The algebraic combinatorics of snakes
ABSTRACT Snakes are analogues of alternating permutations defined for any Coxeter
group. We study these objects from the point of view of combinatorial Hopf
algebras, such as noncommutative symmetric functions and their generalizations.
The main purpose is to show that several properties of the generating functions
of snakes, such as differential equations or closed form as trigonometric
functions, can be lifted at the level of noncommutative symmetric functions or
free quasisymmetric functions. The results take the form of algebraic
identities for type B noncommutative symmetric functions, noncommutative
supersymmetric functions and colored free quasisymmetric functions.
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Article: Remarks on a combinatorial problem
 SourceAvailable from: ArXiv[show abstract] [hide abstract]
ABSTRACT: This article is devoted to the study of several algebras which are related to symmetric functions, and which admit linear bases labelled by various combinatorial objects: permutations (free quasisymmetric functions), standard Young tableaux (free symmetric functions) and packed integer matrices (matrix quasisymmetric functions). Free quasisymmetric functions provide a kind of noncommutative Frobenius characteristic for a certain category of modules over the 0Hecke algebras. New examples of indecomposable $H_n(0)$modules are discussed, and the homological properties of $H_n(0)$ are computed for small $n$. Finally, the algebra of matrix quasisymmetric functions is interpreted as a convolution algebra.05/2001;  SourceAvailable from: arxiv.org
Article: The algebra of binary search trees.
Theor. Comput. Sci. 01/2005; 339:129165.
Page 1
arXiv:1110.5272v1 [math.CO] 24 Oct 2011
THE ALGEBRAIC COMBINATORICS OF SNAKES
MATTHIEU JOSUATVERG`ES, JEANCHRISTOPHE NOVELLI,
AND JEANYVES THIBON
Abstract. Snakes are analogues of alternating permutations defined for any Cox
eter group. We study these objects from the point of view of combinatorial Hopf
algebras, such as noncommutative symmetric functions and their generalizations.
The main purpose is to show that several properties of the generating functions
of snakes, such as differential equations or closed form as trigonometric functions,
can be lifted at the level of noncommutative symmetric functions or free quasi
symmetric functions. The results take the form of algebraic identities for type B
noncommutative symmetric functions, noncommutative supersymmetric functions
and colored free quasisymmetric functions.
1. Introduction
Snakes, a term coined by Arnol’d [2], are generalizations of alternating permuta
tions. These permutations arose as the solution of what is perhaps the first example
of an inverse problem in the theory of generating functions: given a function whose
Taylor series has nonnegative integer coefficients, find a family of combinatorial ob
jects counted by those coefficients. For example, in the expansions
(1)tanz =
?
n≥0
E2n+1
z2n+1
(2n + 1)!
andsecz =
?
n≥0
E2n
z2n
(2n)!,
the coefficients Enare nonnegative integers.
It was found in 1881 by D. Andr´ e [1] that En was the number of alternating
permutations in the symmetric group Sn.
Whilst this result is not particularly difficult and can be proved in several ways,
the following explanation is probably not far from being optimal: there exists an
associative (and noncommutative) algebra admitting a basis labelled by all permu
tations, and such that the map φ sending any σ ∈ Sntozn
this algebra, the formal series
n!is a homomorphism. In
(2)C =
?
n≥0
(−1)nid2n
andS =
?
n≥0
(−1)nid2n+1
(alternating sums of even and odd identity permutations) are respectively mapped
to cosz and sinz by φ. The series C is clearly invertible, and one can see by a direct
calculation that C−1+ C · S−1is the sum of all alternating permutations [8].
Date: October 25, 2011.
M. JosuatVerg` es was supported by the Austrian Science Foundation (FWF) via the grant Y463.
1
Page 2
2M. JOSUATVERG`ES, J.C. NOVELLI, AND J.Y. THIBON
Such a proof is not only illuminating, it says much more than the original statement.
For example, one can now replace φ by more complicated morphisms, and obtain
generating functions for various statistics on alternating permutations.
The symmetric group is a Coxeter group, and snakes are generalizations of al
ternating permutations to arbitrary Coxeter groups. Such generalizations were first
introduced by Springer [22]. For the infinite series An, Bn, Dn, Arnol’d [2] related
the snakes to the geometry of bifurcation diagrams.
The aim of this article is to study the snakes of the classical Weyl groups (types A,
B and D) by noncommutative methods, and to generalize the results to some series
of wreath products (colored permutations).
The case of symmetric groups (type A) is settled by the algebra of Free quasi
symmetric functions FQSym (also known the MalvenutoReutenauer algebra) which
is based on permutations, and its subalgebra Sym (noncommutative symmetric func
tions), based on integer compositions. To deal with the other types, we need an
algebra based on signed permutations, and some of its subalgebras defined by means
of the superization map introduced in [17].
