A Long Pseudo-Comparison of Premice in $ L [X] $

Home , Core model

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Before proceeding, we give a brief summary of some relevant definitions and facts. We assume familiarity with the fundamentals of inner model theory; see [6], [3]. One does not really need to know the analysis of HODL[x,G], but familiarity does help in terms of motivation; the system F described below relates to that analysis. We do rely on some smaller facts from [7, §3]. Let us give some terminology, and recall some facts from [7]. We say that a premouse N is pre-M1-like iff N is proper class, 1-small, and has a (unique) N Woodin cardinal, denoted δ . (The notion M1-like of [7] is stronger; it has some iterability built in.) Let P,Q be pre-M1-like. Given a normal iteration tree T on P , T is maximal iff lh(T ) is a limit and L[M(T )] has no Q-structure for M(T ) (so L[M(T )] is pre-M1-like with Woodin δ(T )). A premouse R is a (non-dropping) pseudo-normal iterate of P iff there is a T normal tree T on P such that either T has successor length and R = M∞, the last model of T (and [0, ∞]T does not drop), or T is maximal and R = L[M(T )]. A pseudo-comparison of (P,Q) is a pair (T , U) of normal iteration trees formed according to the usual rules of comparison, such that either (T , U) is a successful comparison, or either T or U is maximal. A (z-)pseudo-genericity iteration is defined similarly, formed according to the rules for genericity iterations making a real (z) generic for Woodin’s extender algebra. We say that P is normally short-tree-iterable iff for every normal, non-maximal iteration tree T on P of limit length, there is a T - cofinal wellfounded branch through T , and every putative normal tree T on P of length α + 2 has wellfounded last model (that is, we never en- counter an illfounded model at a successor stage). If P |δP ∈ HCL[x], then normal short-tree-iterability is absolute between L[x] and V . If P,Q are normally short-tree-iterable then there is a pseudo-comparison (T , U) of (P,Q), and if T has a last model then [0, ∞]T does not drop, and likewise for U.

It has been suggested1 that one might analyze HODL[x] using an ODL[x] directed system F such that: – thenodesof F are pairs (N,s) such that s ∈ OR<ω and N is a normally

1For example, at the AIM Workshop on Descriptive inner model theory, June, 2014.

2 N L[x] short-tree iterable, pre-M1-like premouse with N|δ ∈ HC and such that there is an L[N]-generic ﬁlter G for Coll(ω, δN ) in L[x],2

– for (P,t), (Q, u) ∈ F, we have (P,t) ≤F (Q, u) iﬀ t ⊆ u and Q is a pseudo-iterate of P , and

– (M1, ∅) ∈ F. There are also further conditions, regarding the sets s, strengthening the iterability requirements; these and other details regarding how the direct limit is formed from F are not relevant here. The main difficulty in analyzing HODL[x] in this manner is in arranging that F be directed. For this, it seems most obvious to try to arrange that F be closed under pseudo-comparison of pairs. However, we show here that, given sufficient large cardinals, there is a cone of reals x such that if F is as above, then F is not closed under pseudo-comparison. The proof proceeds by finding a node (N, ∅) ∈ F such that, letting (T , U) be the pseudo-comparison of (M1,N), then T , U are in fact pseudo-genericity iterations of M1,N respectively, making reals y, z L[y] L[z] L[x] generic, where ω1 = ω1 = ω1 . Letting W be the output of the W W [z] L[x] pseudo-comparison, we have W |δ ∈ L[x], so ω1 = ω1 , which implies W L[x] that δ = ω1 , so (W, ∅) ∈/ F. We now proceed to the details.

2 The comparison

For a formula ϕ in the language of set theory (LST), ζ ∈ OR, and x ∈ R, x L[x] let Aϕ,ζ be the set of all M ∈ HC such that L[x] |= ϕ(ζ, M), and L[M] L[M] M is a normally short-tree-iterable pre-M1-like premouse with δ = OR and M = L[M]|δM . Note that ϕ does not use x as a parameter. So by absoluteness of normal L[x] x short-tree-iterability (between L[x] and V , for elements of HC ), Aϕ,ζ is L[x] x OD . So Aϕ,ζ is a collection of premice like those involved in the system F (restricted to their Woodins). Theorem. # Assume Turing determinacy and that M1 exists and is fully iterable. Then for a cone of reals x, for every formula ϕ in the LST and

2The point of G is that we can then use Neeman’s genericity iterations, working inside L[x]. We cannot use Woodin’s, as closure under Woodin’s would produce premice with ωL[x] Woodin cardinal 1 .

3 M1 x x every ζ ∈ OR, if M1|δ ∈ Aϕ,ζ then there is R ∈ Aϕ,ζ such that the L[x] pseudo-comparison of M1 with L[R] has length ω1 . Proof. Suppose not. Then we may ﬁx ϕ such that for a cone of x, the # theorem fails for ϕ, x. Fix z in this cone with z ≥T M1 . Let W be the z-genericity iteration on M1 (making z generic for the extender algebra), W and Q = M∞ . By standard arguments (see [7]), Q[z]= L[z],

