In this last section, we will finally show that if Aut(Ω) is countable then it is isomorphic to Zn for somen ∈
N. We first prove the following theorem.
Theorem 9.49. Let Ω be a total order and let Aut(Ω) be countable. Then Aut(Ω) is a countable abelian group.
Proof. Let f, g ∈ Aut(Ω). By Theorem 9.16, we know that for all infinite orbitalsU off andT ofg, eitherT∩U =∅orT =U. Now, by Theorem 9.48 we also know that Aut(hUi)∼=Zfor eachU. Thus sincef|U andg|U are both
automorphisms ofU by Corollary 9.8, we find that for anyu∈U,uf g =ugf. Thusf andg commute on any mutual orbital. Clearly ifv ∈VΩ is contained in no infinite orbital of f nor g then vf g = vg = vgf. Furthermore, if v
is contained in an infinite orbital of f but not of g, then vgf = vf = vf g
and similarly if u is contained in an infinite orbital of g but not of f then
vf g =vg=vgf. It now it follows thatf andg commute at all pointsu∈VΩ and hence Aut(Ω) is a countable abelian group.
Lemma 9.50. Let Ω be a total order and let
O ={U :U is an infinite orbital of f for some f ∈Aut(Ω)}.
If Aut(Ω) is countable then |O|=n for some n∈N.
Proof. For eachU ∈ O, letgU ∈Aut(Ω) be chosen such thatU is an infinite
orbital of gU. Define a map f :VΩ →VΩ by,
vf =
(
vgU if v ∈U for some U ∈ O,
v otherwise.
By Theorem, 9.16, U ∩T = ∅ for all distinct orbitals U and T and so by Lemma 9.10, f ∈ Aut(Ω). Thus f is an automorphism of Ω whose infinite orbitals are exactly those in O. Now by Lemma 9.15, f has at most a finite number of distinct infinite orbitals. Thus O must be finite as required.
We now have the main result.
Theorem 9.51. Let Ωbe a countable total order such that Aut(Ω) is count- able. Then Aut(Ω)∼=Zn for some n∈N.
Proof. Again let
O ={U :U is an infinite orbital of f for some f ∈Aut(Ω)},
and let |O| = n. By Lemma 9.50, n ∈ N and so let O = {U1, . . . , Un}.
Since Theorem 9.48 tells us that Aut(hUki) ∼= Z for all k = 1, . . . n. So let
hk ∈Aut(hUki) be chosen such thathhki ∼=Z. By Lemma 9.22, we can extend
eachhk to an automorphismgk∈Aut(Ω) such that gk|VΩ\Uk =1|VΩ\Uk. Now
define a map φ : Zn → Aut(Ω) by (i
defined and since we know from Theorem 9.16 that Aut(Ω) is an abelian group, it follows that,
((i1, . . . , in) + (j1, . . . , jn))φ =(i1+j1, . . . , in+jn)φ, =gi1+j1 1 · · ·gnin+jn, =gi1 1 · · ·g in ng j1 1 · · ·g jn n , =(i1, . . . , in)φ+ (j1, . . . , jn)φ.
Thus φ is a group homomorphism. To see that it is injective suppose that (i1, . . . , in),(j1, . . . , jn)∈Znare such that (i1, . . . , in)φ = (j1, . . . , jn)φ. Then
gi1
1 · · ·gnin = g j1
1 · · ·gnjn. Clearly since each gk is such thatgk|VΩ\Uk = 1|VΩ\Uk
and sinceUk∩Ul =∅for allk 6=l, it follows thatgikk =gkjk for allk = 1, . . . , n.
Hence ik = jk for all k = 1, . . . , n and thus (i1, . . . , in) = (j1, . . . , jn). Now
suppose that f ∈ Aut(Ω). Then by Corollary 9.8, f|Uk ∈ Aut(hUki) for
all k = 1, . . . , n and f|VΩ\∪nk=1Uk = 1. Hence f|Uk = g
ik
k for some ik ∈ Z
and f = gi1
1 · · ·gnin. Then (i1, . . . , in)φ = f and so φ is surjective. Since
we have now shown that φ is a bijective group homomorphism, the result is complete.
Corollary 9.52. SupposeH is a countable groupH -class of End(Q). Then
H ∼=Zn for some n∈N.