After reviewing the necessary background and the above mentioned proof of the
result of Andr´ e, we recover results of Chow [4] on type B snakes, and derive some new
generating functions for this type. This suggests a variant of the definition of snakes,
for which the noncommutative generating series is simpler. These considerations
lead us to some new identities satisfied by the superization map on noncommutative
symmetric functions. Finally, we propose a completely different combinatorial model
for the generating function of type B snakes, based on interesting identities in the
algebra of signed permutations. We also present generalizations of (Arnol’d’s) Euler
Bernoulli triangle, counting alternating permutations according to their last value,
and extend the results to wreath products and to type D, for which we propose an
alternative definition of snakes.
2. Permutations and noncommutative trigonometry
2.1. Free quasisymmetric functions. The simplest way to define our algebra
based on permutations is by means of the classical standardization process, familiar
in combinatorics and in computer science. Let A = {a1,a2,...} be an infinite totally
ordered alphabet. The standardized word Std(w) of a word w ∈ A∗is the permu
tation obtained by iteratively scanning w from left to right, and labelling 1,2,...
the occurrences of its smallest letter, then numbering the occurrences of the next
one, and so on. Alternatively, σ = std(w)−1can be characterized as the unique
permutation of minimal length such that wσ is a nondecreasing word. For example,
std(bbacab) = 341625.
We can now define polynomials
(3)
Gσ(A) :=
?
std(w)=σ
w.
It is not hard to check that these polynomials span a subalgebra of C?A?, denoted
by FQSym(A), an acronym for Free QuasiSymmetric functions.
Page 3
THE ALGEBRAIC COMBINATORICS OF SNAKES3
The multiplication rule is, for α ∈ Skand β ∈ Sℓ,
(4)
GαGβ=
?
γ∈α∗β
Gγ,
where α ∗ β is the set of permutations γ ∈ Sk+ℓsuch that γ = u · v with std(u) = α
and std(v) = β. This is the convolution of permutations (see [19]). Note that the
number of terms in this product depends only on k and ℓ, and is equal to the binomial
coefficient?k+ℓ
(5)
k
?. Hence, the map
φ : σ ∈ Sn?−→zn
n!
is a homomorphism of algebras FQSym → C[z].
2.2. Noncommutative symmetric functions. The algebra Sym(A) of noncom
mutative symmetric functions over A is the subalgebra of FQSym generated by the
identity permutations [8, 6]
(6)Sn(A) := G12...n(A) =
?
i1≤i2≤···≤in
ai1ai2...ain.
These polynomials are obviously algebraically independent, so that the products
(7)SI:= Si1Si2...Sir
where I = (i1,i2,...,ir) runs over compositions of n, form a basis of Symn, the
homogeneous component of degree n of Sym.
Recall that a descent of a word w = w1w2...wn ∈ Anis an index i such that
wi > wi+1. The set of such i is denoted by Des(w). Hence, Sn(A) is the sum of
all nondecreasing words of length n (no descent), and SI(A) is the sum of all words
which may have a descents only at places from the set
(8)Des(I) = {i1, i1+ i2,...,i1+ ··· + ir−1},
called the descent set of I. Another important basis is
(9)RI(A) =
?
Des(w)=Des(I)
w =
?
Des(σ)=Des(I)
Gσ,
the ribbon basis, formed by sums of words having descents exactly at prescribed
places. From this definition, it is obvious that if I = (i1,...,ir), J = (j1,...,js)
(10)RI(A)RJ(A) = RIJ(A) + RI⊲J(A)
with IJ = (i1,...,ir,j1,...,js) and I ⊲ J = (i1,...,ir+ j1,j2,...,js).
2.3. Operations on alphabets. If B is another totally ordered alphabet, we denote
by A + B the ordinal sum of A and B.
symmetric functions of A + B, and
This allows to define noncommutative
(11)Sn(A + B) =
n
?
i=0
Si(A)Sn−i(B)(S0= 1).
Page 4
4M. JOSUATVERG`ES, J.C. NOVELLI, AND J.Y. THIBON
If we assume that A and B commute, this operation defines a coproduct, for which
Sym is a graded bialgebra, hence a Hopf algebra. The same is true of FQSym.
Symmetric functions of the virtual alphabet (−A) are defined by the condition
(12)
?
n≥0
Sn(−A) =
??
n≥0
Sn(A)
?−1
and more generally, for a difference A − B,
(13)
?
n≥0
Sn(A − B) =
??
k≥0
Sk(B)
?−1?
l≥0
Sl(A)
(note the reversed order, see [13] for detailed explanations).
2.4. Noncommutative trigonometry.
2.4.1. Andr´ e’s theorem. One can now define “noncommutative trigonometric func
tions” by
(14)
cos(A) =
?
n≥0
(−1)nS2n(A)and
sin(A) =
?
n≥0
(−1)nS2n+1(A).
The image by φ of these series are the usual trigonometric functions. With the help
of the product formula for the ribbon basis, it is easy to see that
(15)
sec := cos−1=
?
n≥0
R(2n)
and
tan := cos−1sin =
?
n≥0
R(2n1)
which implies Andr´ e’s theorem: the coefficient ofzn
of alternating permutations of Sn(if we choose to define alternating permutations
as those of shape (2n) and (2n1)). In FQSym,
n!in sec(z)+tan(z) is the number
(16)
sec + tan =
?
