A situs is a notion of generalised topological space, and defined as a simplicial object of the category of filters on sets, i.e. a simplicial set equipped, for each $n\geq 0$, with a filter on the set of $n$-simplices such that under any face or degeneration map the preimage of a large set is large. We denote the category of situses by sዋ.
Intuitively, these filters are viewed as additional structure of topological nature on a simplicial set (“the situs structure on a simplicial set”) giving a meaning to the phrase “a simplex is sufficiently small”: by definition, we say “a property holds for all small enough $n$-simplices” iff it holds on a set in the filter, and we refer to sets in the filter as neighbourhoods. This extends to simplicial language the standard intuition of topology: given a topological structure on a set, the precise meaning of the phrase “a property $P_{x_0}(x)$ holds for all points $x$ sufficiently close to a given point $x_0$” is that the property holds on a neighbourhood of $x_0$; intuitively, $x$ near $x_0$ is thought of as small because $x$ approximates $x_0$ up to a small error.
Situses generalise metric and topological spaces, filters, and simplicial sets, and the concept is designed to be flexible enough to formulate categorically a number of standard basic elementary definitions in various fields, e.g. in analysis, limit, (uniform) continuity and convergence, equicontinuity of sequences of functions; in algebraic topology, being locally trivial and geometric realisation; in geometry, quasi-isomorphism; in model theory, stability and simplicity and several Shelah’s dividing lines, e.g. NIP, NOP, NSTOP, $NSOP_i$, $NTP_i$, of a theory.
No homotopy theory for situses has been developed, although the naive definition of an interval object (namely, the simplicial set represented by the linear order $[0,1]$ equipped with some situs structure based on the metric/topology) leads to a directed (not symmetric) notion of homotopy.
We now give a number of examples demonstrating the expressive power of the category of situses.
A simplicial set can be equipped with discrete or indiscrete situs structure: %The (in)discrete situs structure on a simplicial set for each $n$, equip $X_n$ with the discrete or indiscrete filter, respectively.
Let $M$ be a metric space. View $M$ as a simplicial set represented by the set of points of $M$, and equip each $M^n$ with the filter generated by uniform neighbourhoods of the diagonal, i.e. subsets containing all tuples of small enough diameter. With this situs structure, a map $f:M\to N$ is uniformly continuous iff it induces a morphism $f_\bullet:M_\bullet\to N_\bullet$ of situses. In fact this defines a fully faithful embedding of the category of metric spaces with uniformly continuous maps into the category of situses.
We can also consider a different situs structure capturing the notion of quasi-isomorphism in large scale geometry. Equip each $M^n$ with the filter such that a subset of $M^n$ is large iff it contains all $n$-tuples such that the distance between distinct points is at least $D$, for some $D\geq 0$. With this situs structure, for quasi-geodesic metric spaces, a map $f_\bullet:M_\bullet\to N_\bullet$ is an isomorphism of situses iff $f:M\to N$ is an quasi-isometry.
Given a filter $\mathfrak{F}$ on a set $X$, there is a coarsest situs structure on $X$ viewed as a simplicial set (i.e. the simplicial set $|X|_\bullet$ represented by $X$) such that its filter on the set $X$ of $0$-simplices is finer than $\mathfrak{F}$. Dually, there is a finest situs structure on $|X|_\bullet$ such that its filter on the set of $0$-simplices is coarser than $\mathfrak{F}$. We denote these situses by $|X^\mathfrak{F}|_\bullet^{\operatorname{cart}}$ and $|X^\mathfrak{F}|_\bullet^{\operatorname{diag}}$, respectively.
In fact this gives two fully faithful embeddings of the category of filters into the category of situses
In a similar way one can define two fully faithful embeddings of the category of filters on preorders and continuous monotone maps. We denote these by $X^{\leq\mathfrak{F}}_\bullet^{\operatorname{cart}}$ and $X^{\leq\mathfrak{F}}_\bullet^{\operatorname{diag}}$, respectively.
More generally, given an arbitrary simplicial set $X_\bullet$ and a filter $\mathfrak{F}$ on the set of $n$-simplices $X_n$, there is a coarsest/finest situs structure on $X_\bullet$ such that its filter on the set of $n$-simplices is finer/coarser than $\mathfrak{F}$. Taking $n=0$ and the filter $\mathfrak{F}$ always indiscrete gives two fully faithful embeddings of the category of simplicial sets into the category of situses.
