x < y iff x < y, or x and y are incomparable and x χ(x,y) < y χ(x,y).


 Hilary Blankenship
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1 12. Large cardinals The study, or use, of large cardinals is one of the most active areas of research in set theory currently. There are many provably different kinds of large cardinals whose descriptions are different from one another. We restrict ourselves in this chapter to three important kinds: Mahlo cardinals, weakly compact cardinals, and measurable cardinals. All of these large cardinals are regular limit cardinals (which are frequently called weakly inaccessible cardinals), and most of them are strongly inaccessible cardinals. Mahlo cardinals As we mentioned in the elementary part of these notes, one cannot prove in ZFC that uncountable weakly inaccessible cardinals exist (if ZFC itself is consistent). But now we assume that even the somewhat stronger inaccessible cardinals exist, and we want to explore, roughly speaking, how many such there can be. We begin with some easy propositions. Proposition Assume that uncountable inaccessible cardinals exist, and suppose that κ is the least such. Then every strong limit cardinal less than κ is singular. The inaccessibles are a class of ordinals, hence form a wellordered class, and they can be enumerated in a strictly increasing sequence ι α : α O. Here O is an ordinal, or On, the class of all ordinals. The definition of Mahlo cardinal is motivated by the following simple proposition. Proposition If κ = ι α with α < κ, then the set {λ < κ : λ is regular} is a nonstationary subset of κ. Proof. Since κ is regular and α < κ, we must have sup β<α ι β < κ. Let C = {γ : sup β<α ι β < γ < κ and γ is a strong limit cardinal}. Then C is club in κ with empty intersection with the given set. κ is Mahlo iff κ is an uncountable inaccessible cardinal and {λ < κ : λ is regular} is stationary in κ. κ is weakly Mahlo iff κ is an uncountable weakly inaccessible cardinal and {λ < κ : λ is regular} is stationary in κ. Since the function ι is strictly increasing, we have α ι α for all α. Hence the following is a corollary of Proposition 12.2 Corollary If κ is a Mahlo cardinal, then κ = ι κ. Thus a Mahlo cardinal κ is not only inaccessible, but also has κ inaccessibles below it. Proposition For any uncountable cardinal κ the following conditions are equivalent: (i) κ is Mahlo. (ii) {λ < κ : λ is inaccessible} is stationary in κ. Proof. (i) (ii): Let S = {λ < κ : λ is regular}, and S = {λ < κ : λ is inaccessible}. Assume that κ is Mahlo. In particular, κ is uncountable and inaccessible. Suppose that 136
2 C is club in κ. The set D = {λ < κ : λ is strong limit} is clearly club in κ too. If λ S C D, then λ is inaccessible, as desired. (ii) (i): obvious. The following proposition answers a natural question one may ask after seeing Corollary Proposition Suppose that κ is minimum such that ι k = κ. Then κ is not Mahlo. Proof. Suppose to the contrary that κ is Mahlo, and let S = {λ < κ : λ is inaccessible} For each λ S, let f(λ) be the α < κ such that λ = ι α. Then α = f(λ) < λ by the minimality of κ. So f is regressive on the stationary set S, and hence there is an α < κ and a stationary subset S of S such that f(λ) = α for all λ S. But actually f is clearly a oneone function, contradiction. Mahlo cardinals are in a sense larger than ordinary inaccessibles. Namely, below every Mahlo cardinal κ there are κ inaccessibles. But now in principle one could enumerate all the Mahlo cardinals, and then apply the same idea used in going from regular cardinals to Mahlo cardinals in order to go from Mahlo cardinals to higher Mahlo cardinals. Thus we can make the definitions κ is hypermahlo iff κ is inaccessible and the set {λ < κ : λ is Mahlo} is stationary in κ. κ is hyperhypermahlo iff κ is inaccessible and the set {λ < κ : λ is hypermahlo} is stationary in κ. Of course one can continue in