σ alternating
Gσ.
2.4.2. Differential equations. If ∂ is the derivation of Sym such that ∂Sn= Sn−1,
then
?
satisfy the differential equations
∂X = 1 + X2,
(17)X = tan =
m≥0
R(2m1) and Y = sec =
?
m≥0
R(2m)
(18)∂Y = XY .
These equations can be lifted to FQSym, actually to its subalgebra PBT, the Loday
Ronco algebra of planar binary trees (see [9] for details). Solving them in this algebra
provides yet another combinatorial proof of Andr´ e’s result.
Let us sketch it for the tangent. The original proof of Andr´ e relied upon the
differential equation
dx
dt= 1 + x2
(19)
Page 5
THE ALGEBRAIC COMBINATORICS OF SNAKES5
whose x(t) = tan(t) is the solution such that x(0) = 0. Equivalently, x(t) is the
unique solution of the functional equation
?t
which can be solved by iterated substitution.
In general, given an associative algebra R, we can consider the functional equation
for the power series x ∈ R[[t]]
(20)x(t) = t +
0
x(s)2ds
(21)x = a + B(x,x)
where a ∈ R and B(x,y) is a bilinear map with values in R[[t]], such that the
valuation of B(x,y) is strictly greater than the sum of the valuations of x and y.
Then, Equation (21) has a unique solution
(22)x = a + B(a,a) + B(B(a,a),a) + B(a,B(a,a)) + ··· =
?
T∈CBT
BT(a)
where CBT is the set of (complete) binary trees, and for a tree T, BT(a) is the result
of evaluating the expression formed by labeling by a the leaves of T and by B its
internal nodes. Pictorially,
x = a + B(a,a) + B(B(a,a),a) + B(a,B(a,a)) + ...
= a +
B? ?
??
aa
+
B
? ? ?
??
B
?
?
??
a
aa
+
B? ?
???
a
B? ?
??
aa
+ ...
It is proved in [9] that if one defines
(23)
where σ′is obtained from σ by erasing its maximal letter n, then ∂ is a derivation
of FQSym. Its restriction to Sym coincides obviously with the previous definition.
For α ∈ Sk, β ∈ Sℓ, and n = k + ℓ, set
∂Gσ:= Gσ′
(24)
B(Gα,Gβ) =
?
γ=u(n+1)v
std(u)=α,std(v)=β
Gγ.
Clearly,
(25)∂B(Gα,Gβ) = GαGβ,
and our differential equation for the noncommutative tangent is now replaced by the
fixed point problem
(26)X = G1+ B(X,X).
of which it is the unique solution. Again, solving it by iterations gives back the sum
of alternating permutations. As an element of the LodayRonco algebra, tan appears
as the sum of all permutations whose decreasing tree is complete.
The same kind of equation holds for Y :
(27)Y = 1 + B(X,Y ).
Page 6
6M. JOSUATVERG`ES, J.C. NOVELLI, AND J.Y. THIBON
Hence, the noncommutative secant sec is therefore an element of PBT, so a sum
of binary trees. The trees are wellknown: they correspond to complete binary trees
(of odd size) where one has removed the last leaf.
2.5. Derivative polynomials. For the ordinary tangent and secant, the differential
equations imply the existence [11] of two sequences of polynomials Pn, Qnsuch that
dn
dzn(tanz) = Pn(tanz)
Since ∂ is a derivation of Sym, we have as well for the noncommutative lifts
(28)and
dn
dzn(secz) = Qn(tanz)secz.
(29)∂n(X) = Pn(X) and∂n(Y ) = Qn(X)Y .
Hoffman [11] gives the exponential generating functions
(30)
?
The noncommutative version of these identities can be readily derived as follows. We
want to compute
Pn(X)tn
P(u,t) =
n≥0
Pn(u)tn
n!=sint + ucost
cost − usintand Q(u,t) =
?
n≥0
Qn(u)tn
n!=
1
cost − usint.
(31)P(X,t) =
?
n≥0
n!= et∂X .
Since ∂ is a derivation, et∂is an automorphism of Sym. It acts on the generators Sn
by
(32)et∂Sn(A) =
n
?
k=0
Sn−k(A)tk
k!= Sn(A + tE)
where tE is the “virtual alphabet” such that Sn(tE) =tn
n!. Hence,
P(X,t) = tan(A + tE) = cos(A + tE)−1sin(A + tE)
= (cost − X sint)−1(sint + X cost)
(33)
as expected. Similarly,
Qn(X,t) =
?
n≥0
Qn(X)tn
n!= (et∂Y )Y−1
(34)
= cos(A + tE)−1cos(A) = (cost − X sint)−1.(35)
3. The uniform definition of snakes for Coxeter groups
Before introducing the relevant generalizations of Sym and FQSym, we shall com
ment on the definitions of snakes and alternating permutations for general Coxeter
groups.
