26. CJM 2009 (vol 62 pp. 355)
 Král', Daniel; Máčajová, Edita; Pór, Attila; Sereni, JeanSébastien

Characterisation Results for Steiner Triple Systems and Their Application to EdgeColourings of Cubic Graphs
It is known that a Steiner triple system is projective if and only if it does not contain the fourtriple configuration $C_{14}$. We find three configurations such that a Steiner triple system is affine if and only if it does not contain one of these configurations. Similarly, we characterise Hall triple systems using two forbidden configurations. Our characterisations have several interesting corollaries in the area of edgecolourings of graphs. A cubic graph G is Sedgecolourable for a Steiner triple system S if its edges can be coloured with points of S in such a way that the points assigned to three edges sharing a vertex form a triple in S. Among others, we show that all cubic graphs are Sedgecolourable for every nonprojective nonaffine pointtransitive Steiner triple system S.
Categories:05B07, 05C15 

27. CJM 2009 (vol 61 pp. 1300)
 Hubard, Isabel; Orbani\'c, Alen; Weiss, Asia Ivi\'c

Monodromy Groups and SelfInvariance
For every polytope $\mathcal{P}$ there is the universal regular
polytope of the same rank as $\mathcal{P}$ corresponding to the
Coxeter group $\mathcal{C} =[\infty, \dots, \infty]$. For a given
automorphism $d$ of $\mathcal{C}$, using monodromy groups, we
construct a combinatorial structure $\mathcal{P}^d$. When
$\mathcal{P}^d$ is a polytope isomorphic to $\mathcal{P}$ we say that
$\mathcal{P}$ is selfinvariant with respect to $d$, or
$d$invariant. We develop algebraic tools for investigating these
operations on polytopes, and in particular give a criterion on the
existence of a $d$\nobreakdashauto\morphism of a given order. As an application,
we analyze properties of selfdual edgetransitive polyhedra and
polyhedra with two flagorbits. We investigate properties of medials
of such polyhedra. Furthermore, we give an example of a selfdual
equivelar polyhedron which contains no polarity (duality of order
2). We also extend the concept of Petrie dual to higher dimensions,
and we show how it can be dealt with using selfinvariance.
Keywords:maps, abstract polytopes, selfduality, monodromy groups, medials of polyhedra Categories:51M20, 05C25, 05C10, 05C30, 52B70 

28. CJM 2009 (vol 61 pp. 1092)
 Irving, John

Minimal Transitive Factorizations of Permutations into Cycles
We introduce a new approach to an enumerative problem
closely linked with the geometry of branched coverings,
that is, we study the number $H_{\alpha}(i_2,i_3,\dots)$ of ways a
given permutation (with cycles described by the partition $\a$) can be
decomposed into a product of exactly $i_2$ 2cycles, $i_3$ 3cycles,
\emph{etc.}, with certain minimality and transitivity conditions imposed on the factors. The method is to
encode such factorizations as planar maps with certain \emph{descent structure} and apply a new combinatorial
decomposition to make their enumeration more manageable. We apply our technique to determine
$H_{\alpha}(i_2,i_3,\dots)$ when $\a$ has one or two parts, extending earlier work of Goulden and Jackson.
We also show how these methods are readily modified to count \emph{inequivalent} factorizations, where
equivalence is defined by permitting commutations of adjacent disjoint factors. Our technique permits us to
generalize recent work of Goulden, Jackson, and Latour, while allowing for a considerable simplification of
their analysis.
Categories:05A15, 05E10 

29. CJM 2009 (vol 61 pp. 888)
 Novik, Isabella; Swartz, Ed

Face Ring Multiplicity via CMConnectivity Sequences
The multiplicity conjecture of Herzog, Huneke, and Srinivasan
is verified for the face rings of the following classes of
simplicial complexes: matroid complexes, complexes of dimension
one and two,
and Gorenstein complexes of dimension at most four.
The lower bound part of this conjecture is also established for the
face rings of all doubly CohenMacaulay complexes whose 1skeleton's
connectivity does not exceed the codimension plus one as well as for
all $(d1)$dimensional $d$CohenMacaulay complexes.
The main ingredient of the proofs is a new interpretation
of the minimal shifts in the resolution of the face ring
$\field[\Delta]$ via the CohenMacaulay connectivity of the
skeletons of $\Delta$.
Categories:13F55, 52B05;, 13H15;, 13D02;, 05B35 