this vein. Weakly compact cardinals A cardinal κ is weakly compact iff κ > ω and κ (κ, κ) 2. There are several equivalent definitions of weak compactness. The one which justifies the name compact involves infinitary logic, and it will be discussed later. Right now we consider equivalent conditions involving trees and linear orderings. A cardinal κ has the tree property iff every κtree has a chain of size κ. Equivalently, κ has the tree property iff there is no κaronszajn tree. A cardinal κ has the linear order property iff every linear order (L, <) of size κ has a subset with order type κ or κ under <. Lemma For any regular cardinal κ, the linear order property implies the tree property. Proof. We are going to go from a tree to a linear order in a different way from the branch method of Chapter 8. Assume the linear order property, and let (T, <) be a κtree. For each x T and each α ht(x, T) let x α be the element of height α below x. Thus x 0 is the root which is below x, and x ht(x) = x. For each x T, let T x = {y T : y < x}. If x, y are 137
3 incomparable elements of T, then let χ(x, y) be the smallest ordinal α min(ht(x), ht(y)) such that x α y α. Let < be a wellorder of T. Then we define, for any distinct x, y T, x < y iff x < y, or x and y are incomparable and x χ(x,y) < y χ(x,y). We claim that this gives a linear order of T. To prove transitivity, suppose that x < y < z. Then there are several possibilities. These are illustrated in diagrams below. Case 1. x < y < z. Then x < z, so x < z. Case 2. x < y, while y and z are incomparable, with y χ(y,z) < z χ(y,z). Subcase 2.1. ht(x) < χ(y, z). Then x = x ht(x) = y ht(x) = z ht(x) so that x < z, hence x < z. Subcase 2.2. χ(y, z) ht(x). Then x and z are incomparable. In fact, if z < x then z < y, contradicting the assumption that y and z are incomparable; if x z, then y ht(x) = x = x ht(x) = z ht(x), contradiction. Now if α < χ(x, z) then y α = x α = z α ; it follows that χ(x, z) χ(y, z). If α < χ(y, z) then α ht(x), and hence x α = y α = z α ; this shows that χ(y, z) χ(x, z). So χ(y, z) = χ(x, z). Hence x χ(x,z) = y χ(x,z) = y χ(y,z) < z χ(y,z) = z χ(x,z), and hence x < z. Case 3. x and y are incomparable, and y < z. Then x and z are incomparable. Now if α < χ(x, y), then x α = y α = z α ; this shows that χ(x, y) χ(x, z). Also, x χ(x,y) < y χ(x,y) = z χ(x,y), and this implies that χ(x, z) χ(x, y). So χ(x, y) = χ(x, z). It follows that x χ(x,z) = x χ(x,y) < y χ(x,y) = z χ(x,z), and hence x < z. Case 4. x and y are incomparable, and also y and z are incomparable. We consider subcases. Subcase 4.1. χ(y, z) < χ(x, y). Now if α < χ(y, z), then x α = y α = z α ; so χ(y, z) χ(x, z). Also, x χ(y,z) = y χ(y,z) < z χ(y,z), so that χ(x, z) χ(y, z). Hence χ(x, z) = χ(y, z), and x χ(x,z) = y χ(y,z) < z χ(y,z), and hence x < z. Subcase 4.2. χ(y, z) = χ(x, y). Now x χ(x,y) < y χ(x,y) = y χ(y,z) < z χ(y,z) = z χ(x,y). It follows that χ(x, z) χ(x, y). For any α < χ(x, y) we have x α = y α = z α since χ(y, z) = χ(x, y). So χ(x, y) = χ(x, z). Hence x χ(x,z) = x χ(x,y) < y χ(x,y) = y χ(y,z) < z χ(y,z) = z χ(x,z), so x < z. Subcase 4.3. χ(x, y) < χ(y, z). Then x χ(x,y) < y χ(x,y) = z χ(x,y), and if α < χ(x, y) then x α = y α = z α. It follows that x < z Clearly any two elements of T are comparable under <, so we have a linear order. The following property is also needed. (*) If t < x, y and x < a < y, then t < a. In fact, suppose not. If a t, then a < x, hence a < x, contradiction. So a and t are incomparable. Then χ(a, t) ht(t), and hence x < y < a or a < x < y, contradiction. Now by the linear order property, (T, < ) has a subset L of order type κ or κ. First suppose that L is of order type κ. Define B = {t T : x L a L[x a t a]}. 138
