The workshop "Homogeneous Structures, A Workshop in Honour of Norbert Sauer's 70th Birthday" took place at the Banff International Research Station from November 8th to November 13th, 2015. The purpose of the following note is to gather the list of open problems that were posed during the problem sessions. Contributions appear in alphabetical order, according to the name of their author.

A sibling of a relational structure $R$ is any structure $S$ which can be embedded into $R$ and, vice versa, such that $R$ can be embedded into $S$. Let $\operatorname{sib}(R)$ be the number of siblings of $R$, these siblings being counted up to isomorphism. Thomassé conjectured that for countable relational structures made of at most countably many relations, $\operatorname{sib}(R)$ is either one, countably infinite, or the size of the continuum; but even showing the special case $\operatorname{sib}(R)1$ is one or infinite is unsettled when $R$ is a countable tree. We prove that if $R$ is countable and $\aleph_{0}$-categorical, then indeed $\operatorname{sib}(R)$ is one or infinite. Furthermore, $\operatorname{sib}(R)$ is one if and only if $R$ is finitely partitionable in the sense of Hodkinson and Macpherson [14]. The key tools in our proof are the notion of monomorphic decomposition of a relational structure introduced in [35] and studied further in [23], [24], and a result of Frasnay [11].

Let $\mathrm{G}$ be a subgroup of the symmetric group $\mathfrak S(U)$ of all permutations of a countable set $U$. Let $\overline{\mathrm{G}}$ be the topological closure of $\mathrm{G}$ in the function topology on $U^U$. We initiate the study of the poset $\overline{\mathrm{G}}[U]:=\{f[U]\mid f\in \overline{\mathrm{G}}\}$ of images of the functions in $\overline{\mathrm{G}}$, being ordered under inclusion. This set $\overline{\mathrm{G}}[U]$ of subsets of the set $U$ will be called the \emph{poset of copies for} the group $\mathrm{G}$. A denomination being justified by the fact that for every subgroup $\mathrm{G}$ of the symmetric group $\mathfrak S(U)$ there exists a homogeneous relational structure $R$ on $U$ such that $\overline G$ is the set of embeddings of the homogeneous structure $R$ into itself and $\overline{\mathrm{G}}[U]$ is the set of copies of $R$ in $R$ and that the set of bijections $\overline G\cap \mathfrak S(U)$ of $U$ to $U$ forms the group of automorphisms of $\mathrm{R}$.

A relational structure is indivisible if for every partition of its set of elements into two parts there exists an embedding of the structure into one of the parts of the partition. A relational structure is homogeneous if every embedding of a finite induced substructure to a finite induced substructure extends to an automorphism. This article establishes a necessary and sufficient condition for Henson type, see [4], homogeneous structures to be indivisible.

The distinguishing number of a structure is the smallest size of a partition of its elements so that only the trivial automorphism of the structure preserves each cell of the partition. We show that for any countable subset of the positive real numbers, the corresponding countable homogeneous Urysohn metric space, when it exists, has distinguishing number 2 or the distinguishing number is infinite. While it is known that a sufficiently large finite primitive structure has distinguishing number 2, unless its automorphism group is the full symmetric group or alternating group, the infinite case is open and these countable Urysohn metric spaces provide further confirmation toward the conjecture that all primitive homogeneous countably infinite structures have distinguishing number 2 or else the distinguishing number is infinite.

A tree is scattered if it does not contain a subdivision of the complete binary tree as a subtree. We show that every scattered tree contains a vertex, an edge, or a set of at most two ends preserved by every embedding of T. This extends results of Halin, Polat and Sabidussi. Calling two trees equimorphic if each embeds in the other, we then prove that either every tree that is equimorphic to a scattered tree T is isomorphic to T, or there are infinitely many pairwise non-isomorphic trees which are equimorphic to T. This proves the tree alternative conjecture of Bonato and Tardif for scattered trees, and a conjecture of Tyomkyn for locally finite scattered trees.