We use this to define situses corresponding to uniform and topological spaces.
Take a set $X$ and view it as a simplicial set $X_\bullet$ (represented by $X$). A uniform structure on $X$ is a filter on $X\times X$; take the coarsest situs structure with this filter on the set $X\times X$ of $1$-simplices. This is the situs associated with the uniform structure on $X$.
In fact, it is easy to define uniform spaces in terms of situses. A filter on $X\times X$ is a uniform structure iff it is symmetric (i.e. the endomorphism of $X\times X$, $(x,y)\mapsto (y,x)$ permuting the coordinates is continuous) and this construction produces a situs such that the filter on $X\times X\times X$ is the coarsest filter such that the two maps $X^3\to X\times X$, $(x_1,x_2,x_3)\mapsto (x_i,x_{i+1}),i=1,2$ are continuous. This can be used to characterise situses arising from uniform structures as those symmetric situses such that the filter of $X\times X\times X$ has this property. We say that a situs is symmetric iff it factors though the category of non-empty finite sets.
The situs associated to a topological structure on $X$ is defined in the same way starting from the filter of non-uniform neighbourhoods of the diagonal on $X\times X$ defined as consisting of the subsets of form $\bigcup_{x\in X} \{x\}\times U_x$ where $U_x\ni x$ is a not necessarily open neighbourhood of $x\in X$.
A trivial verification shows these constructions define fully faithful embeddings of the categories of topological and of uniform spaces into the category of situses, and in fact there are corresponding forgetful functors to these categories such that the following compositions are the identity:
A filter $\mathfrak{F}$ on a metric space $M$ is Cauchy iff $\mathfrak{F}^{\operatorname{cart}}_\bullet \to M_\bullet$ is continuous.
A sequence of functions $f_i:L \to M$, $i\in \mathbb{N}$ of metric spaces is uniformly equicontinuous iff the map $(\mathbb{ N}^{cofinite})^{\operatorname{diag}}_\bullet\times L_\bullet \to M_\bullet$ is continuous.
A sequence of functions $f_i:X \to M$, $i\in \mathbb {N}$ from a topological space $X$ to a metric space $M$ is equicontinuous iff the map $(\mathbb{ N}^{cofinite})^{\operatorname{diag}}_\bullet\times X_\bullet \to M_\bullet$ is continuous.
An endomorphism of the category $\Delta$ of finite linear orders gives rise to an endomorphism of the category of situses. Of particular interest is the shift endomorphism $\Delta\to\Delta$ adding a new least element (decalage considers the endomorphism adding a new greatest element rather than least) on objects, $n\mapsto 1+n$, and on morphisms, $f:n\to m$ goes to $f[+1]:1+n\to 1+m$, $f(0)=0$, $f[+1](1+i)=1+f(i)$, $0\leq i\leq n$. The object $X_\bullet\circ [+1]$ and morphism $X_\bullet\circ [+1]\to X$ allows one to talk about local properties of $X_\bullet$, e.g. limits and local triviality.
For example, taking a limit of a filter $\mathfrak{F}$ on a topological or metric space $X$ corresponds to taking the factorization
Indeed, the underlying simplicial set of $\mathfrak{F}^{\operatorname{diag}}_\bullet$ is connected and thus maps to a single connected component of $X_\bullet\circ [+1]=\sqcup_{x\in X} \{x\}\times X_\bullet$ (here we consider the equality of the underlying simplicial sets); continuity of the map $\mathfrak{F} \to X\times X$, $x\mapsto (x_0,x)$ means exactly that the first coordinate $x_0$ is a limit point of $\mathfrak{F}$ on $X$.
Let $\mathbb{N}^{cofinite}$ and $\mathbb{N}^{\leq cofinite}$ denote the set, resp. the linear order $\mathbb{N}^\leq$, equipped with the filter of cofinite subsets.
A metric space $M$ is complete iff either of the following equivalent conditions holds:
i. $\bot\to (\mathbb{N}^{\leq cofinite})^{\operatorname{cart}}_\bullet \rightthreetimes M_\bullet\circ [+1]\to M_\bullet$.
ii. $\bot\to (\mathbb{N}^{cofinite})^{\operatorname{cart}}_\bullet \rightthreetimes M_\bullet\circ [+1]\to M_\bullet$.
iii. $(\mathbb{N}^{\leq cofinite})^{\operatorname{cart}}_\bullet\to (\mathbb{N}^{\leq cofinite}\cup\{\infty\})^{\operatorname{cart}}_\bullet \rightthreetimes M_\bullet\to\top$.