4 z y z y z x z y x y x x Case 1 Subcase 2.1 Subcase 2.2 Case 3 x y z x y z x y z Subcase 4.1 Subcase 4.2 Subcase 4.3 We claim that B is a chain in T of size κ. Suppose that t 0, t 1 B with t 0 t 1, and choose x 0, x 1 L correspondingly. Say wlog x 0 < x 1. Now t 0 B and x 0 x 1, so t 0 x 1. And t 1 B and x 1 x 1, so t 1 x 1. So t 0 and t 1 are comparable. Now let α < κ; we show that B has an element of height α. For each t of height α let V t = {x L : t x}. Then {x L : ht(x) α} = ht(t)=α since there are fewer than κ elements of height less than κ, this set has size κ, and so there is a t such that ht(t) = α and V t = κ. We claim that t B. To prove this, take any x V t such that t < x. Suppose that a L and x a. Choose y V t with a < y and t < y. Then t < x, t < y, and x a < y. If x = a, then t a, as desired. If x < a, then t < a by (*). This finishes the case in which L has a subset of order type κ. The case of order type κ is similar, but we give it. So, suppose that L has order type κ. Define V t ; B = {t T : x L a L[a x t a]}. We claim that B is a chain in T of size κ. Suppose that t 0, t 1 B with t 0 t 1, and choose x 0, x 1 L correspondingly. Say wlog x 0 < x 1. Now t 0 B and x 0 x 0, so t 0 x 0. and t 1 B and x 0 x 1, so t 1 x 0. So t 0 and t 1 are comparable. 139
5 Now let α < κ; we show that B has an element of height α. For each t of height α let V t = {x L : t x}. Then {x L : ht(x) α} = V t ; ht(t)=α since there are fewer than κ elements of height less than κ, this set has size κ, and so there is a t such that ht(t) = α and V t = κ. We claim that t B. To prove this, take any x V t such that t < x. Suppose that a L and a x. Choose y V t with y < a and t < y. Then t < x, t < y, and y < a x. If a = x, then t < a, as desired. If a < x, then t < a by (*). Theorem For any uncountable cardinal κ the following conditions are equivalent: (i) κ is weakly compact. (ii) κ is inaccessible, and it has the linear order property. (iii) κ is inaccessible, and it has the tree property. (iv) For any cardinal λ such that 1 < λ < κ we have κ (κ) 2 λ. Proof. (i) (ii): Assume that κ is weakly compact. First we need to show that κ is inaccessible. To show that κ is regular, suppose to the contrary that κ = α<λ µ α, where λ < κ and µ α < κ for each α < λ. By the definition of infinite sum of cardinals, it follows that we can write κ = α<λ M α, where M α = µ α for each α < λ and the M α s are pairwise disjoint. Define f : [κ] 2 2 by setting, for any distinct α, β < κ, { 0 if α, β Mξ for some ξ < λ, f({α, β}) = 1 otherwise. Let H be homogeneous for f of size κ. First suppose that f[[h] 2 ] = {0}. Fix α 0 H, and say α 0 M ξ. For any β H we then have β M ξ also, by the homogeneity of H. So H M ξ, which is impossible since M ξ < κ. Second, suppose that f[[h] 2 ] = {1}. Then any two distinct members of H lie in distinct M ξ s. Hence if we define g(α) to be the ξ < λ such that α M ξ for each α H, we get a oneone function from H into λ, which is impossible since λ < κ. To show that κ is strong limit, suppose that λ < κ but κ 2 λ. Now by Theorem 10.9 we have 2 λ (λ +, λ + ) 2. So choose f : [2 λ ] 2 2 such that there does not exist an X [2 λ ] λ+ with f [X] 2 constant. Define g : [κ] 2 2 by setting g(a) = f(a) for any A [κ] 2. Choose Y [κ] κ such that g [Y ] 2 is constant. Take any Z [Y ] λ+. Then f [Z] 2 is constant, contradiction. So, κ is inaccessible. Now let (L, <) be a linear order of size κ. Let be a well order of L. Now we define f : [L] 2 2; suppose that a, b L with a b. Then { 0 if a < b, f({a, b}) = 1 if b > a. Let H be homogeneous for f and of size κ. If f[[h] 2 ] = {0}, then H is wellordered by <. If f[[h] 2 ] = {1}, then H is wellordered by >. 140
