arXiv subjects

Nenad Morača

Publications and source records attributed to Nenad Morača.

4 recordsLinked to original sources

arXiv2018

Reversibility of Extreme Relational Structures

A relational structure $\mathbb{X}$ is called reversible iff each bijective homomorphism from $\mathbb{X}$ onto $\mathbb{X}$ is an isomorphism, and linear orders are prototypical examples of such structures. One way to detect new reversible structures of a given relational language $L$ is to notice that the maximal or minimal elements of isomorphism-invariant sets of interpretations of the language $L$ on a fixed domain $X$ determine reversible structures. We isolate certain syntactical conditions providing that a consistent $L_{\infty ω}$-theory defines a class of interpretations having extreme elements on a fixed domain and detect several classes of reversible structures. In particular, we characterize the reversible countable ultrahomogeneous graphs.

math.LO↗

arXiv2017

Reversible Disjoint Unions of Well Orders and Their Inverses

A poset ${\mathbb{P}}$ is called reversible iff every bijective homomorphism $f:{\mathbb{P}} \rightarrow {\mathbb{P}}$ is an automorphism. Let ${\mathcal{W}}$ and ${\mathcal{W}} ^*$ denote the classes of well orders and their inverses respectively. We characterize reversibility in the class of posets of the form ${\mathbb{P}} =\bigcup _{i\in I}{\mathbb{L}} _i$, where ${\mathbb{L}} _i, i\in I$, are pairwise disjoint linear orders from ${\mathcal{W}} \cup {\mathcal{W}} ^*$. First, if ${\mathbb{L}} _i \in {\mathcal{W}}$, for all $i\in I$, and ${\mathbb{L}} _i \cong α_i =γ_i+n_i\in Ord$, where $γ_i\in Lim \cup \{0\}$ and $n_i\inω$, defining $I_α:= \{ i\in I : α_i = α\}$, for $α\in Ord$, and $J_γ:= \{ j\in I : γ_j = γ\}$, for $γ\in Lim _0$, we prove that $\bigcup _{i\in I} {\mathbb{L}} _i$ is a reversible poset iff $\langle α_i :i\in I\rangle$ is a finite-to-one sequence, or there is $γ=\max \{ γ_i : i\in I\}$, for $α\leq γ$ we have $|I_α|<ω$, and $\langle n_i : i\in J_γ\setminus I_γ\rangle $ is a reversible sequence of natural numbers. The same holds when ${\mathbb{L}} _i \in {\mathcal{W}} ^*$, for all $i\in I$. In the general case, the reversibility of the whole union is equivalent to the reversibility of the union of components from ${\mathcal{W}}$ and the union of components from ${\mathcal{W}} ^*$.

math.LO↗

arXiv2017

Reversibility of Disconnected Structures

A relational structure is called reversible iff every bijective endomorphism of that structure is an automorphism. We give several equivalents of that property in the class of disconnected binary structures and some its subclasses. For example, roughly speaking and denoting the set of integers by ${\mathbb Z}$, a structure having reversible components is reversible iff its components can not be "merged" by condensations (bijective homomorphisms) and each ${\mathbb Z}$-sequence of condensations between different components must be, in fact, a sequence of isomorphisms. We also give equivalents of reversibility in some special classes of structures. For example, we characterize CSB linear orders of a limit type and show that a disjoint union of such linear orders is a reversible poset iff the corresponding sequence of order types is finite-to-one.

math.LO↗

arXiv2017

Reversible Sequences of Cardinals, Reversible Equivalence Relations, and Similar Structures

A relational structure ${\mathbb X}$ is said to be reversible iff every bijective endomorphism $f:X\rightarrow X$ is an automorphism. We define a sequence of non-zero cardinals $\langle κ_i :i\in I\rangle$ to be reversible iff each surjection $f :I\rightarrow I$ such that $κ_j =\sum_{i\in f^{-1}[\{ j \}]}κ_i$, for all $j\in I $, is a bijection, and characterize such sequences: either $\langle κ_i :i\in I\rangle$ is a finite-to-one sequence, or $κ_i\in {\mathbb N}$, for all $i\in I$, $K:=\{ m\in {\mathbb N} : κ_i =m $, for infinitely many $i\in I \}$ is a non-empty independent set, and $\gcd (K)$ divides at most finitely many elements of the set $\{ κ_i :i\in I \}$. We isolate a class of binary structures such that a structure from the class is reversible iff the sequence of cardinalities of its connectivity components is reversible. In particular, we characterize reversible equivalence relations, reversible posets which are disjoint unions of cardinals $\leq ω$, and some similar structures. In addition, we show that a poset with linearly ordered connectivity components is reversible, if the corresponding sequence of cardinalities is reversible and, using this fact, detect a wide class of examples of reversible posets and topological spaces.

math.LO↗