30. CJM 2009 (vol 61 pp. 904)
 Saliola, Franco V.

The Face Semigroup Algebra of a Hyperplane Arrangement
This article presents a study of an algebra spanned by the faces of a
hyperplane arrangement. The quiver with relations of the algebra is
computed and the algebra is shown to be a Koszul algebra.
It is shown that the algebra depends only on the intersection lattice of
the hyperplane arrangement. A complete system of primitive orthogonal
idempotents for the algebra is constructed and other algebraic structure
is determined including: a description of the projective indecomposable
modules, the Cartan invariants, projective resolutions of the simple
modules, the Hochschild homology and cohomology, and the Koszul dual
algebra. A new cohomology construction on posets is introduced, and it is
shown that the face semigroup algebra is isomorphic to the cohomology
algebra when this construction is applied to the intersection lattice of
the hyperplane arrangement.
Categories:52C35, 05E25, 16S37 

31. CJM 2009 (vol 61 pp. 583)
 Hajir, Farshid

Algebraic Properties of a Family of Generalized Laguerre Polynomials
We study the algebraic properties of Generalized Laguerre Polynomials
for negative integral values of the parameter. For integers $r,n\geq
0$, we conjecture that $L_n^{(1nr)}(x) = \sum_{j=0}^n
\binom{nj+r}{nj}x^j/j!$ is a $\Q$irreducible polynomial whose
Galois group contains the alternating group on $n$ letters. That this
is so for $r=n$ was conjectured in the 1950's by Grosswald and proven
recently by Filaseta and Trifonov. It follows from recent work of
Hajir and Wong that the conjecture is true when $r$ is large with
respect to $n\geq 5$. Here we verify it in three situations: i) when
$n$ is large with respect to $r$, ii) when $r \leq 8$, and iii) when
$n\leq 4$. The main tool is the theory of $p$adic Newton Polygons.
Categories:11R09, 05E35 

32. CJM 2009 (vol 61 pp. 465)
 Woodford, Roger

On Partitions into Powers of Primes and Their Difference Functions
In this paper, we extend the approach first outlined by Hardy and
Ramanujan for calculating the asymptotic formulae for the number of
partitions into $r$th powers of primes, $p_{\mathbb{P}^{(r)}}(n)$,
to include their difference functions. In doing so, we rectify an
oversight of said authors, namely that the first difference function
is perforce positive for all values of $n$, and include the
magnitude of the error term.
Categories:05A17, 11P81 

33. CJM 2008 (vol 60 pp. 1108)
 LopezAbad, J.; Manoussakis, A.

A Classification of Tsirelson Type Spaces
We give a complete classification of mixed Tsirelson spaces
$T[(\mathcal F_i,\theta_i)_{i=1}^{r}]$ for finitely many pairs of
given compact and hereditary families $\mathcal F_i$ of finite sets of
integers and $0<\theta_i<1$ in terms of the CantorBendixson indices
of the families $\mathcal F_i$, and $\theta_i$ ($1\le i\le r$). We
prove that there are unique countable ordinal $\alpha$ and
$0<\theta<1$ such that every block sequence of
$T[(\mathcal F_i,\theta_i)_{i=1}^{r}]$ has a subsequence equivalent to a
subsequence of the natural basis of the
$T(\mathcal S_{\omega^\alpha},\theta)$. Finally, we give a complete criterion of
comparison in between two of these mixed Tsirelson spaces.
Categories:46B20, 05D10 

34. CJM 2008 (vol 60 pp. 960)
35. CJM 2008 (vol 60 pp. 958)
 Chen, Yichao

A Note on a Conjecture of S. Stahl
S. Stahl (Canad. J. Math. \textbf{49}(1997), no. 3, 617640)
conjectured that the zeros of genus polynomial are real.
L. Liu and Y. Wang disproved this conjecture on the basis
of Example 6.7. In this note, it is pointed out
that there is an error in this example and a new generating matrix
and initial vector are provided.
Keywords:genus polynomial, zeros, real Categories:05C10, 05A15, 30C15, 26C10 

36. CJM 2008 (vol 60 pp. 297)
 Bini, G.; Goulden, I. P.; Jackson, D. M.

Transitive Factorizations in the Hyperoctahedral Group
The classical Hurwitz enumeration problem has a presentation in terms of
transitive factorizations in the symmetric group. This presentation suggests
a generalization from type~$A$ to other
finite reflection groups and, in particular, to type~$B$.
We study this generalization both from a combinatorial and a geometric
point of view, with the prospect of providing a means of understanding more
of the structure of the moduli spaces of maps with an $\gS_2$symmetry.
The type~$A$ case has been well studied and connects Hurwitz numbers
to the moduli space of curves. We conjecture an analogous setting for the
type~$B$ case that is studied here.
Categories:05A15, 14H10, 58D29 