6 (ii) (iii): By Lemma (iii) (iv): Assume (iii). Suppose that F : [κ] 2 λ, where 1 < λ < κ; we want to find a homogeneous set for F of size κ. We construct by recursion a sequence t α : α < κ of members of <κ κ; these will be the members of a tree T. Let t 0 =. Now suppose that 0 < α < κ and t β <κ κ has been constructed for all β < α. We now define t α by recursion; its domain will also be determined by the recursive definition, and for this purpose it is convenient to actually define an auxiliary function s : κ κ+1 by recursion. If s(η) has been defined for all η < ξ, we define F({β, α}) where β < α is minimum such that s ξ = t β, if there is such a β, s(ξ) = κ if there is no such β. Now eventually the second condition here must hold, as otherwise s ξ : ξ < κ would be a oneone function from κ into {t β : β < α}, which is impossible. Take the least ξ such that s(ξ) = κ, and let t α = s ξ. This finishes the construction of the t α s. Let T = {t α : α < κ}, with the partial order. Clearly this gives a tree. By construction, if α < κ and ξ < dmn(t α ), then t α ξ T. Thus the height of an element t α is dmn(t α ). (2) The sequence t α : α < κ is oneone. In fact, suppose that β < α and t α = t β. Say that dmn(t α ) = ξ. Then t α = t α ξ = t β, and the construction of t α gives something with domain greater than ξ, contradiction. Thus (2) holds, and hence T = κ. (3) The set of all elements of T of level ξ < κ has size less than κ. In fact, let U be this set. Then U η<ξ λ = λ ξ < κ since κ is inaccessible. So (3) holds, and hence, since T = κ, T has height κ and is a κtree. (4) If t β t α, then β < α and F({β, α}) = t α (dmn(t β )). This is clear from the definition. Now by the tree property, there is a branch B of size κ. For each ξ < λ let H ξ = {α < κ : t α B and t α ξ B}. We claim that each H ξ is homogeneous for F. In fact, take any distinct α, β H ξ. Then t α, t β B. Say t β t α. Then β < α, and by construction t α (dmn(t β )) = F({α, β}). So F({α, β}) = ξ by the definition of H ξ, as desired. Now {α < κ : t α B} = ξ<λ{α < κ : t α H ξ }, 141
7 so since B = κ it follows that H ξ = κ for some ξ < λ, as desired. (iv) (i): obvious. Now we go into the connection of weakly compact cardinals with logic, thereby justifying the name weakly compact. This is optional material. Let κ and λ be infinite cardinals. The language L κλ is an extension of ordinary first order logic as follows. The notion of a model is unchanged. In the logic, we have a sequence of λ distinct individual variables, and we allow quantification over any oneone sequence of fewer than λ variables. We also allow conjunctions and disjunctions of fewer than κ formulas. It should be clear what it means for an assignment of values to the variables to satisfy a formula in this extended language. We say that an infinite cardinal κ is logically weakly compact iff the following condition holds: (*) For any language L κκ with at most κ basic symbols, if Γ is a set of sentences of the language and if every subset of Γ of size less than κ has a model, then also Γ has a model. Notice here the somewhat unnatural restriction that there are at most κ basic symbols. If we drop this restriction, we obtain the notion of a strongly compact cardinal. These cardinals are much larger than even the measurable cardinals discussed later. We will not go into the theory of such cardinals. Theorem An infinite cardinal is logically weakly compact iff it is weakly compact. Proof. Suppose that κ is logically weakly compact. (1) κ is regular. Suppose not; say X κ is unbounded but X < κ. Take the language with individual constants c α for α < κ and also one more individual constant d. Consider the following set Γ of sentences in this language: {d c α : α < κ} (d = c α ). β X α<β If [Γ] <κ, let A be the set of all α < κ such that d = c α is in. So A < κ. Take any α κ\a, and consider the structure M = (κ, γ, α) γ<κ. There is a β X with α < β, and this shows that M is a model of. Thus every subset of Γ of size less than κ has a model, so Γ has a model; but this is clearly impossible. (2) κ is strong limit. In fact, suppose not; let λ < κ with κ 2 λ. We consider the language with distinct individual constants c α, d i α for all α < κ and i < 2. Let Γ be the following set of sentences in this language: { } { } [(c α = d 0 α c α = d 1 α) d 0 α d 1 α] (c α d f(α) α ) : f λ 2. α<λ 142 α<λ