Such a reformulation raises the question whether the notion of completeness may be defined with help of the archetypal counterexample: is a metric space $M$ complete iff
It also allows to define the completion of a metric space in terms of a weak factorisation system
A topological space $X$ is compact iff for each ultrafilter $\mathfrak{U}$ either of the following equivalent conditions holds:
i. $\bot\to \mathfrak{U}^{\operatorname{diag}}_\bullet \rightthreetimes X_\bullet\circ [+1]\to X_\bullet$.
ii. $\mathfrak{U}^{\operatorname{diag}}_\bullet \to (\mathfrak{U}\cup\{\infty\})^{\operatorname{diag}}_\bullet \rightthreetimes X_\bullet\to \top$.
A topological space $K$ is compact iff
Here $\{\{o\},\{o,1\}\}$, $\{\{1\},\{o,1\}\}$, and $\{\{o,1\}\}$ are viewed as filters on the set $\{o,1\}$. (needs verification)
A map $f:X\to Y$ of topological or metric spaces is locally trivial with fibre $F$ iff in sዋ becomes a direct product with $F_\bullet$ (“globally trivial”) after base-change $Y_\bullet\circ [+1]\to Y_\bullet$. That is,
$f_\bullet:(Y_\bullet\circ [+1])\times_{Y_\bullet} X_\bullet \to Y_\bullet\circ [+1]$ is of form $(Y_\bullet\circ [+1])\times F_\bullet \to Y_\bullet\circ [+1]$.
The notion of geometric realization involves topological spaces and simplicial sets, which both are situses. This allows one to interpret the Besser-Drinfeld-Grayson construction of geometric realisation in sዋ, as follows.
View the standard geometric simplex in $\mathbb{R}^n$ as the space of monotone maps $[0,1]^\leq \to (n+1)^\leq$ with Skorokhod-type metric $dist(f,g):= sup_{x} inf_{y} \{ |x-y| : f(x)=g(y) \}$. The category of situses allows us to view both linear orders as situses: the situs structure on $[0,1]_\bullet$ “remembers” the metric, and the situs structure on $\Delta_n=Hom(-,(n+1)^\leq)$ “remembers” the equality $f(x)=g(x)$, i.e. is the finest situs structure such that the filter on the set of 0-simplices is indiscrete.
Then one may define a situs structure on the inner hom
of the underlying simplicial sets motivated by the definition of Skorokhod metric.
This gives the construction of geometric realisation due to
Besser, Drinfeld, and Grayson. See details at section 3.2 of geometric realization.
Let us now describe how situses can be used to reformulate two notions of model theory: stability and simplicity of a first-order theory.
Consider a model $M$ in a language $\mathcal {L}$, and a linear order $I$. For an $r$-ary $\mathcal {L}$-formula $\phi(x_1,...,x_r)$, we say that a sequence $(a_i)_{i\in I}$ of elements of $M$ is $\phi$-indiscernible (with repetitions) iff for either all or none of the subsequences $(a_{i_1},...,a_{i_r}), i_1\le ... \le i_r$ (of distinct elements) the formula $\phi(a_{i_1},...,a_{i_r})$ holds in $M$.
Equip $M^n$ with the filter generated by the sets of all $n$-tuples which are $\phi$-indiscernible with repetitions, where $\phi$ varies through all $\mathcal{L}$-formulas. The situs so obtained is called {the generalised pre-Stone space of $M$ in sዋ} because the forgetful functor $sዋ\to Top$ takes it to the set of elements of $M$ equipped with the preimage of Stone topology?; by this we mean the topology on $M$ generated by sets of elements realising unary $\mathcal{L}$-formulas. There are many variants of this definition, notably instead of being $\phi$-indiscernible one may require being a part of an {infinite} $\phi$-indiscernible sequence, and instead of $M^n$ consider the set of $n$-types $S_n(\emptyset)$ or $S_n(M)$.
We shall reformulate the following characterisation of stable theories as a lifting property in sዋ.
A first-order theory is stable iff in a saturated enough model it holds that each $\phi$-indiscernible sequence of $n$-tuples is in fact a $\phi$-indiscernible set, for each $n\ge 0$ and each formula $\phi$ of the language of the theory.