It is apparent in Springer’s article [22] that alternating permutations can be defined
in a uniform way for any Coxeter group. Still, little attention has been given to this
fact. For example, 20 years later, Arnol’d [2] gives separately the definitions of snakes
of type A, B and D, even though there is no doubt that he was aware of the uniform
Page 7
THE ALGEBRAIC COMBINATORICS OF SNAKES7
definition. The goal of this section is to give some precisions and to simplify the proof
of Springer’s result in [22].
Let (W,S) be an irreducible Coxeter system. Recall that s ∈ S is a descent of
w ∈ W if ℓ(ws) < ℓ(w) where ℓ is the length function. When J ⊂ S, we denote
by DJ the descent class defined as {w ∈ W : ℓ(ws) < ℓ(w) ⇔ s ∈ J}. Following
Arnol’d, let us consider the following definition.
Definition 3.1. The Springer number of the Coxeter system (W,S) is
(36) K(W) := max
J⊆S(#DJ).
The aim of [22] is to give a precise description of sets J realizing this maximum.
The result is as follows:
Theorem 3.2 (Springer [22]). Let J ⊆ S. Then, K(W) = #DJif and only if J and
S\J are independent subsets of the Coxeter graph S (i.e., they contain no two adja
cent vertices). In particular, there are two such subsets J which are complementary
to each other.
Therefore we can choose a subset J such that K(W) = #DJand call snakes of W
the elements of DJ. The other choice S\J would essentially lead to the same objects,
since there is a simple involution on W exchanging the subsets DJand DS\J.
We are thus led to the following definition:
Definition 3.3. Let (W,S) be a Coxeter group, and J be a maximal independent
subset of S. The snakes of (W,S) are the elements of the descent class DJ.
This definition depends on the choice of J, so that we can consider two families
of snakes for each W. In the case of alternating permutations, these are usually
called the updown and downup permutations, and are respectively defined by the
conditions
(37)σ1< σ2> σ3< ...orσ1> σ2< σ3> ...
It is natural to endow a descent class with the restriction of the weak order, and
this defines what we can call the snake poset of (W,S). Known results show that this
poset is a lattice [3]. Let us now say a few words about the proof of Theorem 3.2,
which relies upon the following lemma.
Lemma 3.4 (Springer [22]). Let J ⊆ S, and assume that there is an edge e of the
Coxeter graph whose endpoints are both in J or both not in J. Let S = S1∪S2be the
connected components obtained after removing e. Let J′= (S1∩ J) ∪ (S2∩ (S\J)).
Then, #DJ< #DJ′.
Using the above lemma, we see that if J or complementary is not independent, we
can find another subset J′having a strictly bigger descent class, and Theorem 3.2
then follows. Whereas Lemma 3.4 is the last one out of a series of 5 lemmas in
Springer’s article, we give here a simple geometric argument.
Let R be a root system for (W,S), and let Π = (αs)s∈Sbe a set of simple roots.
There is a bijection w ?→ i(w) between W and the set of Weyl chambers, so that s ∈ S
Page 8
8M. JOSUATVERG`ES, J.C. NOVELLI, AND J.Y. THIBON
is a descent of w ∈ W if and only if i(w) lies in the halfspace {v ∈ Rn: ?v,αs? ≥ 0}.
For any J ⊂ S, let
(38)CJ= {v ∈ Rn: ?v,αs? ≥ 0 if s ∈ J, and ?v,αs? ≤ 0 if s / ∈ J}.
It is the closure of the union of Weyl chambers i(w) where w ∈ DJ. Now, let e, S1,
S2 and J′be as in the lemma, and let x ∈ S1and y ∈ S2be the endpoints of e.
Note that either x or y is in J′but not both. Let σ be the orthogonal symmetry
through the linear span of {αj : j ∈ S1}. We claim that σ(CJ) ? CJ′, and this
implies #DJ< #DJ′ since CJcontains strictly less Weyl chambers than CJ′.
So it remains to show that σ(CJ) ? CJ′. It is convenient to use the notion of dual
cone, which is defined for any closed convex cone C ⊂ Rnas
(39)C∗:= {v ∈ Rn: ?v,w? ≥ 0 for any w ∈ C}.
The map C ?→ C∗is an inclusionreversing involution on closed convex cones, and it
commutes with any linear isometry, so that we have to prove that σ(C∗
(C1∩ C2)∗= C∗
the halfline R+w, the dual of CJ′ is
J′) ? C∗
J. Since
1+ C∗
2and since the dual of the halfspace {v ∈ Rn: ?v,w? ≥ 0} is
(40)C∗
J′ =
??
s∈S
usαs : us≥ 0 if s ∈ J′, and us≤ 0 if s / ∈ J′
?
,
and the same holds for C∗
ity since σ(αs) = αs if s ∈ S1, σ(αs) = −αs if s ∈ S2\y, and σ(αy) = −αy+
2?αx,αy??αx,αx?−1αx. Indeed, let v =?
(41)σ(v) =
?
Since uxand uyhave different signs and ?αx,αy? < 0, we obtain σ(v) ∈ C∗
thus proved σ(C∗
in C∗
σ(v), if uy?= 0 there is a nonzero term in αxas well. This completes the proof.