8 Suppose that [Γ] <κ. We may assume that has the form { } { } [(c α = d 0 α c α = d 1 α ) d0 α d1 α ] (c α d f(α) ) : f M, α<λ where M [ λ 2] <κ. Fix g λ 2\M. Let d 0 α = α, d1 α = α + 1, and c α = dα g(α), for all α < λ. Clearly (κ, c α, d i α) α<λ,i<2 is a model of. Thus every subset of Γ of size less than κ has a model, so Γ has a model, say (M, u α, vα) i α<λ,i<2. By the first part of Γ there is a function f λ 2 such that u α = d f(α) α for every α < λ. this contradicts the second part of Γ. Hence we have shown that κ is inaccessible. Finally, we prove that the tree property holds. Suppose that (T, ) is a κtree. Let L be the language with a binary relation symbol, unary relation symbols P α for each α < κ, individual constants c t for each t T, and one more individual constant d. Let Γ be the following set of sentences: all L κκ sentences holding in the structure M def = (T, <, Lev α (T), t) α<κ,t T ; x[p α x x d] for each α < κ. Clearly every subset of Γ of size less than κ has a model. Hence Γ has a model N def = (A, <, S α, a t, b) α<κ,t T. For each α < κ choose e α S α with e α < b. Now the following sentence holds in M and hence in N: x P α x (x = c s ). s Lev α (T) Hence for each α < κ we can choose t(α) T such that e a = a t(α). Now the sentence α<λ x, y, z[x < z y < z x and y are comparable] holds in M, and hence in N. Now fix α < β < κ. Now e α, e β < b, so it follows that e α and e β are comparable under. Hence a t(α) and a t(β) are comparable under. It follows that t(α) and t(β) are comparable under. So t(α) < t(β). Thus we have a branch of size κ. Now suppose that κ is weakly compact. Let L be an L κκ language with at most κ symbols, and suppose that Γ is a set of sentences in L such that every subset of Γ of size less than κ has a model M. We will construct a model of Γ by modifying Henkin s proof of the completeness theorem for firstorder logic. First we note that there are at most κ formulas of L. This is easily seen by the following recursive construction of all formulas: F 0 = all atomic formulas; { F α+1 = F α { ϕ : ϕ F α } Φ : Φ [Fα ] <κ} { xϕ : ϕ F α, x of length < κ}; F α = F β for α limit. β<α 143 α
9 By induction, F α κ for all α κ, and F κ is the set of all formulas. (One uses that κ is inaccessible.) Expand L to L by adjoining a set C of new individual constants, with C = κ. Let Θ be the set of all subformulas of the sentences in Γ. Let ϕ α : α < κ list all sentences of L which are of the form xψ α (x) and are obtained from a member of Θ by replacing variables by members of C. Here x is a oneone sequence of variables of length less than κ; say that x has length β α. Now we define a sequence d α : α < κ ; each d α will be a sequence of members of C of length less than κ. If d β has been defined for all β < α, then β<α rng(d β ) {c C : c occurs in ϕ β for some β < α} has size less than κ. We then let d α be a oneone sequence of members of C not in this set; d α should have length β α. Now for each α κ let Ω α = { xψ β (x) ψ β (d β ) : β < α}. Note that Ω α Ω γ if α < γ κ. Now we define for each [Γ] <κ and each α κ a model Nα of Ω α. Since Ω 0 =, we can let N0 = M. Having defined Nα, since the range of d α consists of new constants, we can choose denotations of those constants, expanding Nα to Nα+1, so that the sentence xψ α (x) ψ α (d α ) holds in N α+1. For α κ limit we let N α = β<α N β. It follows that N κ is a model of Ω κ. So each subset of Γ Ω κ of size less than κ has a model. It suffices now to find a model of Γ Ω κ in the language L. Let ψ α : α < κ be an enumeration of all sentences obtained from members of Θ by replacing variables by members of C, each such sentence appearing κ times. Let T consist of all f satisfying the following conditions: (3) f is a function with domain α < κ. (4) β < α[(ψ β Γ Ω κ f(β) = ψ β ) and ψ β / Γ Ω κ f(β) = ψ β )]. (5) rng(f) has a model. Thus T forms a tree. (6) T has an element of height α, for each α < κ. In fact, def = {ψ β : β < α, ψ β Γ Ω κ } { ψ β : β < α, ψ β Γ Ω κ } is a subset of Γ Ω κ of size less than κ, so it has a model P. For each β < α let f(β) = { ψβ if P = ψ β, ψ γ if P = ψ β. 144