For $n=1$ this can be reformulated as a lifting property in sዋ as follows.
Fix a linear order $I$.
Let $I^\leq_\bullet:=(T^\leq)^{\operatorname{cart}}_\bullet$ be the situs associated with the preorder $I^\leq$ with the indiscrete filter. Recall that this is the simplicial set $n^\leq \mapsto Hom_{preorders} (n^\leq, I^\leq)$ represented by $I^\leq$ as a linear order, equipped with indiscrete filters. Let $(I^{\leq tails})^{\operatorname{cart}}_\bullet$ denote the situs associated with the preorder $I^\leq$ with the {filter of tails} generated by the subsets containing all elements large enough.
Let $(|I|^{tails})^{\operatorname{cart}}_\bullet$ denote the situs associated with the filter of tails on the set of elements of $I$.
An indiscernible sequence indexed by a linear order $I$ is an injective continuous map $(I^{\leq})^{\operatorname{cart}}_\bullet \to M_\bullet$.
An indiscernible set indexed by $I$ is an injective continuous map $|I|^{\operatorname{cart}}_\bullet \to M_\bullet$.
An eventually indiscernible sequence indexed on a linear order $I$ is an injective continuous map $(I^{\leq tails})^{ cart}_\bullet \to M_\bullet$.
Let $M$ be a model. The following are equivalent:
i. each $\phi$-indiscernible sequence of elements is in fact a $\phi$-indiscernible set.
ii. the situs $M_\bullet$ is symmetric
iii. the following lifting property holds in sዋ:
(Simon,2021) implies that a theory is stable iff the lifting property iii. holds for the situs associated with $M\times M$ considered in the language with arbitrary parameters, for a saturated enough model $M$ of the theory.
The definition of simplicity is not as simple combinatorially.
First let us introduce the situs associated with a model for this purpose; this situs structure is defined to talk about consistency of instances of a formula. Fix a formula $\phi$. As usual, the situs is based on the simplicial set represented by the set of elements of $M$. The filter on $M^n$ is generated by a single set of those tuples $(a_1,...,a_n)$ such that
$M\models \exists x \bigwedge_{1\leq i\leq n} \phi(x,a_i)$. Let us denote this situs by $M_\bullet^{\exists\phi}$ and call it the $\phi$-characteristic situs of model $M$. Note that $\phi$-characteristic situs captures the same structure as the characteristic sequence of a first order formula in (M.Malliaris. The characteristic sequence of a first-order formula. 2010), see also (M.Malliaris. “Edge distribution and density in the characteristic sequence).
Let $P_n$ denote the formula $\exists x \wedge_{i\leq n} \phi(x,y_i)$.
$\phi$ has no finite cover property
the characteristic sequence $\lt P_n\gt$ has finite support (in terminology of Malliaris, Def.2.5(2))
the situs $M^{\{\phi\}}_\bullet$ has finite dimension, i.e. for some $k$ for each $n$ the filter on $M^{\{\phi\}}_\bullet(n)=M^n$ is the coarsest filter such that all the simplicial maps $M^{\{\phi\}}_\bullet(n)=M^n\to M^{\{\phi\}}_\bullet(k)=M^k$ are continuous
there is $k$ such that for each $n$ $\exists x \wedge_{i\leq n} \phi(x,y_i)$ holds iff $\exists x \wedge_{i\leq n} \phi(x,y_i)$ for any $k$-element subset $y_{i_1},...,y_{i_k}$
there is $k$ such that for each $n$ $\exists x \wedge_{i\leq n} \phi(x,y_i)\leftrightarrow \wedge_{1\leq i_1\le ...\le i_k\le n}\exists x \wedge_{1\leq l\leq k} \phi(x,y_{i_l})$
Items 1 and 2 are Remark 2.6 of Malliaris, item 4 and 5 are both item 2 and item 3 written explicitly.
The reformulation in terms of situses uses the definition of a simple first-order theory which says that each formula of the theory has NTP (“not the tree property”) see Tent-Ziegler, Def.7.2.1, or 3,\S9 NTP is defined as a lifting property with respect to a morphism involving the following combinatorial structures.
We recall the definition of NTP and a simple theory.
