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Tarek Sayed Ahmed

Publications and source records attributed to Tarek Sayed Ahmed.

At least 37 records · Page 2Linked to original sources
arXiv2013

Cylindric and polyadic algebras, new perspectives

We generalize the notion of Monk's schema in such a way to integrate finite dimensions. This allows us to lift a plathora of deep results proved for finite dimensions to the infinite dimensional case, like the solution to problem 2.12 in Henkin Monk and Tarski part one, solved by Hirsch and Hodkinson. This lifting argument was already used in a joint paper with Robin Hirsch, but in a narrower context, accepted for publication in the Journal of Symbolic Logic. We also give a general new definition of a schema for infinite dimensions covering Monk's schema and Halmos' schema. Several algebraic properties (like amalgamation) are proved for instances of systems of varieties definable by such a schema like MV algebras, reducts of Heyting polyadic algebras and Ferenczi's cylindric polyadic algebras. Finally, two serious errors in two publications in prestigeous journals are pointed out. One is fixed, the alledged result in the second is weakened.

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arXiv2013

Atom-canonicity, relativized representations and omitting types for clique guarded semantics and guarded logics

We study atom canonicity for several varieties of cylindric like algebras that contain properly the variety of representable algebras. The algebras in such varieties have relativized representations, and we thereby obtain many omitting types theorems, both negative and positive for finite variable fragments and / or modifications of first order logic. Negative results are obtained when we keep usual syntax and relativize models (so that they witness commutativity of quantifiers only locally) and positive ones are obtained when we weaken 'commutativity of quantifiers' in the syntax and relativize semantics differently. Such algebras have weak neat embedding properties, too, in the sense that they embed into neat reducts of algebras in higher dimensions possibly finite. In the second part of the paper various notions of representability originally formulated for atom structure are lifted in an obvious way to the algebra level, like weak and strong representability. Such classes of algebras (that are atomic) are characterized completely via neat embeddings. Finally, several model theoretic questions on such classes consisting of algebras having weak neat embedding properties and relativized representations, like decidability of their equational or universal theory, their finite axiomatizability if first order definable, are posed and answered.

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arXiv2013

Strongly representable atom structures and neat embeddings

In this paper we give an alternative construction using Monk like algebras that are binary generated to show that the class of strongly representable atom structures is not elementary. The atom structures of such algebras are cylindric basis of relation algebras, both algebras are based on one graph such that both the relation and cylindric algebras are representable if and only if the chromatic number of the graph is infinite. We also relate the syntactic notion of algebras having a (complete) neat embedding property to the semantical notion of having various forms of (complete) relativized representations. Finally, we show that for n>5, the problemn as to whether a finite algebra is in the class SNr_3CA_6 is undecidable. In contrast, we show that for a finite algebra of arbitary finite dimensions that embed into extra dimensions of a another finite algebra, then this algebra have a finite relativized representation. Finally we devise what we call neat games, for such a game if \pe\ has a \ws \ on an atomic algebra \A in certain atomic game and \pa has a \ws in another atomic game, then such algebras are elementary equivalent to neat reducts, but do not have relativized (local) complete represenations. From such results, we infer that the omitting types theorem for finite variable fragments fails even if we consider clique guarded semantics. The size of cliques are determined by the number of pebbles used by \pa\.

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arXiv2013

On completions, neat embeddings and omittings types, yet again

In this paper we investigate using the methodology of algebraic logic, deep algebraic results to prove three new omitting types theorems for finite variable fragments of first order logic. As a sample, we show that it T is an L_n theory and |T|=lambda, lambda a regular cardinal, if T admits elimination of quantifiers, then T omits < 2^λ many non isolated {\it maximal} types. This is basically a result of Shelah's restricted to L_n. that is not completely representable. We also show, using a rainbow construction for cylindric algebras, that the omitting types theorem fails for L_n even if we consider clique guarded semantics. This is done by constructing a an atomic \A\in \PEA_n with countably many atoms (which are coloured graphs) who Sc (Pinter's) reduct is not in S_c\Nr_n\Sc_{n+3}, but $A$ is elementary equivalent to a countable completely representable (polyadic equality) algebra. Various connections between the notions of strong representability and complete representability are given in terms of neat embeddings. Several examples, using rainbow constructions and Monk-like algebras are also given to show that our results are best possible. As a sample we show that, assuming the existence of certain finite relation algebras, that for any k\in ω, there exists \A\in {\sf RPEA}_n\cap \Nr_n\PEA_{n+k} such that Rd_{\sf Sc}\Cm\At\A\notin S\Nr_n\Sc_{n+k+1}. This implies that for any finite n\geq 3, for any k\geq 0, there is an L_n theory and a type Γsuch that Gamma is realized in every n+k+1 relativized smooth model, but cannot be isolated by a witness using n+k variables.