J. A description of σ(C∗
J′) is obtained easily by linear
s∈Susαs∈ C∗
J′, we have:
s∈S1
usαs−
?
s∈S2\y
usαs− uyαy+ 2uy?αx,αy??αx,αx?−1αx.
J. We have
J′) ⊂ C∗
J. To show the strict inclusion, note that either αyor −αyis
J. But none of these two elements is in σ(C∗
J′), because in the above formula for
4. Signed permutations and combinatorial Hopf algebras
Whereas the constructions of the Hopf algebras Sym, PBT, and FQSym ap
pearing when computing the usual tangent are almost straightforward, the situation
is quite different in type B. First, there are at least three different generalizations
of Sym to a pair of alphabets, each with its own qualities either combinatorial or
algebraic. Moreover, there are also two different ways to generalize FQSym. The
generalizations of PBT are not (yet) defined in the literature but the computations
done in the present paper give a glimpse of what they should be.
Here follows how they embed each in the other. All embeddings are embeddings
of Hopf algebras except the two embeddings concerning BSym which is not itself an
algebra. However, the embedding of Sym(A¯A) into Sym(2)obtained by composing
the two previous embeddings is a Hopf embedding:
Page 9
THE ALGEBRAIC COMBINATORICS OF SNAKES9
(42)
Sym(A¯A) ֒→ BSym ֒→ Sym(2)֒→ FQSym(A¯A) ֒→ FQSym(2).
4.1. The MantaciReutenauer algebra of type B. The most straightforward
definition of Sym in type B is to generalize the combinatorial objects involved in the
definition: change compositions into signed compositions.
We denote by Sym(2)= MR [15] the free product Sym⋆Sym of two copies of the
Hopf algebra of noncommutative symmetric functions. In other words, MR is the
free associative algebra on two sequences (Sn) and (S¯ n) (n ≥ 1). We regard the two
copies of Sym as noncommutative symmetric functions on two auxiliary alphabets:
Sn= Sn(A) and S¯ n= Sn(¯A). We denote by F ?→¯F the involutive automorphism1
which exchanges Snand S¯ n. And we denote the generators of Sym(2)by S(k,ǫ)where
ǫ = {±1}, so that S(k,1)= Skand S(k,−1)= S¯k.
4.2. Noncommutative supersymmetric functions. The second generalization of
Sym comes from the transformation of alphabets sending A to a combination of A
and¯A. It is the algebra containing the type B alternating permutations.
We define Sym(A¯A) as the subalgebra of Sym(2)generated by the S#
any F ∈ Sym(A),
nwhere, for
(43)F#= F(A¯A) = F(A − q¯A)q=−1,
called the supersymmetric version, or superization, of F [17].
The expansion of an element of Sym(A¯A) as a linear combination in Sym(2)is
done thanks to generating series. Indeed,
(44)σ#
1=¯λ1σ1=
??
k≥0
Λk
???
m≥0
Sm
?
where Λk=?
(45)
I=k(−1)ℓ(I)−kSI, as follows from¯λ1= (¯ σ−1)−1(see [13]). For example,
S#
1= S1+ S1,S#
2= S2+ S11− S2+ S11,
(46)S#
3= S3+ S12+ S111− S21+ S111− S21− S12+ S3.
4.3. Noncommutative symmetric functions of type B. The third generaliza
tion of Sym is not an algebra but only a cogebra but is the generalization one gets
with respect to the group Bn: its graded dimension is 2nand, as we shall see later
in this paragraph, a basis of BSym is given by sums of permutations having given
descents in the type B sense. This algebra contains the snakes of type B.
Noncommutative symmetric functions of type B were introduced in [4] as the right
Symmodule BSym freely generated by another sequence (˜Sn) (n ≥ 0,˜S0= 1) of
homogeneous elements, with ˜ σ1grouplike. This is a coalgebra, but not an algebra.
1This differs from the convention used in some references.
Page 10
10 M. JOSUATVERG`ES, J.C. NOVELLI, AND J.Y. THIBON
We embed BSym as a subcoalgebra and right subSymmodule of MR as follows.
The basis element˜SIof BSym, where I = (i0,i1,...,ir) is a Bcomposition (that
is, i0may be 0), can be embedded as
˜SI= Si0(A)Si1i2...ir(A¯A).
In the sequel, we identify BSym with its image under this embedding.
As in Sym, one can define by triangularity the analog of the ribbon basis ([4]):
(47)
(48)
˜SI=
?
J≤I
˜RJ,
where J ≤ I if the Bdescent set of J is a subset of the Bdescent set of I. Note that
we have in particular˜S0n=˜R0n+˜Rn.
Note also that, thanks to that definition, SI#=˜SIand, thanks to the transitions
between all bases,
R#
(49)
I=˜R0I+˜RI.