[Tent-Ziegler,7.2.1] 1. A formula $\varphi(x, y)$ has the tree property (with respect to k) if there is a tree of parameters $(a_s\,\,|\,\,\emptyset \neq s \in {}^{\lt\omega}\omega )$ such that:
a) For all $s\in {}^{\lt\omega}\omega$, $(\varphi(x, a_{si} )\,\,|\,\,i \lt\omega )$ is $k$-inconsistent.
b) For all $\sigma\in {}^{\omega}\omega$ $\{\varphi(x, a_s )\,\,|\,\,\emptyset \neq s\subseteq \sigma \}$ is consistent.
Let $T^\leq$ be an infinitely branching tree of infinite depth, viewed as preorder, and equipped with the indiscrete filter.
We may take $T^\leq$ to be ${}^{\lt\omega}\omega$. Let $T^\leq_\bullet:=(T^\leq)^{cart}_\bullet$ denote the corresponding situs. Recall that by definition $T^\leq_\bullet(n)$ is the set of ordered (weakly increasing) $n$-tuples of vertices of $T$, and there is only one large subset, namely the whole set.
Note that to give a morphism $T^\leq_\bullet \to M_\bullet^{\exists\phi}$ of the underlying simplicial sets is the same as to give a a tree of parameters $(a_s\,\,|\,\,\emptyset \neq s \in {}^{\lt\omega}\omega )$. This morphism is continious iff these parameters satisfy item b, in notation of the definition: indeed, continuity means that the preigame of the large (by definition) set of $\phi$-consistent tuples is large, i.e. the whole set $T^\leq_\bullet(n)$ of ordered tuples, for each $n$.
Let $|T|_\bullet$ be the simplicial set represented by the set $|T|$ of vertices of $T$, namely $|T|_\bullet(n^\leq)=|T|^n$.
Let $|T|^{TP}_\bullet$ denote the simplicial set $|T|_\bullet$ equipped with the $TP$-tautological filter on $|T|_\bullet(n^\leq)$ defined as follows: a subset is not small iff it either contains
1) some tuple in weakly increasing order, or
2) all the lexicographically ordered tuples required to be inconsistent by the tree property with respect to a subtree-counterexample to the tree property.
In more detail, a subset $\epsilon$ is large iff
1’) it contains the subset of tuples in weakly increasing order
2’) for each isomorphic copy of $T'={}^{\lt\omega}\omega$ in $T^\leq$ there is a vertex $v\in T'$ and its immediate (in $T'$) descendants $v_1\leq_{lex}...\leq_{lex} v_k$ such that $(v_1,..,v_k) \in \epsilon \cap T'$. %A verification shows that this indeed defines a filter.
Note that by item (i) the map $T^\leq_\bullet \to |T|_\bullet^{TP}$ is continuous. Also note that no tuple of increasing elements is required to be $\phi$-inconsistent by the tree property.
The following are equivalent:
i. the formula $\phi$ has NTP with respect $k$ in the model $M$
ii. in sዋ there is no morphism $\tau:T^\leq_\bullet\to M_\bullet^{\exists\phi}$ such that for each tuple $k$-tuple $v_1,..,v_k$, for each $k$, of immediate descendants of the same vertex, $M \models\neg \exists x (\phi(x,\tau(v_1))\wedge ... \wedge \phi(x,\tau(v_n))$
iii. In sዋ the following lifting property holds: $T^\leq_\bullet \to |T|^{TP}_\bullet \rightthreetimes M_\bullet^{\exists\phi}\to \top$
ii. is exactly the definition of NTP for formula $\phi$ as stated in (Tent-Ziegler, Def.7.2.1), cf. 3,\S9. In iii., one only needs to check that the unique lifting is continuous, namely that the set of tuples $(v_1,...,v_k)$ such that $M \models \exists x (\phi(x,\tau(v_1))\wedge ... \wedge \phi(x,\tau(v_k))$ is large. By the definition of the filter, this set is large iff there is an infinitely branching subtree of infinite depth satisfying ii. This implies that ii. and iii. are equivalent.