10 Clearly f is an element of T with height α. So (6) holds. Thus T is clearly a κtree, so by the tree property we can let B be a branch in T of size κ. Let Ξ = {f(α) : α < κ, f B, f has height α + 1}. Clearly Γ Ω κ Ξ and for every α < κ, ψ α Ξ or ψ α Ξ. (7) If ϕ, ϕ χ Ξ, then χ Ξ. In fact, say ϕ = f(α) and ϕ χ = f(β). Choose γ > α, β so that ψ γ is χ. We may assume that dmn(f) γ + 1. Since rng(f) has a model, it follows that f(γ) = χ. So (7) holds. Let S be the set of all terms with no variables in them. We define σ τ iff σ, τ S and (σ = τ) Ξ. Then is an equivalence relation on S. In fact, let σ S. Say that σ = σ is ψ α. Since ψ α holds in every model, it holds in any model of {f(β) : β α}, and hence f(α) = (σ = σ). So (σ = σ) Ξ and so σ σ. Symmetry and transitivity follow by (7). Let M be the collection of all equivalence classes. Using (7) it is easy to see that the function and relation symbols can be defined on M so that the following conditions hold: (8) If F is an mary function symbol, then F M (σ 0 /,..., σ m 1 / ) = F(σ 0,..., σ m 1 )/. (9) If R is an mary relation symbol, then σ 0 /,..., σ m 1 / R M iff R(σ 0,..., σ m 1 ) Ξ. Now the final claim is as follows: (10) If ϕ is a sentence of L, then M = ϕ iff ϕ Ξ. Clearly this will finish the proof. We prove (10) by induction on ϕ. It is clear for atomic sentences by (8) and (9). If it holds for ϕ, it clearly holds for ϕ. Now suppose that Q is a set of sentences of size less than κ, and (10) holds for each member of Q. Suppose that M = Q. Then M = ϕ for each ϕ Q, and so Q Ξ. Hence there is a [κ] <κ such that Q = f[ ], with f B. Choose α greater than each member of such that ψ α is the formula Q. We may assume that α dmn(f). Since rng(f) has a model, it follows that f(α) = Q. Hence Q Ξ. Conversely, suppose that Q Ξ. From (7) it easily follows that ϕ Ξ for every ϕ Q, so by the inductive hypothesis M = ϕ for each ϕ Q, so M = Q. Finally, suppose that ϕ is xψ, where (10) holds for shorter formulas. Suppose that M = xψ. Then there are members of S such that when they are substituted in ψ for x, obtaining a sentence ψ, we have M = ψ. Hence by the inductive hypothesis, ψ Ξ. (7) then yields xψ Ξ. Conversely, suppose that xψ Ξ. Now there is a sequence d of members of C such that xψ Ξ ψ(d) is also in Ξ, and so by (7) we get ψ(d) Ξ. By the inductive hypothesis, M = ψ(d), so M = xψ Ξ. Next we want to show that every weakly compact cardinal is a Mahlo cardinal. To do this we need two lemmas. 145
11 Lemma Let A be a set of infinite cardinals such that for every regular cardinal κ, the set A κ is nonstationary in κ. Then there is a oneone regressive function with domain A. Proof. We proceed by induction on γ def = A. Note that γ is a cardinal; it is 0 if A =. The cases γ = 0 and γ = ω are trivial, since then A = or A = {ω} respectively. Next, suppose that γ is a successor cardinal κ +. Then A = A {κ + } for some set A of infinite cardinals less than κ +. Then A < κ +, so by the inductive hypothesis there is a oneone regressive function f on A. We can extend f to A by setting f(κ + ) = κ, and so we get a oneone regressive function defined on A. Suppose that γ is singular. Let µ ξ : ξ < cf(γ) be a strictly increasing continuous sequence of infinite cardinals with supremum γ, with cf(γ) < µ 0. Note then that for every cardinal λ < γ, either λ < µ 0 or else there is a unique ξ < cf(γ) such that µ ξ λ < µ ξ+1. For every ξ < cf(γ) we can apply the inductive hypothesis to A µ ξ to get a oneone regressive function g ξ with domain A µ ξ. We now define f with domain A. In case cf(γ) = ω we define, for each λ A, g 0 (λ) + 2 if λ < µ 0, µ ξ + g