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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 $L_{\infty \omega }$-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., Comment: 24 pages

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 ${\mathbb{L}} _i \cong \alpha _i =\gamma_i+n_i\in Ord$, where $\gamma_i\in Lim \cup \{0\}$ and $n_i\in\omega$, defining $I_\alpha := \{ i\in I : \alpha_i = \alpha \}$, for $\alpha \in Ord$, and $J_\gamma := \{ j\in I : \gamma_j = \gamma \}$, for $\gamma\in Lim _0$, we prove that $\bigcup _{i\in I} {\mathbb{L}} _i$ is a reversible poset iff $\langle \alpha _i :i\in I\rangle$ is a finite-to-one sequence, or there is $\gamma =\max \{ \gamma _i : i\in I\}$, for $\alpha \leq \gamma$ we have $|I_\alpha |<\omega $, and $\langle n_i : i\in J_\gamma \setminus I_\gamma \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 ${\mathcal{W}} ^*$., Comment: 12 pages

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., Comment: 16 pages

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 $\langle \kappa_i :i\in I\rangle$ to be reversible iff each surjection $f :I\rightarrow I$ such that $\kappa_j =\sum_{i\in f^{-1}[\{ j \}]}\kappa_i$, for all $j\in I $, is a bijection, and characterize such sequences: either $\langle \kappa_i :i\in I\rangle$ is a finite-to-one sequence, or $\kappa_i\in {\mathbb N}$, for all $i\in I$, $K:=\{ m\in {\mathbb N} : \kappa_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 $\{ \kappa_i :i\in I \}$. 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 $\leq \omega$, 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., Comment: 18 pages

We first show that sufficient conditions for a diagonally dominant matrix to be a nonsingular one (and also an H-matrix), obtained independently by Shivakumar and Chew in 1974, and Farid in 1995, are equivalent. Then we simplify the characterization of diagonally dominant H-matrices obtained by Huang in 1995, and using it prove that the Shivakumar-Chew-Farid sufficient condition for a diagonally dominant matrix to be an H-matrix, is also necessary., Comment: 7 pages, 0 figures This manuscript is part of my Diploma Thesis defended in 2006