Finally, let us prove our $TP$-tautological filters are well-defined. We need only to show that the union of any two small sets $X\cup Y$ is small. Assume it is not small. Label each vertex of the tree with the largest $n\lt\omega$ such that the first small subset contains above the vertex all tuples required to be inconsisent in some copy of ${}^{\lt n}\omega$. Above each vertex in $X$ there are at most finitely many vertices in $X$ labelled by the same or greater number. Removing them leaves $X\cup Y$ not small. But then we get that vertices of $X$ are labelled by numbers strictly decreasing along any branch, hence $X$ is of finite depth. Now pick a vertex labelled $0$. This means that below that vertex there is no infinite set of siblings that each lexicographically ordered tuple is in $X$, hence among any infinite set of siblings by Ramsey theorem there is an infinite set of siblings not in $X$, i.e. in $Y$. Hence, $Y$ is not small.
One can similarly define $TP_i$-tautological situs of a tree $T$, for $i=1$, and see that $TP_i$ is defined by a lifting property. The same argument gives lifting properties related to cdt, inp, and sct patterns in classification theory.
This raises the question whether $NTP=NTP_1\&NTP_2$ holds in the category of situses. It seems the standard proof would go through if one defines the corresponding lifting properties carefully enough. In particular, to reflect the use of Ramsey theorem, it may be necessary to replace $|M|_\bullet$ by the simplicial set of types $S^M_\bullet$ where $S^M(n)$ is the set of $n$-types, and also do the same for $T^\leq_\bullet$ and $|T|^{TP}_\bullet$ for quantifier-free types in an appropriate language.
In fact, it would seem that the standard proof of $NTP=NTP_1\&NTP_2$ gives that $T^\leq_\bullet\to |T|_\bullet^{TP}$ is the pushout of $T^\leq_\bullet\to |T|_\bullet^{TP_1}$ and $T^\leq_\bullet\to |T|_\bullet^{TP_2}$, just as diagram chasing considerations show would be sufficient for the corresponding relation between the lifting properties. Though, possibly one needs to modify the definitions of the filters appropriately modified to reflect the need to use Ramsey theorem and consider the tree properties with respect all the finite conjunctions $\&_i\phi(x,y_i)$ of $\phi(x,y)$ at the same time.
The notion of a $\phi$-characteristic situs captures the same structure as the characteristic sequence of a first order formula in (M.Malliaris. The characteristic sequence of a first-order formula. 2010), see also (M.Malliaris. “Edge distribution and density in the characteristic sequence). Moreover, it appears that several properties of characteristic sequences can be defined as lifting properties in the category of situses.
We show that several dividing lines of Shelah can also be defined by lifting properties in . Namely, we show that properties NOP, NSOP, and $NSOP_n$, $n\gt 3$, can be defined by lifting properties using tautological filters.
This subsection is preliminary.
Let $\phi(-,-)$ be a binary formula, and let $M^{\{\phi\}}_\bullet$ be the situs on simplicial set $|M|_\bullet$ represented by the set of elements of $M$ where $|M|^n$ is equipped with the filter generated by the set of all $\phi$-indiscernible sequences
Recall a formula $\phi(-,-)$ has NOP (no order property) iff there no sequence $(a_i)_{i\in\omega}$ such that
Let $I^{\leq tails}_\bullet$ be the situs associated with the filter of final segments on the linear order $I$.
Let $|I|^{NOP}_\bullet$ be the simplicial set $|I|_\bullet$ represented by the set $|I|$ of elements of $I$, equipped with NOP-tautological filters defined as follows.
a. it contains each triple $(a_i,a_j,a_k)$ for $i\leq j\leq k$
b. for each infinite increasing subsequence $a_i\in I, i\in\omega$ there is a pair $i\lt j$ such that $(a_i,a_j,a_i)\in X$
We call this filter tautological because by definition any large subset contains a “witness” $\phi(a_i,a_j)\leftrightarrow \phi(a_j,a_i)$ of failure of the order property. More precisely, by very definition both maps $a_\bullet: I^{\leq tails}_\bullet\to M^{\{\phi\}}$ and $a_\bullet: |I|^{NOP}_\bullet\to M^{\{\phi\}}$ are continuous iff (a) $a_i$ is a sequence such that $\phi(a_i,a_j)$ whenever $i\leq j$, and (b) each (infinite) subsequence of $(a_i)_{i\in I}$ does not have the order property for $\phi$. A little argument using Ramsey theorem shows that the NOP-tautological filter is indeed a filter, as follows. Assume that the intersection $X\cap Y$ of two large subsets is not large, i.e. there is an infinite subsequence $a_{i_l}$ such that for each $k\lt l$ either $(a_{i_k},a_{i_l},a_{i_k})\notin X$ or $(a_{i_k},a_{i_l},a_{i_k})\notin Y$. This gives a colouring of pairs $i\lt j$ in two colours, and by Ramsey theory there is an infinite clique of the same colour, which by definition means that either $X$ or $Y$ is not large.
