ADI JARDEN. Abstract. We introduce a connection between tameness and non-forking frames. We assume the existence of a semi-good non-forking λ-frame,.
arXiv:1406.4251v1 [math.LO] 17 Jun 2014
A SEMI-GOOD FRAME WITH AMALGAMATION AND TAMENESS IN λ+ ADI JARDEN Abstract. We introduce a connection between tameness and non-forking frames. We assume the existence of a semi-good non-forking λ-frame, (λ, λ+ )-tameness and amalgamation in λ+ and present sufficient additional conditions for the existence of a good non-forking λ+ -frame. Moreover, we improve results of [JrSi3] about independence and dimension.
1. Introduction In [Sh:h].III Shelah presented an axiomization of a non-forking relation in the context of AECs. But this non-forking relation relates to models of a specific cardinality, λ, only. Shelah presented a way to to extend a nonforking relation to models of cardinality > λ and proved that several axioms preserved. The extension, uniqueness and symmetry are the main problem. But uniqueness for models of cardinality > λ is actually tameness. The connection between extension and uniqueness is known. We conjectured that the symmetry holds too, but it is still an open problem (and now we have no idea if this conjecture holds). In [?], Boney presented a variant of tameness that is a sufficient condition for symmetry. Later, Sebastien proved a non-structure theorem assuming the failure of symmetry (in similar context). Here, we present new sufficient conditions for symmetry. On one hand, we show that a variant of the continuity property for independence (studied in [JrSi3]) is a sufficient condition and proved it under reasonable hypothesis. On the other hand, we apply the non-structure theorem of Sebastien. In [Sh:h].III and [?] we derive good λ+ -frames too. Let us compare the main results of those papers with the main result in the current paper. The disadvantages in those papers are: (1) The restriction to saturated models, (2) the restriction of the relation to N F , ++ (3) the hypothesis that I(λ++ , K) < 2λ . The disadvantages in the current paper are the following hypotheses: (1) the amalgamation property in λ+ , (2) tameness. This is actually a draft and it has to be checked and improved. complete... 1
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2. Non-forking frames S Definition 2.0.1. s = (K, , S bs , ) is a good λ-frame if: (0) (a) (K, ) is an AEC. (b) LST (K, ) ≤ λ. (1) (a) (Kλ , ↾ Kλ ) satisfies the joint embedding property. (b) (Kλ , ↾ Kλ ) satisfies the amalgamation property. (c) There is no -maximal model in Kλ . (2) S bs is a function with domain Kλ , which satisfies the following axioms: (a) S bs (M ) ⊆ S na (M ) = {tp(a, M, N ) : M ≺ N ∈ Kλ , a ∈ N − M }. (b) It respects isomorphisms: If tp(a, M, N ) ∈ S bs (M ) and f : N → N ′ is an isomorphism, then tp(f (a), f [M ], N ′ ) ∈ S bs (f [M ]). (c) Density of the basic types: If M, N ∈ Kλ and M ≺ N , then there is a ∈ N − M such that tp(a, M, N ) ∈ S bs (M ). (d) Basic stability: For every M ∈ Kλ , the cardinality of S bs (M ) is ≤ λ. S satisfies the following axioms: (3) the relation S is a set of quadruples (M0 , M1 , a, M3 ) where M0 , M1 , M3 ∈ Kλ , (a) a ∈ M3 − M1 and for nS= 0, 1 tp(a, Mn , M3 ) ∈ S bs (Mn ) and it re′ spects isomorphisms: S If (M0 , M1 , a, M3 ) ′ and f : M3 → M3 is an isomorphism, then (f [M0 ], f [M1 ], f (a), M3 ). S ∗ ∗ ∗ ∗ {a} ⊆ (b) Monotonicity: If M 0 M0 M1 M 1 M3 M3 , M1 S S M3∗∗ M3∗ , then (M0 , M1 , a, M3 ) ⇒ (M0∗ , M1∗ , a, M3∗∗ ). From now on, ‘p ∈ S bs (N ) does not