4.4. Type B permutations and descents in Bn. The hyperoctahedral group Bnis
the group of signed permutations. A signed permutation can be denoted by w = (σ,ǫ)
where σ is an ordinary permutation and ǫ ∈ {±1}n, such that w(i) = ǫiσ(i). If we
set w(0) = 0, then, i ∈ [0,n − 1] is a Bdescent of w if w(i) > w(i + 1). Hence, the
Bdescent set of w is a subset D = {i0,i0+ i1,...,i0+ ··· + ir−1} of [0,n − 1]. We
then associate with D the typeB composition (i0− 0,i1,...,ir−1,n − ir−1).
4.5. Free quasisymmetric functions of level 2. Let us now move to generaliza
tions of FQSym. As in the case of Sym, the most natural way is to change the usual
alphabet into two alphabets, one of positive letters and one of negative letters and
to define a basis indexed by signed permutations as a realization on words on both
alphabets. This algebra is FQSym(2), the algebra of free quasisymmetric functions
of level 2, as defined in [16].
Let us set
A(0)= A = {a1< a2< ··· < an< ...},(50)
A(1)=¯A = {··· < ¯ an< ··· < ¯ a2< ¯ a1},(51)
and order A =¯A ∪ A by ¯ ai< ajfor all i,j. Let us also denote by std the standard
ization of signed words with respect to this order.
We shall also need the signed standardization Std, defined as follows. Represent a
signed word w ∈ Anby a pair (w,ǫ), where w ∈ Anis the underlying unsigned word,
and ǫ ∈ {±1}nis the vector of signs. Then Std(w,ǫ) = (std(w),ǫ).
Then, FQSym(2)is spanned by the polynomials in A ∪¯A
(52)
Gσ,u:=
?
w∈An;Std(w)=(σ,u)
w∈ Z?A?.
Let (σ′,u′) and (σ′′,u′′) be signed permutations. Then (see [16, 17])
(53)
Gσ′,u′ Gσ′′,u′′ =
?
σ∈σ′∗σ′′
Gσ,u′·u′′.
Page 11
THE ALGEBRAIC COMBINATORICS OF SNAKES 11
We denote by m(ǫ) the number of entries −1 in ǫ.
4.6. Free superquasisymmetric functions. The second algebra generalizing the
algebra FQSym is FQSym(A¯A). It comes from the transformation of alphabets
applied to FQSym as Sym(A¯A) comes from Sym. To do this, we first need to
recall that FQSym(2)is equipped with an internal product.
Indeed, viewing signed permutations as elements of the group {±1} ≀ Sn, we have
the internal product
(54)
Gα,ǫ∗ Gβ,η= G(β,η)◦(α,ǫ)= Gβ◦α,(ηα)·ǫ,
with ηα = (ηα(1),...,ηα(n)) and ǫ · η = (ǫ1η1,...,ǫnηn).
We can now embed FQSym into FQSym(2)by
(55)
Gσ?→ G(σ,1n),
which allows us to define
(56)
so that FQSym(A¯A) is the algebra spanned by the Gσ(A¯A).
G#
σ:= Gσ(A¯A) = Gσ∗ σ#
1,
Theorem 4.1 ([17], Thm. 3.1). The expansion of Gσ(A¯A) on the basis Gτ,ǫis
(57)
Gσ(A¯A) =
?
std(τ,ǫ)=σ
Gτ,ǫ.
4.6.1. Embedding Sym#and BSym into FQSym(2). One can embed BSym into
FQSym(2)as one embeds Sym into FQSym (see [4]) by
(58)
˜RI=
?
Bdes(π)=I
Gπ,
where I is any Bcomposition.
Given Equation (49) relating R#
Iand the˜RI, one has
?
(59)R♯
I=
Des(π)=I
Gπ,
where I is any (usual) composition.
5. Algebraic theory in type B
5.1. Alternating permutations of type B.
5.1.1. Alternating shapes. Let us say that a signed permutation π ∈ Bnis alternating
if π1<π2>π3<... (shape 2mor 2m1).
Here are the alternating permutations of type B for n ≤ 4:
(60)
¯1,1,
12,¯12,¯21,¯2¯1
(61) 12¯3,¯12¯3,132,13¯2,¯132,¯13¯2,¯21¯3,¯2¯1¯3,231,23¯1,¯231,¯23¯1,¯31¯2,¯3¯1¯2,¯321,¯32¯1,
Page 12
12 M. JOSUATVERG`ES, J.C. NOVELLI, AND J.Y. THIBON
12¯34,¯12¯34,12¯43,12¯4¯3,¯12¯43,¯12¯4¯3,1324,13¯24,¯1324,¯13¯24,13¯42,13¯4¯2,
¯13¯42,¯13¯4¯2,1423,14¯23,¯1423,¯14¯23,14¯32,14¯3¯2,¯14¯32,¯14¯3¯2,¯21¯34,¯2¯1¯34,
¯21¯43,¯21¯4¯3,¯2¯1¯43,¯2¯1¯4¯3,2314,23¯14,¯2314,¯23¯14,23¯41,23¯4¯1,¯23¯41,¯23¯4¯1,
2413,24¯13,¯2413,¯24¯13,24¯31,24¯3¯1,¯24¯31,¯24¯3¯1,¯31¯24,¯3¯1¯24,¯31¯42,¯31¯4¯2,
¯3¯1¯42,¯3¯1¯4¯2,¯3214,¯32¯14,¯32¯41,¯32¯4¯1,¯3¯2¯41,¯3¯2¯4¯1,3412,34¯12,¯3412,¯34¯12,
34¯21,34¯2¯1,¯34¯21,¯34¯2¯1,¯41¯23,¯4¯1¯23,¯41¯32,¯41¯3¯2,¯4¯1¯32,¯4¯1¯3¯2,¯4213,¯42¯13,
¯42¯31,¯42¯3¯1,¯4¯2¯31,¯4¯2¯3¯1,¯4312,¯43¯12,¯43¯21,¯43¯2¯1
(62)
Hence, π is alternating iff ¯ π is a βsnake in the sense of [2]. Hence, the sum in
FQSym(2)of all Gπlabeled by alternating signed permutations is, as already proved
in [4]
(63)
X = (X + Y )#= sec#+ tan#= sec#(1 + sin#) =
?