37. CJM 2008 (vol 60 pp. 266)
 Bergeron, Nantel; Reutenauer, Christophe; Rosas, Mercedes; Zabrocki, Mike

Invariants and Coinvariants of the Symmetric Group in Noncommuting Variables
We introduce a natural Hopf algebra structure on the space of noncommutative
symmetric functions.
The bases for this algebra are indexed
by set partitions. We show that there exists a natural inclusion of the Hopf
algebra of noncommutative symmetric functions
in this larger space. We also consider this algebra as a subspace of
noncommutative polynomials and use it to
understand the structure of the spaces of harmonics and coinvariants
with respect to this collection of noncommutative polynomials and conclude
two analogues of Chevalley's theorem in the noncommutative setting.
Categories:16W30, 05A18;, 05E10 

38. CJM 2008 (vol 60 pp. 64)
39. CJM 2007 (vol 59 pp. 828)
 Ortner, Ronald; Woess, Wolfgang

NonBacktracking Random Walks and Cogrowth of Graphs
Let $X$ be a locally finite, connected graph without vertices of
degree $1$. Nonbacktracking random walk moves at each step with equal
probability to one of the ``forward'' neighbours of the actual state,
\emph{i.e.,} it does not go back along
the preceding edge to the preceding
state. This is not a Markov chain, but can be turned into a Markov
chain whose state space is the set of oriented edges of $X$. Thus we
obtain for infinite $X$ that the $n$step nonbacktracking transition
probabilities tend to zero, and we can also compute their limit when
$X$ is finite. This provides a short proof of old results concerning
cogrowth of groups, and makes the extension of that result to
arbitrary regular graphs rigorous. Even when $X$ is nonregular, but
\emph{small cycles are dense in} $X$, we show that the graph $X$ is
nonamenable if and only if the nonbacktracking $n$step transition
probabilities decay exponentially fast. This is a partial
generalization of the cogrowth criterion for regular graphs which
comprises the original cogrowth criterion for finitely generated
groups of Grigorchuk and Cohen.
Keywords:graph, oriented line grap, covering tree, random walk, cogrowth, amenability Categories:05C75, 60G50, 20F69 

40. CJM 2007 (vol 59 pp. 225)
41. CJM 2007 (vol 59 pp. 36)
 Develin, Mike; Martin, Jeremy L.; Reiner, Victor

Classification of Ding's Schubert Varieties: Finer Rook Equivalence
K.~Ding studied a class of Schubert varieties $X_\lambda$
in type A partial
flag manifolds, indexed by
integer partitions $\lambda$ and in bijection
with dominant permutations. He observed that the
Schubert cell structure of $X_\lambda$ is indexed by maximal rook
placements on the Ferrers board $B_\lambda$, and that the
integral cohomology groups $H^*(X_\lambda;\:\Zz)$, $H^*(X_\mu;\:\Zz)$ are
additively isomorphic exactly when the Ferrers boards $B_\lambda, B_\mu$
satisfy the combinatorial condition of \emph{rookequivalence}.
We classify the varieties $X_\lambda$ up to isomorphism, distinguishing them
by their graded cohomology rings with integer coefficients. The crux of our approach
is studying the nilpotence orders of linear forms in
the cohomology ring.
Keywords:Schubert variety, rook placement, Ferrers board, flag manifold, cohomology ring, nilpotence Categories:14M15, 05E05 

42. CJM 2006 (vol 58 pp. 1026)
 Handelman, David

Karamata Renewed and Local Limit Results
Connections between behaviour of real analytic functions (with no
negative Maclaurin series coefficients and radius of convergence one)
on the open unit interval, and to a lesser extent on arcs of the unit
circle, are explored, beginning with Karamata's approach. We develop
conditions under which the asymptotics of the coefficients are related
to the values of the function near $1$; specifically, $a(n)\sim
f(11/n)/ \alpha n$ (for some positive constant $\alpha$), where
$f(t)=\sum a(n)t^n$. In particular, if $F=\sum c(n) t^n$ where $c(n)
\geq 0$ and $\sum c(n)=1$, then $f$ defined as $(1F)^{1}$ (the
renewal or Green's function for $F$) satisfies this condition if $F'$
does (and a minor additional condition is satisfied). In come cases,
we can show that the absolute sum of the differences of consecutive
Maclaurin coefficients converges. We also investigate situations in
which less precise asymptotics are available.
Categories:30B10, 30E15, 41A60, 60J35, 05A16 

43. CJM 2005 (vol 57 pp. 82)
 Fallat, Shaun M.; Gekhtman, Michael I.

Jordan Structures of Totally Nonnegative Matrices
An $n \times n$ matrix is said to be totally nonnegative if every
minor of $A$ is nonnegative. In this paper we completely
characterize all possible Jordan canonical forms of irreducible
totally nonnegative matrices. Our approach is mostly combinatorial
and is based on the study of weighted planar diagrams associated
with totally nonnegative matrices.
Keywords:totally nonnegative matrices, planar diagrams,, principal rank, Jordan canonical form Categories:15A21, 15A48, 05C38 