The following theorem summarises the considerations above.
formula $\phi$ has NOP
there no sequence $(a_i)_{i\in\omega}$ such that $\phi(a_i,a_j) \leftrightarrow i\leq j$
the following lifting property holds in sዋ:
Each map $I^\leq_\bullet \to M^{\{\phi\}}_\bullet$ corresponds to a sequence $(a_i)_{i\in I}$ such that $i\leq j\implies \phi(a_i,a_j)$.
If NOP fails, take $(a_i)_{i\in I}$ to be a witness of this. Then evidently the induced map $|I|^{NOP}_\bullet \to M^{\{\phi\}}_\bullet$ is not continuous as there are no witnesses for (b).
Assume NOP. Then for each infinite subsequence there is a witness of NOP, i.e. a tuple as in (b) contained in the preimage of the set of $\phi$-consistent tuples in $M$. Hence, this preimage is large and the induced diagonal map is continuous.
The trick behind the definition of tautological filter works for some other dividing lines such as NSOP and tree properties. We discuss the tree properties in the next section.
To define NSOP-tautological filters we need to define a notion of a witness to failure of NSOP formulated in terms of $\phi$-indiscernible sequences.
The following reformulation is convenient for us: we say that a formula $\phi(-,-)$ has NSOP’ iff there is an infinite sequence $(a_i)_I$ such that
(a) $\phi(a_i,a_j)$ iff $i\leq j$
(b) $\phi(M,a_i)\subset \phi(M,a_j)$ for $i\leq j$.
Failure of this is witnessed by
(a’) a sequence $(a_i,a_j,a_i)$ being $\phi$-indiscernible for $i\lt j$
(b’) both sequences $(x,a_i,a_j)$ and $(x,a_l,a_k)$ being $\phi$-indiscernible for $i\lt j, k\lt l$, and $j\lt l$.
More precisely:
1. If items (a) and (b) hold for $\phi$, then items (a’) and (b’) never hold
Indeed, assuming (a), item (b’) means that $\phi(x,a_i),\phi(x,a_j)$, $\neg \phi(x,a_k)$, and $\neg \phi(x,a_l)$, for some $i\lt j, k\lt l$, and $j\lt l$, which does contradict (b).
Item (a’) means that for each $i\lt j$ $\phi(a_i,a_j)\leftrightarrow \neg\phi(a_j,a_i)$, hence by Ramsey theory (a) holds for an infinite subsequence, possibly replacing $\phi$ by $\neg\phi$. So without loss of generality we may assume (a) holds for $\phi$.
Now let us prove that (a) and (b’) imply SOP’ holds for some infinite subsequence $(a_i)_{0\lt i\lt \omega}$ and
We only need to prove that (a) and (b’) implies that (b’’) $\phi'(M,a_i)\subset \phi'(M,a_j)$ for $i\leq j$. Indeed, let $i\lt j$ and $x$ be a counterexample, i.e.
$\phi(x,a_i) \wedge \neg \phi(x,a_0)\wedge \phi(x,a_\omega)$ and $\neg (\phi(x,a_j) \wedge \neg \phi(x,a_0)\wedge \phi(x,a_\omega))$. These formulae imply that $\neg\phi(x,a_j)$ and thus both $(x,a_0,a_i)$ and $(x,a_\omega,a_j)$ are $\phi$-indiscernible.
Hence we define NSOP-tautological filters on $|I\cup I^4|_\bullet$ as:
1. A subset $U\subset |I\cup I^4|^3$ is large in the {\em $NSOP$-tautological filter} on $|I\cup I^4|^3$ iff each infinite subsequence $I'\subset I$ contains a witness of (a’) and (b’), i.e.
i. there is a pair $i\lt j,i,j\in I'$ such that $(a_i,a_j,a_i)\in U$
ii. there are $i\lt j, k\lt l$, and $j\lt l$, $i,j,k,l\in I'$ such that $((i,j,k,l),i,j)\in U$ and $((i,j,k,l),a_l,a_k)\in U$
theory $T$ has NSOP
there no formula $\phi$ and an infinite sequence $(a_i)_{i\in\omega}\in M$ satisfying (a’) and (b’) above, for some saturated model $M$
for each linear order $I$, model $M$ and formula $\phi$ it holds
Let $(a_i)_{i\in I}$ be a witness for SOP. Take the corresponding map $|I|^\leq_\bullet \to M^{\{\phi\}}_\bullet$. By SOP there are no witnesses of (b’) in $M$, thus wherever I^4 is sent to, the preimage of the subset of $\phi$-consistent tuples will not contain a witness of ii., hence by definition is large in the NTP-tautological filter. Therefore the lifting property fails.