m≥0
(R#
(2m)+ R#
(2m1)).
5.1.2. Quasidifferential equations. Let d be the linear map acting on Gπas follows:
(64)dGπ=
?
Guv
Gu¯ v
if π = unv,
if π = u¯ nv.
This map lifts to FQSym(2)the derivation ∂ of (23), although it is not itself a
derivation. We then have
Theorem 5.1. The series X satisfies the quasidifferential equation
(65)dX = 1 + X2.
Proof – Indeed, let us compute what happens when applying d to R#
being the same with dR#
in σ, let us write σ = unv. Then dGσappears in the product GStd(u)GStd(v)and u
and v are of respective shapes (2n1) and (2m−n−1). If n appears in σ, let us write
again σ = unv. Then dGσappears in the product GStd(u)GStd(v)and u and v are of
respective shapes (2n) and (2m−n−11). Conversely, any permutation belonging to u∗v
with u and v of shapes (2n1) and (2m−n−1) has a shape (2m) if one adds n in position
2n + 2. The same holds for the other product, hence proving the statement.
(2m), the property
(2m1). Let us fix a permutation σ of shape (2m). If n appears
This is not enough to characterize X but we have the analog of fixed point equa
tion (26)
(66)
X = 1 + G1+ B(X,X),
where
(67)B(Gα,Gβ) =
??
γ=u(n+1)v, Std(u)=α, Std(v)=βGγ
?
if α is odd,
if α is even.
γ=u(n+1)¯ v, Std(u)=α, Std(v)=βGγ
Indeed, applying d to the fixed point equation brings back Equation (65) and it is
clear from the definition of B that all terms in B(Gα,Gβ) are alternating signed
permutations.
Page 13
THE ALGEBRAIC COMBINATORICS OF SNAKES13
Solving this equation by iterations gives back the results of [12, Section 4]. Indeed,
the iteration of Equation (66) yields the solution
(68)
X =
?
T∈CBT
BT(G0= 1,G1),
where, for a tree T, BT(a,b) is the result of the evaluation of all expressions formed
by labeling by a or b the leaves of T and by B its internal nodes. This is indeed the
same as the polynomials Pndefined in [12, Section 4] since one can interpret the G0
leaves as empty leaves in this setting, the remaining nodes then corresponding to all
increasing trees of the same shape, as can be seen on the definition of the operator B.
5.1.3. Alternating signed permutations counted by number of signs. Under the spe
cialization¯A = tA, X goes to the series
(69)X(t;A) =
?
I
?
π alternating, C(std(π))=I
tm(π)
RI(A)
where m(π) is the number of negative letters of π. If we further set A = zE, we
obtain
x(t,z) =1 + sin((1 + t)z)
cos((1 + t)z)
(70)
which reduces to
(71)x(1,z) =1 + sin2z
cos2z
=cosz + sinz
cosz − sinz
for t = 1, thus giving a tanalogue different from the one of [12].
5.1.4. A simple bijection. From (70), we have
(72)
?
n
zn
?
π alternating in Bn
tm(π)= sec((1 + t)z) + tan((1 + t)z).
But another immediate interpretation of the series in the righthand side is
(73)
?
n
zn
?
π s.t. π is alternating in An
tm(π)= sec((1 + t)z) + tan((1 + t)z).
It is thus in order to give a bijection proving the equality of the generating func
tions. Let π be an alternating signed permutation. We can associate with π the pair
(std(π),ǫ) where ǫ is the sign vector such that ǫi = 1 if π−1(i) > 0 and ǫi = −1
otherwise. The image of {1,...,n} by π is {ǫii : 1 ≤ i ≤ n}. Since π can be recov
ered from std(π) and the image of {1,...,n}, this map is a bijection between signed
alternating permutations and pairs (σ,ǫ) where σ is alternating and ǫ is a sign vector.
Then, with such a pair (σ,ǫ), one can associate a signed permutation τ such that τ
is alternating simply by taking τi= σiǫi. The composition π ?→ (σ,ǫ) ?→ τ gives the
desired bijection.