Proof. Let f ∈ End(Q) be the idempotent contained in H. Then, by The- orem 2.7, H ∼= Aut(imf). Thus if H is countable, Aut(imf) countable. Hence by Theorem 9.51, Aut(imf)∼=Zn for somen∈N and the result now
Chapter 10
Questions and Open Problems
In this final chapter we will discuss some questions and possible directions for further research which arise from the results presented in this thesis.
Most obviously, this thesis has dealt with only a handful of the most common Fra¨ıss´e limits. Given time, we could also ask the same type of questions about the maximal subgroups, regular D-classes andJ-classes of other Fra¨ıss´e limits. For example one could consider:
The random poset – the Fra¨ıss´e limit of the class of all finite partial orders.
The random n-independence free directed graph – the Fra¨ıss´e limit of the class of directed graphs with no independent sets of size n.
The ordered Urysohn space – the Fra¨ıss´e limit of the class of finite ordered metric spaces with rational distances.
Focussing on the Fra¨ıss´e limits that are covered in this thesis, there are many additional questions we could ask about Green’s relations on End(Ω) where Ω =R, D, T, B, Gn,Q. For example, we produced many results about
the regularD-classes in each setting, but we might naturally ask the following questions.
Question 10.1. How many non-regular D-classes of End(Ω) are there? What sizes are they?
Question 10.2. Can we gain any information on the number of H-classes contained in non-regular D-classes of End(Ω)?
The primary focus on this thesis was on groupH -classes. However might also want to investigate the H-classes which do not contain an idempotent
and are therefore not groups. In a natural way we can associate a group to such an H -class as follows.
LetH be a H -class of a semigroup S and let TH ={s ∈S1 :Hs⊆H}.
Then for each s∈TH we can define a function fs :H →H, where hfs =hs
for all h ∈ H. It is not hard to see that the set {fs : s ∈ TH} forms a
group under composition of mappings. In fact this group is known as the Schutzenberger group of the H -class H. It can be shown that if K is a maximal group H -class then the Schutzenberger group of K is isomorphic toK itself. If we let S = End(Ω) for some relational structure Ω then it can be shown that for anyD-classD, all Schutzenberger groups associated to an
H -class of D are isomorphic and are isomorphic to the group H -classes in
D (see [Mag75, Theorem 3.1]). Furthermore, if K is anyH -class of End(Ω) and k ∈ K, then the Schutzenberger groups associated to K is isomorphic to a subgroup of Aut(imk), [Mag75, Theorem 3.2]. We might now ask the following question.
Question 10.3. Which groups arise as Schutzenberger groups associated to
H -classes from non regular D-classes of End(Ω)?
In Chapter 6, we briefly discussed triangle free graphs which have property
? (recall Definition 6.9). We were able to classify the finite triangle free graphs with property ? which have exactly two maximal independent sets. As a result we provided a complete description of the groups which occur as the automorphism group of such finite triangle free graphs. However, as already mentioned in that chapter, the following is still an open problem.
Question 10.4. Which groups can occur as the automorphism group of a finite triangle-free graph with property ? which has three or more maximal independent sets?
Similarly, we also showed in Chapter 6 that the automorphism group of a countably infinite triangle-free graph with property ? which has finitely many vertices of infinite degree has cardinality 2ℵ0. A natural open problem which the arose was the following.
Question 10.5. What is the cardinality of the automorphism group of a countably infinite triangle-free graph with property ? which has infinitely many vertices of infinite degree?
In Chapter 8 we were able to show that if a total order Ω can be embedded into Q via an embedding f such that imf was a retract of Q, then Aut(Ω) was isomorphic to 2ℵ0 maximal subgroups of End(
Q). The following still
Question 10.6. Exactly which total orders Ω can be embedded into Q via an embedding f such that imf is a retract of Q? Can we find an example of a total which cannot be embedded into Q via an embedding f such that imf is a retract of Q?
Bibliography
[AG07] Geir Agnarsson and Raymond Greenlaw, Graph theory: modelling, applications and algorithms, Pearson, 2007.
[BD00] Anthony Bonato and Dejan Deli´c,The monoid of the random graph, Semigroup Forum 61 (2000), 138–148.
[BDD10] Anthony Bonato, Dejan Deli´c, and Igor Dolinka, All countable monoids embed into the monoid of the infinite random graph, Dis- crete Mathematics 310 (2010), 373–375.
