also closed under pairing. The key technique is to encode hereditarily definable sets over $J_{\alpha }$ by codes; then $J_{\alpha +1}$ will contain all sets whose codes are in $J_{\alpha }$.
Like $L_{\alpha }$, $J_{\alpha }$ is defined recursively. For each ordinal $\alpha$, we define $W_{n}^{\alpha }$ to be a universal $\Sigma _{n}$ predicate for $J_{\alpha }$. We encode hereditarily definable sets as $X_{\alpha }(n+1,e)=\{X_{\alpha }(n,f)\mid W_{n+1}^{\alpha }(e,f)\}$, with $X_{\alpha }(0,e)=e$. Then set $J_{\alpha ,n}:=\{X_{\alpha }(n,e)\mid e\in J_{\alpha }\}$ and finally, $J_{\alpha +1}:=\bigcup _{n\in \omega }J_{\alpha ,n}$.

## Properties

Each sublevel J$_{α,n}$ is transitive and contains all ordinals less than or equal to ωα + n. The sequence of sublevels is strictly ⊆-increasing in n, since a Σ$_m$ predicate is also Σ$_n$ for any n > m. The levels J$_α$ will thus be transitive and strictly ⊆-increasing as well, and are also closed under pairing, $\Delta _{0}$-comprehension and transitive closure. Moreover, they have the property that
    $J_{\alpha +1}\cap {\mathcal {P}}(J_{\alpha })={\text{Def}}(J_{\alpha }),$
as desired. (Or a bit more generally, $L_{\omega +\alpha }=J_{1+\alpha }\cap V_{\omega +\alpha }$.)
The levels and sublevels are themselves Σ$_1$ uniformly definable (i.e. the definition of J$_{α,n}$ in J$_β$ does not depend on β), and have a uniform Σ$_1$ well-ordering. Also, the levels of the Jensen hierarchy satisfy a condensation lemma much like the levels of Gödel's original hierarchy.
For any $J_{\alpha }$, considering any $\Sigma _{n}$ relation on $J_{\alpha }$, there is a Skolem function for that relation that is itself definable by a $\Sigma _{n}$ formula.

## Rudimentary functions

A rudimentary function is a V$^n$→V function (i.e. a finitary function accepting sets as arguments) that can be obtained from the following operations:
* F(x$_1$, x$_2$, ...) = x$_i$ is rudimentary (see projection function)
* F(x$_1$, x$_2$, ...) = {x$_i$, x$_j$} is rudimentary
* F(x$_1$, x$_2$, ...) = x$_i$ − x$_j$ is rudimentary
* Any composition of rudimentary functions is rudimentary
* ∪$_{z∈y}$G(z, x$_1$, x$_2$, ...) is rudimentary, where G is a rudimentary function
For any set M let rud(M) be the smallest set containing M∪{M} closed under the rudimentary functions. Then the Jensen hierarchy satisfies J$_{α+1}$ = rud(J$_α$).

## Projecta

Jensen defines $\rho _{\alpha }^{n}$, the $\Sigma _{n}$ projectum of $\alpha$, as the largest $\beta \leq \alpha$ such that $(J_{\beta },A)$ is amenable for all $A\in \Sigma _{n}(J_{\alpha })\cap {\mathcal {P}}(J_{\beta })$, and the $\Delta _{n}$ projectum of $\alpha$ is defined similarly. One of the main results of fine structure theory is that $\rho _{\alpha }^{n}$ is also the largest $\gamma$ such that not every $\Sigma _{n}(J_{\alpha })$ subset of $\omega \gamma$ is (in the terminology of α-recursion theory) $\alpha$-finite.
Lerman defines the $S_{n}$ projectum of $\alpha$ to be the largest $\gamma$ such that not every $S_{n}$ subset of $\beta$ is $\alpha$-finite, where a set is $S_{n}$ if it is the image of a function $f(x)$ expressible as $\lim _{y_{1}}\lim _{y_{2}}\ldots \lim _{y_{n}}g(x,y_{1},y_{2},\ldots ,y_{n})$ where $g$ is $\alpha$-recursive. In a Jensen-style characterization, $S_{3}$ projectum of $\alpha$ is the largest $\beta \leq \alpha$ such that there is an $S_{3}$ epimorphism from $\beta$ onto $\alpha$. There exists an ordinal $\alpha$ whose $\Delta _{3}$ projectum is $\omega$, but whose $S_{n}$ projectum is $\alpha$ for all natural $n$.