Page 14
14M. JOSUATVERG`ES, J.C. NOVELLI, AND J.Y. THIBON
For example, here follow the 16 permutations obtained by applying the bijection
to the 16 alternating permutations of size 3 (see Equation (61)):
23¯1,¯23¯1,132,2¯31,¯132,¯2¯31,2¯3¯1,¯2¯3¯1,231,¯231,1¯32,¯1¯32,1¯3¯2,¯1¯3¯2,13¯2,¯13¯2. (74)
5.2. Type B snakes.
5.2.1. An alternative version. The above considerations suggest a new definition of
type B snakes, which is a slight variation of the definition of [2]. We want to end up
with the generating series
1
cosz − sinz=cosz + sinz
after the same sequence of specializations. A natural choice, simple enough and given
by a series in BSym, is to set
(75)y(1,z) =
cos2z
(76)
Y = (cos + sin) · sec#=
??
k≥0
(−1)k(S2k+ S2k+1)
?
·
?
n≥0
R#
2n.
Now, Y lives in BSym and expands in the ribbon basis˜R of BSym as
Y =
??
?
?
?
k≥0
(R#
(−1)k(˜R2k+˜R2k+1)
??
n≥0
R#
2n
=
n≥0
2n +˜R12n +˜R32n)
+
k≥1;n≥0
(−1)k(˜R2k2n +˜R2k+22n−1 +˜R2k+12n +˜R2k+32n−1)
=
n≥0
(R#
2n +˜R12n +˜R32n) −
?
n≥0
(˜R2n+1 +˜R32n)
(77)
which simplifies into
(78)
Y = 1 +
?
n≥0
(˜R12n +˜R02n+1).
In FQSym(2), this is the sum of all Gπsuch that
?
Thus, for n odd, π is exactly a Bnsnake in the sense of [2], and for n even, ¯ π is a
Bnsnake. Clearly, the number of sign changes or of minus signs in snakes and in
these modified snakes are related in a trivial way so we have generating series for
both statistics in all cases.
(79)
0 > π1< π2> ... if n is even,
0 < π1> π2< ... if n is odd.
Here are these modified snakes for n ≤ 4:
(80)1,
¯12,¯21,¯2¯1,
(81)1¯23,1¯32,1¯3¯2,213,2¯13,2¯31,2¯3¯1,312,3¯12,3¯21,3¯2¯1,
Page 15
THE ALGEBRAIC COMBINATORICS OF SNAKES15
¯12¯34,¯12¯43,¯12¯4¯3,¯1324,¯13¯24,¯13¯42,¯13¯4¯2,¯1423,¯14¯23,¯14¯32,¯14¯3¯2,¯21¯34,
¯2¯1¯34,¯21¯43,¯21¯4¯3,¯2¯1¯43,¯2¯1¯4¯3,¯2314,¯23¯14,¯23¯41,¯23¯4¯1,¯2413,¯24¯13,¯24¯31,
¯24¯3¯1,¯31¯24,¯3¯1¯24,¯31¯42,¯31¯4¯2,¯3¯1¯42,¯3¯1¯4¯2,¯3214,¯32¯14,¯32¯41,¯32¯4¯1,¯3¯2¯41,
¯3¯2¯4¯1,¯3412,¯34¯12,¯34¯21,¯34¯2¯1,¯41¯23,¯4¯1¯23,¯41¯32,¯41¯3¯2,¯4¯1¯32,¯4¯1¯3¯2,¯4213,
¯42¯13,¯42¯31,¯42¯3¯1,¯4¯2¯31,¯4¯2¯3¯1,¯4312,¯43¯12,¯43¯21,¯43¯2¯1.
(82)
5.2.2. Snakes as particular alternating permutations. Note that in the previous defi
nition, snakes are not alternating permutations for odd n. So, instead, let us consider
the generating series
(83)
Y = cos · sec#+ sin · sec#,
where f ?→¯f is the involution of FQSym(2)inverting the signs of permutations.
Expanding Y in the˜R basis, one gets
(84)
Y = 1 +
?
n≥0
(˜R02n1+˜R02n+1).
As for type B alternating permutations (see Equation (65)), the series Y satisfies
a differential equation with the same linear map d as before (see Equation (64)):
(85)dY = YX.
It is then easy to see that Y also satisfies a fixed point equation similar to (26):
(86)
Y = 1 + B(Y,X).
The iteration of (86) brings up a solution close to (68):
(87)
Y =
?
T∈CBT
BT(G0= 1,G1),
where, for a tree T, BT(a,b) is now the result of the evaluation of all expressions
formed by labeling by a or b the leaves of T and by B its internal nodes. Note that
in this case, the first leaf needs to have label a. This is the same as the trees defined
in [12, Section 4] since one can again interpret the G0leaves as empty leaves in this
setting, the remaining nodes then corresponding to all increasing trees of the same
shape.
5.2.3. Snakes from [2]. The generating series of the snakes of [2], also in BSym is
(88)
cos · sec#+ sin · sec#,
and can be written as
(89)
Y = 1 +
?
n≥0
(˜R12n +˜R12n1)
on the ribbon basis.
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