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arXiv2013

For finite n\geq 3, and k\geq 4, the variety SNr_n\CA_{n+k} is not atom canonical

We show that there exists an atomic representable polyadic equality algebra of finite dimension n\geq 3, such that the cylindric reduct of its completion is not in SNr_n\CA_{n+4}, hence the result in the title. This solves an open problem in algebraic logic, though the values for k=n+1, n+2, n+3, is still, to the best of our knowlege unknown.

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arXiv2013

A polyadic algebra of infinite dimension is completely representable if and only if it is atomic and completely additive

We prove the result in the title. We infer, that unlike cylindric algebras, there is a first order axiomatization of the class of completely representable polyadic algebras of infinite dimension, though the one we obtain is infinite; in fact uncountable, but shares a single schema, stipulating that the (uncountably many)substitution operators are completely additive. Similar results are obtained for non commutative reducts of polyadic equality algebras of infinite dimensions, where we can drop complete additivity. However, it remains unknown to us whether there are atomic polyadic algebras of infinite dimension that are not completely additive; but we strongly conjecture that there are.

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arXiv2013

There is no finite variable axiomatization for various diagonal free algebras

We show, using a ranbow construction for cylindric algebras, that for any class K between diagonal free cylindric algebras and polyadic equality algebras of finite dimension > 2, there is no finite variable universal axiomatization for the class of representable algebras. This solves an old open problem in algebraic logic, formulated by Sain and Thompson back in 1990.

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arXiv2013

Blowing up and blurring finite Monk and rainbow algebras

We use Monk like algebras to give a new proof that the classes of strongly representable relation algebras and finite dimensional cylindric algebras of dimension >2 are not elementary. Our construction is based on relation algebras have cylindric basis, so that we obtain the result for both in one go, since they are both based on the same graph. The proof also uses Erdos' graphs. We also show using a rainbow construction that the class SNr_n\CA_{n+k} is not atom canonical, for any k\geq 4.

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arXiv2013

Strongly representable algebras

We give a simpler proof of a result of Hodkinson in the context of a blow and blur up construction argueing that the idea at heart is similar to that adopted by Andréka et all \cite{sayed}. The idea is to blow up a finite structure, replacing each 'colour or atom' by infinitely many, using blurs to represent the resulting term algebra, but the blurs are not enough to blur the structure of the finite structure in the complex algebra. A reverse of this process exists in the literature, it builds algebras with infinite blurs converging to one with finite blurs. This idea due to Hirsch and Hodkinson, uses probabilistic methods of Erdos to construct a sequence of graphs with infinite chromatic number one that is 2 colourable. This construction, which works for both relation and cylindric algebras, further shows that the class of strongly representable atom structures is not elementary. We will generalize such a result for any class of algebras between diagonal free algebras and polyadic algebras with and without equality, then we further discuss possibilities for the infinite dimensional case. Finally, we suggest a very plausible equivalence, and that is: If $n>2$, is finite, and $\A\in \CA_n$ is countable and atomic, then $\Cm\At\A$ is representable if and only if $\A\in \Nr_n\CA_ω$. We could prove one side.

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arXiv2013

Various interplays between relation and cylindric algebras

Using model theoretic techniques that proved that the class of $n$ neat reducts of $m$ dimensional cylindric algebras, $\Nr_n\CA_m$, is not elementary, we prove the same result for $\Ra\CA_k$, $k\geq 5$, and we show that $\Ra\CA_k\subset S_c\Ra\CA_k$ for all $k\geq 5$. Conversely, using the rainbow construction for cylindric algebra, we show that several classes of algebras, related to the class $\Nr_n\CA_m$, $n$ finite and $m$ arbitrary, are not elementary. Our results apply to many cylindric-like algebras, including Pinter's substitution algebras and Halmos' polyadic algebras with and without equality. The techniques used are essentially those used by Hirsch and Hodkinson, and later by Hirsch in \cite{hh} and \cite{r}. In fact, the main result in \cite{hh} follows from our more general construction. Finally we blow up a little the {\it blow up and blur construction} of Andréka nd Németi, showing that various constructions of weakly representable atom structures that are not strongly representable, can be formalized in our blown up, blow up and blur construction, both for relation and cylindric algebras. Two open problems are discussed, in some detail, proposing ideas. One is whether class of subneat reducts are closed under completions, the other is whether there exists a weakly representable $ω$ dimensional atom structure, that is not strongly representable. For the latter we propose a lifting argument, due to Monk, applied to what we call anti-Monk algebras (the algebras, constructed by Hirsch and Hodkinson, are atomic, and their atom structure is stongly representable.)