Now assume NOP and let us show the lifting property holds. The map $|I|^\leq_\bullet \to M^{\{\phi\}}_\bullet$ corresponds to a sequence $(a_i)_{i\in I}$ witnessing (a). If this sequence has only finitely many distinct elements, then (b) is witnessed by all tuples where $i,j,k,l$ belong to an infinite constant subsequence, and therefore the preimage of the subset of $\phi$-consistent tuples is large.
Thus we may assume that all $a_i$‘s are distinct. Take the diagonal map sending each $(i,j,k,l)\in I^4$ into a witness of (b’) whenever it exists. Each infinite subsequence also fails SOP, hence there is a witness of (b’) for this. Hence, the preimage of the subset of $\phi$-consistent tuples is large.
Recall that a formula $\phi$ has $SOP_n$ iff there is an infinite sequence $(a_i)_{i\in \omega}$ such that
(a) $\phi(a_i,a_j)$ iff $i\leq j$
($b_n$) there are $(a_i)_{0\leq i \leq n-1}$ such that $\phi((a_i,a_{(i+1)mod\,n})$ for any $0\leq i\leq n$
Say a formula $\phi$ has $SOP'_n$ iff there is an infinite sequence $(a_i)_{i\in \omega}$ such that
(a) $\phi(a_i,a_j)$ iff $i\leq j$
($b'_n$) there are $(a_i)_{0\leq i \leq n-1}$ such that for any $0\leq i\leq n$ the sequence $(a_i,a_{(i+1)mod\,n},a_{(i+2)mod\,n})$ is $\phi$-indiscernible.
It is easy to see that for $n\gt 3$ $SOP'_n(\phi)\leftrightarrow SOP_n(\phi)\vee SOP_n(\neg\phi)$.
Indeed, either there is a 3-cycle $\phi(a_i,a_{(i+1)mod\,n}), \phi(a_{(i+1)mod\,n},a_{(i+2)mod\,n}), \phi(a_{(i+2)mod\,n},a_{i})$ or for each $0\leq i\less n$ $\neg\phi(a_{(i+2)mod\,n},a_{i})$, hence there is an $n$-cycle $n-1,..,(n-1-2k)mod\,n,..(n-1-2n)mod\,n$ for $\neg\phi$.
Let us reformulate this as a lifting property.
Let $|\{0,...,n-1\}|^{cycle}_\bullet$ be the situs associated with the filter on $|\{0,...,n-1\}|^3$ generated by the set of triples $(a_i,a_{(i+1)mod\,n},a_{(i+2)mod\,n})$, $0\leq i\leq n-1$.
A formula $\phi$ has $NSOP'_n$ iff $\phi$ has NOP and the following lifting property holds:
The proof is straightforward.
Some of these constructions are sketched in the drafts below.
Topology and analysis:
Geometric realisation:
Stability and simplicity:
[TentZiegler] K.Tent, M.Ziegler. A Course in Model Theory. CUP. 2012.
Maryanthe Malliaris. The characteristic sequence of a first-order formula. J Symb Logic 75, 4 (2010) 1415-1440. (pdf)
Maryanthe Malliaris. “Edge distribution and density in the characteristic sequence,” Ann Pure Appl Logic 162, 1 (2010) 1-19. pdf
[Scow2012] Lynn Scow. Characterization of nip theories by ordered graph-indiscernibles. Annals of Pure and Applied Logic, 163(11):1624 – 1641, 2012. (pdf)
[Simon2021] Pierre Simon. A note on stability and NIP in one variable. (pdf)
[3] Misha Gavrilovich. Remarks on Shelah’s classification theory and Quillen’s negation. (pdf)
Last revised on May 8, 2021 at 07:58:54. See the history of this page for a list of all contributions to it.