FOM: 154:Orderings on theories
friedman@math.ohio-state.edu
friedman at math.ohio-state.edu
Tue Jun 25 01:28:45 EDT 2002
1. ORDERINGS ON SET THEORIES.
By a set theory we will mean a theory in predicate calculus with equality and
membership.
Let S,T be two finitely axiomatized set theories. We define
S < T if and only if for every model M of T there is an element x of M such
that ({y: y epilson_M x},epsilon_M) satisfies S.
Let T be a recursively axiomatized extension of ZF\P = ZF without power set.
The provable ordinal of T is the sup of the ordinals of the recursive well
orderings which are given by an index for which it is provable in T that the
index defines a recursive well ordering.
THEOREM 1.1. Let T be a recursively axiomatized extension of ZF\P with
reflection, which has a well founded model. Then < on finitely axiomatized
fragments of T is r.e. and well founded, with ordinal = the provable ordinal of
T.
We can consider set theories axiomatizated by finitely many schemes. For such
S,T, define
S < T if and only if for every model M of T there is an element x of M such
that according to M, ({y: y epilson_M x},epsilon_M) satisfies S.
THEOREM 1.2. Let T be a recursively axiomatized extension of ZF\P with
reflection, which has a well founded model. Then < on the fragments of T given
by finitely many schemes is well founded, with ordinal > the provable ordinal
of T. The ordinal is a recursive ordinal.
2. ORDERINGS ON FRAGMENTS OF Z_2.
There is a similar treatment for fragments of Z_2. We only state this for Z_2.
Let S,T be finite fragments of Z_2, proving ACA_0. We define
S < T if and only if every model M of T has a second order object x such that
according to M, x codes a beta model of S.
THEOREM 2.1. < is r.e. and well founded, with ordinal = the provable ordinal of
Z_2.
3. ORDERINGS ON SENTENCES IN LINEARLY ORDERED PREDICATE CALCULUS.
We use linearly ordered predicate calculus with one binary relation. All models
are of the form (A,<,R), where A is a set, < is a strict linear ordering on A,
and R is a binary relation on A. It will be convenient to allow A to be empty.
We say that (A,<,R) is an initial submodel of (B,<',S) if and only if
i) (A,<,R) is a submodel of (B,<',S);
ii) for all x in A and y in B\A, x <' y.
We say that (B,<',S) is a single point extension of (A,<,R) if and only if (A,<
,R) is an initial submodel of (B,<',S) and B\A has cardinality 1.
If two single point extensions of (A,<,R) are isomorphic by an isomorphism
which is the identity on A, then we identify those extensions.
In a direct union of models, we take the union of a (possibly empty) set of
models which is linearly ordered by initial submodelhood.
We order the sentences in our language as follows.
phi < psi if and only if every direct union of countable models of phi is
countable and has a unique single point extension satisfying psi.
THEOREM 3.1. < is r.e. and well founded, and the ordinal of < is the provable
ordinal of Z_2.
More generally, let kappa be an infinite cardinal.
phi <kappa psi if and only if every direct union of models of phi, each of
cardinality < kappa, is of cardinality < kappa, and has a unique single point
extension satisfying psi.
THEOREM 3.2. For all infinite cardinals kappa, <kappa is well founded and has
recursive ordinal. Assuming large cardinals, there is a maximum ordinal among
the <kappa, and it is the provable ordinal of MAH. In particular, if kappa is
n-Mahlo for all n < omega, then the ordinal of <kappa is the provable ordinal
of MAH.
Here MAH = ZFC + {there exists an n-Mahlo cardinal}_n.
*********************************************
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This is the 154th in a series of self contained postings to FOM covering
a wide range of topics in f.o.m. Previous ones counting from #100 are:
100:Boolean Relation Theory IV corrected 3/21/01 11:29AM
101:Turing Degrees/1 4/2/01 3:32AM
102: Turing Degrees/2 4/8/01 5:20PM
103:Hilbert's Program for Consistency Proofs/1 4/11/01 11:10AM
104:Turing Degrees/3 4/12/01 3:19PM
105:Turing Degrees/4 4/26/01 7:44PM
106.Degenerative Cloning 5/4/01 10:57AM
107:Automated Proof Checking 5/25/01 4:32AM
108:Finite Boolean Relation Theory 9/18/01 12:20PM
109:Natural Nonrecursive Sets 9/26/01 4:41PM
110:Communicating Minds I 12/19/01 1:27PM
111:Communicating Minds II 12/22/01 8:28AM
112:Communicating MInds III 12/23/01 8:11PM
113:Coloring Integers 12/31/01 12:42PM
114:Borel Functions on HC 1/1/02 1:38PM
115:Aspects of Coloring Integers 1/3/02 10:02PM
116:Communicating Minds IV 1/4/02 2:02AM
117:Discrepancy Theory 1/6/02 12:53AM
118:Discrepancy Theory/2 1/20/02 1:31PM
119:Discrepancy Theory/3 1/22.02 5:27PM
120:Discrepancy Theory/4 1/26/02 1:33PM
121:Discrepancy Theory/4-revised 1/31/02 11:34AM
122:Communicating Minds IV-revised 1/31/02 2:48PM
123:Divisibility 2/2/02 10:57PM
124:Disjoint Unions 2/18/02 7:51AM
125:Disjoint Unions/First Classifications 3/1/02 6:19AM
126:Correction 3/9/02 2:10AM
127:Combinatorial conditions/BRT 3/11/02 3:34AM
128:Finite BRT/Collapsing Triples 3/11/02 3:34AM
129:Finite BRT/Improvements 3/20/02 12:48AM
130:Finite BRT/More 3/21/02 4:32AM
131:Finite BRT/More/Correction 3/21/02 5:39PM
132: Finite BRT/cleaner 3/25/02 12:08AM
133:BRT/polynomials/affine maps 3/25/02 12:08AM
134:BRT/summation/polynomials 3/26/02 7:26PM
135:BRT/A Delta fA/A U. fA 3/27/02 5:45PM
136:BRT/A Delta fA/A U. fA/nicer 3/28/02 1:47AM
137:BRT/A Delta fA/A U. fA/beautification 3/28/02 4:30PM
138:BRT/A Delta fA/A U. fA/more beautification 3/28/02 5:35PM
139:BRT/A Delta fA/A U. fA/better 3/28/02 10:07PM
140:BRT/A Delta fA/A U. fA/yet better 3/29/02 10:12PM
141:BRT/A Delta fA/A U. fA/grammatical improvement 3/29/02 10:43PM
142:BRT/A Delta fA/A U. fA/progress 3/30/02 8:47PM
143:BRT/A Delta fA/A U. fA/major overhaul 5/2/02 2:22PM
144: BRT/A Delta fA/A U. fA/finesse 4/3/02 4:29AM
145:BRT/A U. B U. TB/simplification/new chapter 4/4/02 4:01AM
146:Large large cardinals 4/18/02 4:30AM
147:Another Way 7:21AM 4/22/02
148:Finite forms by relativization 2:55AM 5/15/02
149:Bad Typo 1:59PM 5/15/02
150:Finite obstruction/statisics 8:55AM 6/1/02
151:Finite forms by bounding 4:35AM 6/5/02
152:sin 10:35PM 6/8/02
153:Large cardinals as general algebra 1:21PM 6/17/02
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