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arXiv2013

On neat atom structures for cylindric like algebras

(1) Let 1\leq k\leq ω. Call an atom structure αweakly k neat representable, the term algebra is in \RCA_n\cap \Nr_n\CA_{n+k}, but the complex algebra is not representable. Call an atom structure neat if there is an atomic algebra \A, such that \At\A=α, \A\in \Nr_n\CA_ω and for every algebra $\B$ based on this atom structure there exists k\in ω$, k\geq 1, such that \B\in \Nr_n\CA_{n+k}. (2) Let k\leq ω. Call an atom structure αk complete, if there exists \A such that \At\A=αand \A\in S_c\Nr_n\CA_{n+k}. (3) Let k\leq ω. Call an atom structure $α$ k neat if there exists \A such that \At\A=α, and \A\in \Nr_n\CA_{n+k}. (4) Let K\subseteq \CA_n, and Łbe an extension of first order logic. We say that \K is well behaved w.r.t to Ł, if for any \A\in \K, A atomic, and for any any atom structure βsuch that \At\A is elementary equivalent to β, for any \B, \At\B=β, then B\in K. We investigate the existence of such structures, and the interconnections. We also present several K's and L's as in the second definition. All our results extend to Pinter's algebras and polyadic algebras with and without equality.

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arXiv2013

Strongly representable atom structures

Using constructions of Hirsch and Hodkinson, we show that the class of strongly atom structures for various cylindric-like algebras is not elementary. This applies to diagonal free reducts and polyadic algebras with and without equality. This follows from the simple observation, that the cylindric atom structures of dimension n, constructed by Hirsch and Hodkinson, are built from algebras that can be easily expanded to polyadic equality algebras, and that such expansions are generated by elements whose dimension sets <n.

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arXiv2013

Blow up and Blur constructions in Algebraic Logic

The idea in the title is to blow up a finite structure, replacing each 'colour or atom' by infinitely many, using blurs to represent the resulting term algebra, but the blurs are not enough to blur the structure of the finite structure in the complex algebra. Then, the latter cannot be representable due to a {finite- infinite} contradiction. This structure can be a finite clique in a graph or a finite relation algebra or a finite cylindric algebra. This theme gives examples of weakly representable atom structures that are not strongly representable. Many constructions existing in the literature are placed in a rigorous way in such a framework, properly defined. This is the essence too of construction of Monk like-algebras, one constructs graphs with finite colouring (finitely many blurs), converging to one with infinitely many, so that the original algebra is also blurred at the complex algebra level, and the term algebra is completey representable, yielding a representation of its completion the complex algebra. A reverse of this process exists in the literature, it builds algebras with infinite blurs converging to one with finite blurs. This idea due to Hirsch and Hodkinson, uses probabilistic methods of Erdos to construct a sequence of graphs with infinite chromatic number one that is 2 colourable. This construction, which works for both relation and cylindric algebras, further shows that the class of strongly representable atom structures is not elementary.

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arXiv2013

The elementary closure of the class Nr_nCA_m for m\geq n+1 is not finitely axiomatizable, futhermore for any finite k\geq 1, there is A\in Nr_ωCA_{\omeg+k}that is not SNr_ωCA_{ω+k+1}

We show that for 1<n<m, the class Nr_nCA_m known to be non-elementary is pseudo elementary. When n and m are finite we use a two sorted theory, when n is finite and m infinite we use a three sorted one, and finally when both are infinite we use a four sorted defining theory. Our non finite axiomatizability result, follows from the fact that for 2<n<m, and any r\in ωthere exists a finite (Monk like) algebra C(m,n,r), such that C(m,n,r)\in Nr_nCA_m C(m,n,r)\notin SNr_nCA_{m+1}, and any non trivial ultraproduct on r of such algebras in in ElNr_nCA_m. Finally we use such algebras, to show that for infinite dimension there is an algebra A\in Nr_α\CA_{\apha+k} that is not in SNr_αCA_{α+k+1} (αan infinite ordinal).

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arXiv2013

Logics to which the class of neat reducts is sensitive to

Let L be a quantifier predicate logic. Let K be a class of algebras. We say that K is sensitive to L, if there is an algebra in K, that is L interpretable into an another algebra, and this latter algebra is elementary equivalent to an algebra not in K. (In particular, if L is L_{ω,ω}, this means that K is not elementary). We show that the class of neat reducts of every dimension is sensitive to quantifier free predicate logics with infinitary conjunctions; for finite dimensions, we do not need infinite conjunctions.

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arXiv2013

Results on Polyadic Algebras

While every polyadic algebra ($\PA$) of dimension 2 is representable, we show that not every atomic polyadic algebra of dimension two is completely representable; though the class is elementary. Using higly involved constructions of Hirsch and Hodkinson we show that it is not elementary for higher dimensions a result that, to the best of our knowledge, though easily destilled from the literature, was never published. We give a uniform flexible way of constructing weak atom structures that are not strong, and we discuss the possibility of extending such result to infinite dimensions. Finally we show that for any finite $n>1$, there are two $n$ dimensional polyadic atom structures $\At_1$ and $\At_2$ that are $L_{\infty,ω}$ equivalent, and there exist atomic $\A,\B\in \PA_n$, such that $\At\A=\At_1$ and $\At\B= \At_2$, $\A\in \Nr_n\PA_ω$ and $\B\notin \Nr_n\PA_{n+1}$. This can also be done for infinite dimensions (but we omit the proof)

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