There is a permutation of the vertices of a tree for which no proper subtree on at least two vertices is mapped to a subtree, if and only if twice the number of its endpoints is less than or equal to the number of points of the tree;Theorem 4.1. The following more general result follows:Let G = (V (G), E(G)) be a simple graph and let C(G) be the set of subsetsA  V (G) which induce a connected subgraph of G containing at least two vertices and let Π(G) be the set of permutations of V (G) which do not map an element of C(G) to an element of C(G). In the case where G has n vertices and at most n − 1 edges we give a necessary and sufficient condition on G so that Π(G) [SPECIAL CHARACTER]= ∅.AMS subject classification (2000): 05C70

A relational structure $\mathrm{R}$ is {\em rainbow Ramsey} if for every finite induced substructure $\mathrm{C}$ of $\mathrm{R}$ and every colouring of the copies of $\mathrm{C}$ with countably many colours, such that each colour is used at most $k$ times for a fixed $k$, there exists a copy $\mathrm{R}^\ast$ of $\mathrm{R}$ so that the copies of $\mathrm{C}$ in $\mathrm{R^\ast}$ use each colour at most once. We show that certain ultrahomogenous binary relational structures, for example the Rado graph, are rainbow Ramsey. Via compactness this then implies that for all finite graphs $\mathrm{B}$ and $\mathrm{C}$ and $k \in \omega$, there exists a graph $\mathrm{A}$ so that for every colouring of the copies of $\mathrm{C}$ in $\mathrm{A}$ such that each colour is used at most $k$ times, there exists a copy $\mathrm{B}^\ast$ of $\mathrm{B}$ in $\mathrm{A}$ so that the copies of $\mathrm{C}$ in $\mathrm{B^\ast}$ use each colour at most once.

A metric space M is homogeneous if every isometry between finite subsets extends to a surjective isometry defined on the whole space. We show that if M is an ultrametric space, it suffices that isometries defined on singletons extend, i.e that the group of isometries of M acts transitively. We derive this fact from a result expressing that the arity of the group of isometries of an ultrametric space is at most 2. An illustration of this result with the notion of spectral homogeneity is given. With this, we show that the Cauchy completion of a homogeneous ultrametric space is homogeneous. We present several constructions of homogeneous ultrametric spaces, particularly the countable homogeneous ultrametric space, universal for rational distances, and its Cauchy completion. From a general embeddability result, we prove that every ultrametric space is embeddable into a homogeneous ultrametric space with the same set of distances values and we also derive three embeddability results due respectively to F. Delon, A. Lemin and V. Lemin, and V. Fevinberg. Looking at ultrametric spaces as 2-structures, we observe that the nerve of an ultrametric space is the tree of its robust modules.

We characterize pairs of orthogonal countable ordinals. Two ordinals α and β are orthogonal if there are two linear orders A and B on the same set V with order types α and β respectively such that the only maps preserving both orders are the constant maps and the identity map. We prove that if α and β are two countable ordinals, with α≤β, then α and β are orthogonal if and only if either ω+1≤α or α=ω and β<ωβ.

A relational structure H is homogeneous if every isomorphism between finite induced substructures has an extension to an automorphism of H.A relational structure R is indivisible if for every partition (A, B) of the set of elements of R, there is a copy of R in A or in B. The structure R is weakly indivisible if for every partition (A, B) of the set of elements of R for which some finite induced substructure of R does not have a copy in A, there exists a copy of R in B. The structure R is age indivisible if for every partition (A, B) of the elements of R every finite induced substructure of R has an embedding into A or every finite induced substructure of R has an embedding into B. It follows that indivisibility implies weak indivisibility which in turn implies age indivisibility. There are many examples of countable infinite homogeneous structures which are weakly indivisible but not indivisible; see Sauer (2000) [17] and the end of Section 3.1 of this paper.In general, it is difficult to find structures which are age indivisible but not weakly indivisible. One such example (see Pouzet et al. (2011) [16]) is obtained via the inhomogeneous general linear group of vector spaces over finite fields. Finding such an example for homogeneous structures seems to be even more difficult. The only one that I obtained, together with L Nguyen Van The, is a countable homogeneous metric subspace of the Hilbert sphere l(2) (see Nguyen Van The and Sauer (2010) [13]). Both of the examples above are relational structures with infinitely many relations. One of the still open questions then is: Are there countable infinite homogeneous structures with finite signature which are age but not weakly indivisible? We will show that the generic "free amalgamation" homogeneous structures are weakly indivisible; see Section 3.1.Countable "Urysohn metric spaces" (see the next section), with finite distance sets, are indivisible (see Sauer (2013) [19]). Countable Urysohn metric spaces are age indivisible, which follows from the Hales-Jewett theorem (Hales and Jewett (1963) [7]; see Delhomme et al. (2007) [2]). For non-Urysohn homogeneous countable metric spaces, except for the example mentioned above, the weak indivisibility question is completely open. We will show, in this paper, that a countable homogeneous ultrametric space is age indivisible if and only if it is weakly indivisible. (C) 2013 Elsevier Ltd. All rights reserved.