fork over M ’ S will be interpreted as ‘for some a, N + we have p = tp(a, N, N + ) and (M, N, a, N + )’. See Proposition ??. (c) Local character: For every limit ordinal δ < λ+ if hMα : α ≤ δ + 1i is an increasing continuous sequence of models in Kλ , and tp(a, Mδ , Mδ+1 ) ∈ S bs (Mδ ), then there is α < δ such that tp(a, Mδ , Mδ+1 ) does not fork over Mα . (d) Uniqueness of the non-forking extension: If M, N ∈ Kλ , M N , p, q ∈ S bs (N ) do not fork over M , and p ↾ M = q ↾ M , then p = q. (e) Symmetry: If M0 , M1 , M3 ∈ Kλ , M0 M1 M3 , a1 ∈ M1 , tp(a1 , M0 , M3 ) ∈ S bs (M0 ), and tp(a2 , M1 , M3 ) does not fork over M0 , then there are M2 , M3∗ ∈ Kλ such that a2 ∈ M2 , M0 M2 M3∗ , M3 M3∗ , and tp(a1 , M2 , M3∗ ) does not fork over M0 . (f) Existence of non-forking extension: If M, N ∈ Kλ , p ∈ S bs (M ) and M ≺ N , then there is a type q ∈ S bs (N ) such that q does not fork over M and q ↾ M = p. (g) Continuity: Let δ < λ+ and hMα : α ≤ δi an increasing continuous sequence of models in Kλ and let p ∈ S(Mδ ). If for every α ∈ δ, p ↾ Mα does not fork over M0 , then p ∈ S bs (Mδ ) and does not fork over M0 .
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While in [Sh:h].II we study good frames, so basic stability is assumed; here we assume basic almost stability so the following definition is central: s S S Definition 2.0.2. s = (K s , s , S bs,s , ) = (K, , S bs , ) is a semi -good λ-frame, if s satisfies the axioms of a good λ-frame except that instead of assuming basic stability, we assume that s satisfies basic almost stability, namely, for every M ∈ Kλ S bs (M ) is of cardinality at most λ+ . s is said to be a semi-good frame if it is a semi-good λ-frame for some λ. Remark 2.0.3. If for each M ∈ Kλ S bs (M ) = S na (M ), then the continuity axiom is an easy consequence of the local character. Recall (from [Sh E46]): Definition 2.0.4. Let p0 ∈ S(M0 ), p1 ∈ S(M1 ). We say that p0 , p1 are conjugate if for some a0 , M0+ , a1 , M1+ , f , the following hold: (1) For n = 0, 1, tp(an , Mn , Mn+ ) = pn . (2) f : M0+ → M1+ is an isomorphism. (3) f ↾ M0 : M0 → M1 is an isomorphism. (4) f (a0 ) = a1 . Proposition 2.0.5. Assume that p0 , p1 are conjugate and the types p1 , p2 are conjugate. Then the types p0 , p2 are conjugate. Proof. Compose the isomorphisms.
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Definition 2.0.6. Let p = tp(a, M, N ). Let f be a bijection with domain M . Define f(p) := tp(f (a), f [M ], f + [N ]), where f + is an extension of f (and the relations and functions on f + [N ] are defined such that f + : N → f + [N ] is an isomorphism). Remark 2.0.7. The definition of f (p) in Definition 2.0.6 does not depend on the representative (M, N, a) ∈ p. Definition 2.0.8. Let s be a semi-good λ-frame. We say that s satisfies the conjugation property when: Kλ is categorical and if M1 , M2 ∈ Kλ , M1 M2 and p2 ∈ S bs (M2 ) is the non-forking extension of p1 ∈ S bs (M1 ), then the types p1 , p2 are conjugate. By Claim 2.18 in [Sh:h].II: Fact 2.0.9 (The transitivity proposition). Suppose s is a semi-good λ-frame. Then: If M0 M1 M2 , p ∈ S bs (M2 ) does not fork over M1 and p ↾ M1 does not fork over M0 , then p does not fork over M0 . By Claim 2.16 in [Sh:h].II: Fact 2.0.10. Suppose (1) s satisfies the axioms of a semi-good λ-frame. (2) n < 3 ⇒ M0 Mn . (3) For n = 1, 2, an ∈ Mn − M0 and tp(an , M0 , Mn ) ∈ S bs (M0 ). Then there is an amalgamation (f1 , f2 , M3 ) of M1 , M2 over M0 such that for n = 1, 2 tp(fn (an ), f3−n [M3−n ], M3 ) does not fork over M0 .