L[z] Q lh(W)= ω1 +1= δ + 1,

Q Q Q Q|δ = M(W ↾δ ), and T =def W ↾δ is the z-pseudo-genericity iteration, and T ∈ L[z]. Let B be the extender algebra of Q and let P be the ﬁnite support ω-fold th product of B. For p ∈ P let pi be the i component of p. Let G ⊆ P be Q-generic, with z0 = z where x =def hziii<ω is the generic sequence of reals. Then Q[G]= Q[x]= L[x] and x>T z. Let ζ ∈ OR witness the failure of the theorem with respect to M1 x ϕ, x. So M1|δ ∈ Aϕ,ζ . By [1, Lemma 3.4] (essentially due to Hjorth), P is δQ-cc in Q, so δQ ≥ L[x] Q L[z] Q L[x] ω1 , but δ = ω1 , so δ = ω1 . So it suﬃces to see that there is some x Q R ∈ Aϕ,ζ such that the pseudo-comparison of M1 with L[R] has length δ . y For e ∈ ω and y ∈ R let Φe : ω → ω be the partial function coded by the th z e Turing program using the oracle y. Let e ∈ ω be such that Φe is total M1 and codes M1|δ . Letx ˙ be the P-name for the P-generic sequence of reals, th and for n<ω letz ˙n be the P-name for the n real. Let p ∈ G be such Q v that p P ψ(z ˙0), where ψ(v) asserts “Φe is total and codes a premouse R such that R ∈ Ax˙ , and the v-pseudo-genericity iteration of L[R] produces ϕ,ˇ ζˇ a maximal tree U of length δˇQ with M(U) = L[Eˇ]|δˇQ”. In the notation of this formula,

Q ˇ Q U p P “R∈ / V ”, because p P “E0 ∈/ M(U)”. By genericity, we may ﬁx q ∈ G such that q ≤ p and for some m > 0, Q qm = q0. Note that q P ψ(z ˙m). ˙ P z˙i Let Ri be the -name for the premouse coded by Φe (or for ∅ if this ′ ′ B B does not code a premouse). Also letz ˙0, z˙1 be the × -names for the two B×B-generic reals (in order), and let R˙ ′ be the B×B-name for the premouse ′ i z˙i coded by Φe .

4 We may ﬁx r ≤ q, r ∈ G, such that

Q r P “R˙ 0 6= R˙ m”. (1)

Q ˙ ˙ For otherwise there is r ≤ q, r ∈ G, such that r P “R0 = Rm”. But since

M1 ˙ G M1|δ = R0 ∈/ Q, there are s,t ∈ B, s,t ≤ r0, such that

Q ˙ ′ ˙ ′ (s,t) B×B “R0 6= R1”.

Therefore there are u, v ∈ B, with u ≤ r0 and v ≤ rm, such that

Q ˙ ′ ˙ ′ (u, v) B×B “R0 6= R1”.

Let w ≤ r be the condition with wi = ri for i 6= 0,m, and w0 = u and wm = v. Then Q w P “R˙ 0 6= R˙ m”, a contradiction. ˙ G M1 x Q So letting R = Rm, we have R 6= M1|δ and R ∈ Aϕ,ζ and Q|δ = G M(U), where U is the zm-pseudo-genericity iteration of L[R], and lh(U) = δQ. We deﬁned T earlier. Let T ∗, U ∗ be the padded trees equivalent to T , U, T ∗ U ∗ T ∗ U ∗ such that for each α, either Eα 6= ∅ or Eα 6= ∅, and if Eα 6= ∅= 6 Eα then T ∗ U ∗ ′ ′ lh(Eα ) = lh(Eα ). Let (T , U ) be the pseudo-comparison of (M1,L[R]). We claim that (T ′, U ′) = (T ∗, U ∗); this completes the proof. For this, we prove by induction on α that

(T ′, U ′)↾(α + 1) = (T ∗, U ∗)↾(α + 1).

This is immediate if α is a limit, so suppose it holds for α = β; we prove T ∗ U ∗ it for α = β + 1. Let λ = lh(Eβ ) or λ = lh(Eβ ), whichever is deﬁned. ∗ Q ∗ T ∗ Because M(T ) = Q|δ = M(U ), the least disagreement between Mβ U ∗ T ∗ U ∗ and Mβ has index ≥ λ, so we just need to see that Eβ 6= Eβ . T ∗ U ∗ So suppose that Eβ = Eβ . In particular, both are non-empty. Then Q there is s ∈ G such that s ≤ r (see line (1)) and s P σ where σ asserts “For i = 0,m, let Ti be thez ˙i-pseudo-genericity iteration of L[R˙ i]. Then T0 and Tm use identical non-empty extenders E of index λˇ.” Because

Q s P ψ(z ˙0)& ψ(z ˙m),

5 Q ′ ′ ˇ ˇ also s P σ , where σ asserts “Letting E be as above, E ⊆ L[E]|λ, but ˇ T ∗ G E∈ / V ”; here Eβ ∈/ Q because λ is a cardinal of Q. But since Ti is G computed in Q[zi ] (for i = 0,m) we can argue as before (as in the proof of the existence of r as in line (1)) to reach a contradiction.

A slightly simpler argument, using B × B instead of P, proves the weak- ening of the theorem given by dropping the parameter ζ.

References

2 [1] Ilijas Farah. The extender algebra and Σ1-absoluteness. To appear in Cabal Seminar: Volume 4. Available on author’s website.

[2] Peter Koellner and W. Hugh Woodin. Large cardinals from determinacy. In Handbook of set theory. Vols. 1, 2, 3, pages 1951–2119. Springer, Dordrecht, 2010.

[3] William J. Mitchell and John R. Steel. Fine structure and iteration trees, volume 3 of Lecture Notes in Logic. Springer-Verlag, Berlin, 1994.

[4] G. Sargsyan. Hod Mice and the Mouse Set Conjecture. Memoirs of the American Mathematical Society Series. American Mathematical Society, 2015. R [5] John R. Steel. HODL( ) is a core model below Θ. Bull. Symbolic Logic, 1(1):75–84, 1995.

[6] John R. Steel. An outline of inner model theory. In Handbook of set theory. Vols. 1, 2, 3, pages 1595–1684. Springer, Dordrecht, 2010.

[7] John R. Steel and W. Hugh Woodin. HOD as a core model. To appear in Cabal Seminar: Volume 3. I imagine that a preprint would be available upon request from John R. Steel.