A metric space M = (M;d ) is homogeneous if for every isometry α of a finite subspace of M to a subspace of M there exists an isometry of M onto M extending α . The metric space M is universal if it isometrically embeds every finite metric space F with dist (F) ⊆ dist= (M) . ( dist (M) being the set of distances between points of M .) A metric space M is oscillation stable if for every ϵ > 0 and every uniformly continuous and bounded function f : M →ℜ there exists an isometric copy M^∗ = (M^∗;d ) of M in M for which: sup{| f(x) - f(y)|| x,y ∈M^∗} < ϵ. Theorem. Every bounded, uncountable, separable, complete, homogeneous, universal metric space M = (M;d ) is oscillation stable. (Theorem 12.)

We investigate infinite versions of vector and affine space partition results, and thus obtain examples and a counterexample for a partition problem for relational structures. In particular we provide two (related) examples of an age indivisible relational structure which is not weakly indivisible.

For alpha an ordinal, a graph with vertex seta may be represented by its characteristic function, f : [alpha](2) -> 2, where f ({gamma, delta}) = 1 if and only if the pair ({gamma, delta} is joined in the graph. We call these functions alpha-colorings.We introduce a quasi order on the a-colorings (graphs) by setting f <= g if and only if there is an order-preserving mapping t : alpha -> alpha such that f ({gamma, delta}) = g({t(gamma), t(delta)}) for all (gamma, delta) is an element of [alpha](2). An alpha-coloring f is an atom if g <= f implies f <= g.We show that for alpha = omega(omega) below every coloring there is an atom and there are continuum many atoms. For alpha < omega(omega) below every coloring there is an atom and there are finitely many atoms. (C) 2010 Elsevier B.V. All rights reserved.

Two orders on the same set are orthogonal if the constant maps and the identity map are the only maps preserving both orders. We construct linear orders orthogonal to the order on the rationals.

A metric space $\mathrm{M}=(M;\de)$ is {\em homogeneous} if for every isometry $f$ of a finite subspace of $\mathrm{M}$ to a subspace of $\mathrm{M}$ there exists an isometry of $\mathrm{M}$ onto $\mathrm{M}$ extending $f$. A metric space $\boldsymbol{U}$ is an {\em Urysohn} metric space if it is homogeneous and separable and complete and if it isometrically embeds every separable metric space $\mathrm{M}$ with $\dist(\mathrm{M})\subseteq \dist(\boldsymbol{U})$. (With $\dist(\mathrm{M})$ being the set of distances between points in $\mathrm{M}$.) The main results are: (1) A characterization of the sets $\dist(\boldsymbol{U})$ for Urysohn metric spaces $\boldsymbol{U}$. (2) If $R$ is the distance set of an Urysohn metric space and $\mathrm{M}$ and $\mathrm{N}$ are two metric spaces, of any cardinality with distances in $R$, then they amalgamate disjointly to a metric space with distances in $R$. (3) The completion of a homogeneous separable metric space $\mathrm{M}$ which embeds isometrically every finite metric space $\mathrm{F}$ with $\dist(\mathrm{F})\subseteq \dist(\mathrm{M})$ is homogeneous.

We study finite dimensional partition properties of the countable homogeneous dense local order (a directed graph closely related to the order structure of the rationals). Some of our results use ideas borrowed from the partition calculus of the rationals and are obtained thanks to a strengthening of Milliken’s theorem on trees.

We solve the oscillation stability problem for the Urysohn sphere, an analog of the distortion problem for ℓ 2 in the context of the Urysohn space U. This is achieved by solving a purely combinatorial problem involving a family of countable ultrahomogeneous metric spaces with finitely many distances.

The age of a relational structure A of signature μ is the set age(A) of its finite induced substructures, considered up to isomorphism. This is an ideal in the poset Ωμ consisting of finite structures of signature μ and ordered by embeddability. We shall show that if the structures have infinitely many relations and if, among those, infinitely many are at least binary then there are ideals which do not come from an age. We provide many examples. We particularly look at metric spaces and offer several problems. We also answer a question due to Cusin and Pabion [R. Cusin, J.F. Pabion, Une généralisation de l’âge des relations, C. R. Acad. Sci. Paris, Sér. A-B 270 (1970) A17–A20]: there is an ideal I of isomorphism types of at most countable structures whose signature consists of a single ternary relation symbol such that I does not come from the set ageI(A) of isomorphism types of substructures of A induced on the members of an ideal I of sets.