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2.1. Non-forking with larger models. Now we extend our non-forking notion to include models of cardinality greater than λ. ≥λ S is the class of quadruples (M0 , a, M1 , M2 ) such that: Definition 2.1.1. (1) λ ≤ ||Mi || for each i < 3. (2) M0 M1 M2 and a ∈ M2 − M1 . (3) ForSsome model N0 ∈ K Sλ with N0 M0 for each model N ∈ Kλ , N0 {a} ⊆ N M1 ⇒ (N0 , a, N, M2 ).
Definition 2.1.2. Let M0 , M1 be models in K≥λ with M0 M1 and p ∈ S(M1 ). We say that p does not fork over M0 , when for some triple ≤λ S (M1 , M2 , a) ∈ p we have (M0 , a, M1 , M2 ). Remark 2.1.3. We can replace the quantification ‘for some’ (M1 , M2 , a) in Definition 2.1.2 by ‘for each’. Definition 2.1.4. Let M ∈ K>λ , p ∈ S(M ). p is said to be basic when there is N ∈ Kλ such that N M and p does not fork over N . For bs (M ) is the set of basic types over M . Sometimes we every M ∈ K>λ , S>λ bs (M ), meaning S bs (M ) or S bs (M ) (the unique difference is the write S≥λ >λ cardinality of M ). By the following fact ([?, Theorem 2.complete...]), every non-forking relation in (Kλ , ↾ Kλ ) can be lifted to K≥λ with many properties preserved. Assuming local character, we can prove that density, monotonicity, transitivity, local character and continuity are preserved. Without assuming local character, we can prove that monotonicity, transitivity and continuity are preserved. Fact 2.1.5. Let s be a semi-good λ-frame, except local character. (1) Density: If s satisfies local character and M ≺ N, M ∈ K≥λ , then bs (M ). there is a ∈ N − M such that tp(a, M, N ) ∈ S≥λ (2) Monotonicity: Suppose M0 M1 M2 , n < 3 ⇒ Mn ∈ K≥λ , ||M2 || bs (M ) does not fork over M , then > λ. If p ∈ S≥λ 2 0 (a) p does not fork over M1 . (b) p ↾ M1 does not fork over M0 . (3) Transitivity: Suppose M0 , M1 , M2 ∈ K≥λ and M0 M1 M2 . If bs (M ) does not fork over M , and p ↾ M does not fork over p ∈ S≥λ 2 1 1 M0 , then p does not fork over M0 . (4) About local character: Let δ be a limit ordinal. Suppose s satisfies local character or λ+ ≤ cf (δ). If hMα : α ≤ δi is an increasing bs (M ) then there continuous sequence of models in K>λ , and p ∈ S>λ δ is α < δ such that p does not fork over Mα . (5) Continuity: Suppose hMα : α ≤ δ + 1i is an increasing continuous sequence of models in K≥λ . Let c ∈ Mδ+1 − Mδ . Denote pα = tp(c, Mα , Mδ+1 ). If for every α < δ, pα does not fork over M0 , then pδ does not fork over M0 .
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3. Non-Forking in λ+ Theorem 3.0.6. If s+ satisfies symmetry then it is a good non-forking λ+ frame. Proof. By assumption the amalgamation property is satisfied. By... we have the joint embedding property. By Fact 2.1.5 the following axioms are satisfied: density, monotonicity, local character and continuity. It satisfies the remain axioms by the following propositions: Proposition 3.0.7. s+ satisfies uniqueness. Proof. By uniqueness for s and tameness.
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Proposition 3.0.8. s+ satisfies basic stability. Proof. By basic stability in s and tameness.
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Proposition 3.0.9. s+ satisfies extension. Proof. It is sufficient to prove that if M is a model of cardinality λ, N is a model of cardinality λ+ , M N and p ∈ S bs (M ) then there is a non-forking extension of p to a type over N . Take a filtration hMα : α < λ+ i of N with M0 = M . Let N0 be a model of cardinality λ such that M0 N0 and for some a ∈ N0 − M0 tp(a, M0 , N0 ) = p. We choose by induction on α < λ+ a model Nα and an embedding fα : Mα → Nα such that: (1) f0 is the identity from M to N0 , (2) if α = β + 1 then fβ ⊆ fα and tp(a, fα [Mα ], Nα ) does not fork over fβ [Mβ ] (it is possible by the extension property in s)Sand S (3) if α is a limit ordinal then Nα = β