ξ+1 (λ) + 1 if µ ξ < λ < µ ξ+1, f(λ) = µ ξ if λ = µ ξ+1, 1 if λ = µ 0, 0 if λ = γ A. Here the addition is ordinal addition. Clearly f is as desired in this case. If cf(γ) > ω, let ν ξ : ξ < cf(γ) be a strictly increasing sequence of limit ordinals with supremum cf(γ). Then we define, for each λ A, g 0 (λ) + 1 if λ < µ 0, µ f(λ) = ξ + g ξ+1 (λ) + 1 if µ ξ < λ < µ ξ+1, ν ξ if λ = µ ξ, 0 if λ = γ A. Clearly f works in this case too. Finally, suppose that γ is a regular limit cardinal. By assumption, there is a club C in γ such that C γ A =. We may assume that C ω =. Let µ ξ : ξ < γ be the strictly increasing enumeration of C. Then we define, for each λ A, g 0 (λ) + 1 if λ < µ 0, f(λ) = µ ξ + g ξ+1 (λ) + 1 if µ ξ < λ < µ ξ+1, 0 if λ = γ A. Clearly f works in this case too. Lemma Suppose that κ is weakly compact, and S is a stationary subset of κ. Then there is a regular λ < κ such that S λ is stationary in λ. Proof. Suppose not. Thus for all regular λ < κ, the set S λ is nonstationary in λ. Let C be the collection of all infinite cardinals less than κ. Clearly C is club in κ, so 146
12 S C is stationary in κ. Clearly still S C λ is nonstationary in λ for every regular λ < κ. So we may assume from the beginning that S is a set of infinite cardinals. Let λ ξ : ξ < κ be the strictly increasing enumeration of S. Let T = s : ξ < κ s λ η and s is oneone. η<ξ For every ξ < κ the set S λ ξ is nonstationary in every regular cardinal, and hence by Lemma 12.9 there is a oneone regressive function s with domain S λ ξ. Now S λ ξ = {λ η : η < ξ}. Hence s T. Clearly T forms a tree of height κ under. Now for any α < κ, ( ) α λ β sup λ β < κ. β<α β<α Hence by the tree property there is a branch B in T of size κ. Thus B is a oneone regressive function with domain S, contradicting Fodor s theorem. Theorem Every weakly compact cardinal is Mahlo, hypermahlo, hyperhyper Mahlo, etc. Proof. Let κ be weakly compact. Let S = {λ < κ : λ is regular}. Suppose that C is club in κ. Then C is stationary in κ, so by Lemma there is a regular λ < κ such that C λ is stationary in λ; in particular, C λ is unbounded in λ, so λ C since C is closed in κ. Thus we have shown that S C. So κ is Mahlo. Let S = {λ < κ : λ is a Mahlo cardinal}. Suppose that C is club in κ. Let S = {λ < κ : λ is regular}. Since κ is Mahlo, S is stationary in κ. Then C S is stationary in κ, so by Lemma there is a regular λ < κ such that C S λ is stationary in λ. Hence λ is Mahlo, and also C λ is unbounded in λ, so λ C since C is closed in κ. Thus we have shown that S C. So κ is hypermahlo. Let S = {λ < κ : λ is a hypermahlo cardinal}. Suppose that C is club in κ. Let S iv = {λ < κ : λ is Mahlo}. Since κ is hypermahlo, S iv is stationary in κ. Then C S iv is stationary in κ, so by Lemma there is a regular λ < κ such that C S iv λ is stationary in λ. Hence λ is hypermahlo, and also C λ is unbounded in λ, so λ C since C is closed in κ. Thus we have shown that S C. So κ is hyperhypermahlo. Etc. Measurable cardinals Our third kind of large cardinal is the class of measurable cardinals. Although, as the name suggests, this notion comes from measure theory, the definition and results we give are purely settheoretical. Moreover, similarly to weakly compact cardinals, it is not obvious from the definition that we are dealing with large cardinals. The definition is given in terms of the notion of an ultrafilter on a set. 147