44. CJM 2004 (vol 56 pp. 871)
 Schocker, Manfred

Lie Elements and Knuth Relations
A coplactic class in the symmetric group $\Sym_n$ consists of all
permutations in $\Sym_n$ with a given Schensted $Q$symbol, and may
be described in terms of local relations introduced by Knuth. Any
Lie element in the group algebra of $\Sym_n$ which is constant on
coplactic classes is already constant on descent classes. As a
consequence, the intersection of the Lie convolution algebra
introduced by Patras and Reutenauer and the coplactic algebra
introduced by Poirier and Reutenauer is the direct sum of all
Solomon descent algebras.
Keywords:symmetric group, descent set, coplactic relation, Hopf algebra,, convolution product Categories:17B01, 05E10, 20C30, 16W30 

45. CJM 2002 (vol 54 pp. 1086)
 Polterovich, Iosif

Combinatorics of the Heat Trace on Spheres
We present a concise explicit expression for the heat trace
coefficients of spheres. Our formulas yield certain combinatorial
identities which are proved following ideas of D.~Zeilberger. In
particular, these identities allow to recover in a surprising way
some known formulas for the heat trace asymptotics. Our approach is
based on a method for computation of heat invariants developed in [P].
Categories:05A19, 58J35 

46. CJM 2002 (vol 54 pp. 757)
 Larose, Benoit

Strongly Projective Graphs
We introduce the notion of strongly projective graph, and characterise
these graphs in terms of their neighbourhood poset. We describe certain
exponential graphs associated to complete graphs and odd cycles. We
extend and generalise a result of Greenwell and Lov\'asz \cite{GreLov}:
if a connected graph $G$ does not admit a homomorphism to $K$, where $K$
is an odd cycle or a complete graph on at least 3 vertices, then the
graph $G \times K^s$ admits, up to automorphisms of $K$, exactly $s$
homomorphisms to $K$.
Categories:05C15, 06A99 

47. CJM 2002 (vol 54 pp. 795)
 Möller, Rögnvaldur G.

Structure Theory of Totally Disconnected Locally Compact Groups via Graphs and Permutations
Willis's structure theory of totally disconnected locally compact groups
is investigated in the context of permutation actions. This leads to new
interpretations of the basic concepts in the theory and also to new proofs
of the fundamental theorems and to several new results. The treatment of
Willis's theory is selfcontained and full proofs are given of all the
fundamental results.
Keywords:totally disconnected locally compact groups, scale function, permutation groups, groups acting on graphs Categories:22D05, 20B07, 20B27, 05C25 

48. CJM 2002 (vol 54 pp. 239)
 Cartwright, Donald I.; Steger, Tim

Elementary Symmetric Polynomials in Numbers of Modulus $1$
We describe the set of numbers $\sigma_k(z_1,\ldots,z_{n+1})$, where
$z_1,\ldots,z_{n+1}$ are complex numbers of modulus $1$ for which
$z_1z_2\cdots z_{n+1}=1$, and $\sigma_k$ denotes the $k$th
elementary symmetric polynomial. Consequently, we give sharp
constraints on the coefficients of a complex polynomial all of whose
roots are of the same modulus. Another application is the calculation
of the spectrum of certain adjacency operators arising naturally
on a building of type ${\tilde A}_n$.
Categories:05E05, 33C45, 30C15, 51E24 

49. CJM 2001 (vol 53 pp. 696)
 Currie, J.; Linek, V.

Avoiding Patterns in the Abelian Sense
We classify all 3 letter patterns that are avoidable in the abelian
sense. A short list of four letter patterns for which abelian
avoidance is undecided is given. Using a generalization of Zimin
words we deduce some properties of $\o$words avoiding these
patterns.
Categories:05, 68 

50. CJM 2001 (vol 53 pp. 758)
 Goulden, I. P.; Jackson, D. M.; Latour, F. G.

Inequivalent Transitive Factorizations into Transpositions
The question of counting minimal factorizations of permutations into
transpositions that act transitively on a set has been studied extensively
in the geometrical setting of ramified coverings of the sphere and in the
algebraic setting of symmetric functions.
It is natural, however, from a combinatorial point of view to ask how such
results are affected by counting up to equivalence of factorizations, where
two factorizations are equivalent if they differ only by the interchange of
adjacent factors that commute. We obtain an explicit and elegant result for
the number of such factorizations of permutations with precisely two
factors. The approach used is a combinatorial one that rests on two
constructions.
We believe that this approach, and the combinatorial primitives that have
been developed for the ``cut and join'' analysis, will also assist with the
general case.
Keywords:transitive, transposition, factorization, commutation, cutandjoin Categories:05C38, 15A15, 05A15, 15A18 