[Cam97] Peter Cameron,The random graph, The Mathematics of Paul Erd˝os II, Algorithms and Combinatorics, vol. 14, pp. 333–351, Springer- Verlag, 1997.
[Cam01] , Oligomorphic permutation groups, London Mathematical Lecure Note Series 152, Cambridge University Press, 2001.
[Cam08] , Sets, logic and categories, Springer Undergraduate Math- ematics Series, Springer, London, 2008.
[CP61] Alfred H. Clifford and Gordon B. Preston, The algebraic theory of semigroups (Vol. 1), American Mathematical Society, 1961.
[Dar97] Michael R. Darnel, Theory of lattice ordered groups, Marcel Dekker Inc., 1997.
[DD04] Dejan Deli´c and Igor Dolinka, The endomorphism monoid of the random graph has uncountably many ideals, Semigroup Forum 69
(2004), 75–79.
[Dol07] Igor Dolinka, The endomorphism monoid of the random poset con- tains all countable semigroups, Algebra Universalis 56 (2007), 469– 474.
[Dol12] , A characterization of retracts in certain Fra¨ıss´e limits, Mathematical Logic Quarterly 58 (2012), 46–54.
[EKR76] Paul Erd˝os, Daniel J. Kleitman, and Bruce L. Rothschild, Asymp- totic enumeration of Kn-free graphs, International Colloquium on
Combinatorial Theory, Atti dei Convegni Lincei, vol. 2, pp. 19–27, 1976.
[ER63] Paul Erd˝os and Alfr´ed R´enyi,Asymmetric graphs, Acta Mathematica Academiae Scientiarum Hungaricae 14 (1963), 295–315.
[Fra53] Roland Fra¨ıss´e, Sur certains relations qui g´en´eralisent l’ordre des nombres rationnels, Comptes Rendus de l’Acad´emie des Sciences Paris 237 (1953), 540–542.
[Fru39] Robert Frucht, Herstellung von graphen mit vorgegebener abstrakter gruppe, Compositio Mathematica 6 (1939), 239–250.
[Goo86] Ken R. Goodearl, Partially ordered abelian groups with interpola- tion, Mathematical Surveys and Monographs 20, American Mathe- matical Society, 1986.
[Gro59] Johannes de Groot, Groups represented by homeomorphism groups, Mathematische Annalen 138 (1959), 80–120.
[Hen71] C. Ward Henson, A family of countable homogeneous graphs, Pacific Journal of Mathematics 38 (1971), no. 1, 69–83.
[Hod97] Wilfrid Hodges, A shorter model theory, Cambridge University Press, 1997.
[How95] John Howie,Fundamentals of semigroup theory, London Mathemat- ical Society Monographs, Clarendon Press, 1995.
[Mag75] K. D. Magill, A survey of semigroups of continuous selfmaps, Semi- group Forum 11 (1975), no. 1, 189–282.
[MS74] K. D. Magill and S. Subbiah, Green’s relations for regular elements of semigroups of endomorphisms, Canadian Journal of Mathematics
26 (1974), 1484–1497.
[MT11] Dugald Macpherson and Katrin Tent, Simplicity of some automor- phism groups, Journal of Algebra 342 (2011), 40–52.
[Mud10] Neboj˘sa Mudrinski,Notes on endomorphisms of Henson graphs and their complements, Ars Combinatoria 96 (2010), 173–183.
[PV10] F. Petrov and A. Vershik, Uncountable graphs and invariant mea- sures on the set of universal countable graphs, Random Structures and Algorithms37 (2010), no. 3, 389–406.
[Rad64] Richard Rado, Universal graphs and universal functions, Acta Arithmetica 9 (1964), 331–340.
[Ros99] Kenneth H. Rosen, Handbook of discrete and combinatorial mathe- matics, second ed., CRC Press, Florida, 1999.
[RS09] John Rhodes and Benjamin Steinberg, The q-theory of finite semi- groups, Springer Monographs in Mathematics, Springer, New York, 2009.
[Sab60] Gert Sabidussi, Graphs with given infinite group, Monatshefte fur Mathematik 64 (1960), 64–67.
[Tru85] John Truss, The group of the countable universal graph, Mathemat- ical Proceedings of the Cambridge Philosophical Society 98 (1985), 213–245.