13 Let X be a nonempty set. A filter on X is a family F of subsets of X satisfying the following conditions: (i) X F. (ii) If Y, Z F, then Y Z F. (iii) If Y F and Y Z X, then Z F. A filter F on a set X is proper or nontrivial iff / F. An ultrafilter on a set X is a nontrivial filter F on X such that for every Y X, either Y F or X\Y F. A family A of subsets of X has the finite intersection property, fip, iff for every finite subset B of A we have B. If A is a family of subsets of X, then the filter generated by A is the set {Y X : B Y for some finite B A }. [Clearly this is a filter on X, and it contains A.] Proposition If x X, then {Y X : x Y } is an ultrafilter on X. An ultrafilter of the kind given in this proposition is called a principal ultrafilter. There are nonprincipal ultrafilters on any infinite set, as we will see shortly. Proposition Let F be a proper filter on a set X. Then the following are equivalent: (i) F is an ultrafilter. (ii) F is maximal in the partially ordered set of all proper filters (under ). Proof. (i) (ii): Assume (i), and suppose that G is a filter with F G. Choose Y G \F. Since Y / F, we must have X\Y F G. So Y, X\Y G, hence = Y (X\Y ) G, and so G is not proper. (ii) (i): Assume (ii), and suppose that Y X, with Y / F; we want to show that X\Y F. Let G = {Z X : Y W Z for some W F }. Clearly G is a filter on X, and F G. Moreover, Y G \F. It follows that G is not proper, and so G. Thus there is a W F such that Y W =. Hence W X\Y, and hence X\Y F, as desired. Theorem For any infinite set X there is a nonprincipal ultrafilter on X. Moreover, if A is any collection of subsets of X with fip, then A can be extended to an ultrafilter. Proof. First we show that the first assertion follows from the second. Let A be the collection of all cofinite subsets of X the subsets whose complements are finite. A has fip, since if B is a finite subset of A, then X\ B = Y B (X\B) is finite. By the second assertion, A can be extended to an ultrafilter F. Clearly F is nonprincipal. 148
14 To prove the second assertion, let A be a collection of subsets of X with fip, and let C be the collection of all proper filters on X which contain A. Clearly the filter generated by A is proper, so C. We consider C as a partially ordered set under inclusion. Any subset D of C which is a chain has an upper bound in C, namely D, as is easily checked. So by Zorn s lemma C has a maximal member F. By Proposition 12.13, F is an ultrafilter. Let X be an infinite set, and let κ be an infinite cardinal. An ultrafilter F on X is κ complete iff for any A [F] <κ we have A F. We also say σcomplete synonomously with ℵ 1 complete. This notion is clearly a generalization of one of the properties of ultrafilters. In fact, every ultrafilter is ωcomplete, and every principal ultrafilter is κcomplete for every infinite cardinal κ. Lemma Suppose that X is an infinite set, F is an ultrafilter on X, and κ is the least infinite cardinal such that there is an A [F] κ such that A / F. Then there is a partition P of X such that P = κ and X\Y F for all Y P. Proof. Let Y α : α < κ enumerate A. Let Z 0 = X\Y 0, and for α > 0 let Z α = ( β<α Y β)\y α. Note that Y α X\Z α, and so X\Z α F. Clearly Z α Z β = for α β. Let W = α<λ Y α. Clearly W Z α = for all α < λ. Let P = ({Z α : α < κ} {W })\{ }. So P is a partition of X and X\Z F for all Z P. Clearly P κ. If P < κ, then contradiction. So P = κ. = Z P (X\Z) F, Theorem Suppose that κ is the least infinite cardinal such that there is a nonprincipal σcomplete ultrafilter F on κ. Then F is κcomplete. Proof. Assume the hypothesis, but suppose that F is not κcomplete. So there is a A [F] <κ such that A / F. Hence by Lemma there is a partition P of κ such that P < κ and X\P F for every P P. Let P α : α < λ be a oneone enumeration of P, λ an infinite cardinal. We are now going to construct a nonprincipal σcomplete ultrafilter G on λ, which will contradict the minimality of κ. Define f : κ λ by letting f(β) be the unique α < λ such that β P α. Then we define G = {D λ : f 1 [D] F }. We check the desired conditions for G. / G, since f 1 [ ] = / F. If D G and D E, then f 1 [D] F and f 1 [D] f 1 [E], so f 1 [E] F and hence E G. Similarly, G is closed under. Given D λ, either f 1 [D] F or f 1 [λ\d] = κ\f 1 [D] F, hence D G or λ\d G. So G is an ultrafilter on λ. It is nonprincipal, since for any α < λ 149
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