Model
A model of a theory is a set together with relations (eg.
two: and ) satisfying all axioms of the theory . Symbolically
. According to the Gödel
completeness theorem, in
(Peano arithmetic)
(so also in ) a theory has models iff it is consistent.
According to Löwenheim–Skolem theorem, in if a countable
first-order theory has an infinite model, it has infinite models of all
cardinalities.
A model of a set theory (eg. ) is a set such that
the structure satisfies all axioms of
the set theory. If is base theory’s , the model is
called a transitive model. Gödel completeness theorem and
Löwenheim–Skolem theorem do not apply to transitive models. (But
Löwenheim–Skolem theorem together with Mostowski collapsing lemma show
that if there is a transitive model of ZFC, then there is a countable
such model.) See
Transitive ZFC model.
Class-sized transitive models
One can also talk about class-sized transitive models. Inner model is a
transitive
class (from other point of view, a class-sized transitive model (of ZFC
or a weaker theory)) containing all ordinals.
Forcing
creates outer models, but it can also be used in relation with inner
models.(Fuchs et al., 2015)
Among them are canonical inner models like
Mantle
The mantle is the intersection of all grounds. Mantle
is always a model of ZFC. Mantle is a ground (and is called a
bedrock) iff has only set many
grounds. (Fuchs et al., 2015; Usuba, 2017)
Generic mantle was defined as the intersection of all
mantles of generic extensions, but then it turned out that it is
identical to the
mantle. (Fuchs et al., 2015; Usuba, 2017)
th inner mantle is defined by ,
(mantle of the
previous inner mantle) and for limit . If there is uniform presentation of
for all ordinals as a single class, one can talk
about , etc.
If an inner mantle is a ground, it is called the outer
core.(Fuchs et al., 2015)
It is conjenctured (unproved) that every model of ZFC is the
of another model of ZFC for any desired , in which the sequence of inner mantles does not
stabilise before . It is probable that in the some time there are
models of ZFC, for which inner mantle is undefined (Analogously, a 1974
result of Harrington appearing in (Zadrożny, 1983, section 7), with
related work in (McAloon, 1974), shows that it is relatively consistent
with Gödel-Bernays set theory that exists for each but the intersection is not a
class.).(Fuchs et al., 2015)
For a cardinal , we call a ground of a -ground if there is
a poset of size and a -generic such that . The -mantle is
the intersection of all
-grounds.(Usuba, 2019)
The -mantle is a definable, transitive, and extensional class. It is
consistent that the -mantle is a model of ZFC (e.g. when there are no
grounds), and if is a strong limit, then the -mantle must be a
model of ZF. However it is not known whether the -mantle is always a
model of ZFC.(Usuba, 2019)
Mantle and large cardinals
If is
hyperhuge,
then has many
grounds
(so the mantle is a ground
itself).(Usuba, 2017)
If is
extendible
then the -mantle of is its smallest ground (so of course the
mantle is a ground of
).(Usuba, 2019)
On the other hand, it s consistent that there is a
supercompact
cardinal and class many grounds of (because of the indestructibility
properties of
supercompactness).(Usuba, 2017)
-model
A weak -model is a
transitive
set of size with and satisfying the
theory ( without the axiom of power
set, with collection, not replacement). It is a -model if
additionaly . (Hamkins & Johnstone, 2014; Holy & Schlicht, 2018)
References
- Fuchs, G., Hamkins, J. D., & Reitz, J. (2015). Set-theoretic geology. Annals of Pure and Applied Logic, 166(4), 464–501. https://doi.org/http://web.archive.org/web/20191116153209/https://doi.org/10.1016/j.apal.2014.11.004
- Usuba, T. (2017). The downward directed grounds hypothesis and very large cardinals. Journal of Mathematical Logic, 17(02), 1750009. https://doi.org/10.1142/S021906131750009X
- Usuba, T. (2019). Extendible cardinals and the mantle. Archive for Mathematical Logic, 58(1-2), 71–75. https://doi.org/10.1007/s00153-018-0625-4
- Hamkins, J. D., & Johnstone, T. A. (2014). Resurrection axioms and uplifting cardinals. http://jdh.hamkins.org/resurrection-axioms-and-uplifting-cardinals/
- Holy, P., & Schlicht, P. (2018). A hierarchy of Ramsey-like cardinals. Fundamenta Mathematicae, 242, 49–74. https://doi.org/10.4064/fm396-9-2017
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