G o hendieck bialgeb as, Pa i ion la ices, and
symme ic unc ions in noncommu a i e a iables
N. Be ge on∗1, C. Hohlweg∗2,M.Rosas
∗1, and M. Zab ocki∗1.
∗1Depa men o Ma hema ics and S a is ics,
Yo k Uni e si y
To on o, On a io M3J 1P3, Canada.
be ge on@ma hs a .yo ku.ca, m [email protected], zab ocki@ma hs a .yo ku.ca
∗2The Fields Ins i u e
222 College S ee
To on o, On a io, M5T 3J1, Canada.
chohlweg@ ields.u o on o.ca
Submi ed: Jul 14, 2005; Accep ed: Jul 19, 2006; Published: Aug 25, 2006
Ma hema ics Subjec Classi ica ions: 05E05, 05E10, 16G10, 20C08.
Abs ac
We show ha he G o hendieck bialgeb a o he semi- owe o pa i ion la ice
algeb as is isomo phic o he g aded dual o he bialgeb a o symme ic unc ions in
noncommu a i e a iables. In pa icula his isomo phism singles ou a canonical
new basis o he symme ic unc ions in noncommu a i e a iables which would be
an analogue o he Schu unc ion basis o his bialgeb a.
In oduc ion
Combina o ial Hop algeb as a e g aded connec ed Hop algeb as equipped wi h a mul i-
plica i e linea unc ional ζ:H→kcalled a cha ac e (see [1]). He e we assume ha k
is a ield o cha ac e is ic ze o. The e has been enewed in e es in hese spaces in ecen
pape s (see o example [3, 4, 6, 11, 13] and he e e ences he ein). One pa icula ly
in e es ing aspec o ecen wo k has been o ealize a gi en combina o ial Hop algeb a
as he G o hendieck Hop algeb a o a owe o algeb as.
The p o o ypical example is he Hop algeb a o symme ic unc ions iewed, ia he
F obenius cha ac e is ic map, as he G o hendieck Hop algeb as o he modules o all
∗This wo k is suppo ed in pa by CRC and NSERC. I is he esul s o a wo king semina a Fields
Ins i u e wi h he ac i e pa icipa ion o T. MacHen y, M. Mishna, H. Li and L. Sabou in
he elec onic jou nal o combina o ics 13 (2006), #R75 1
symme ic g oup algeb as kSn o n≥0. The mul iplica ion is gi en ia induc ion
om kSn⊗kSm o kSn+mand he comul iplica ion is he sum o e o he es ic ion
om kSn o kS ⊗kSn− . The enso p oduc o modules de ines a hi d ope a ion on
symme ic unc ions usually e e ed o as he in e nal mul iplica ion o he K onecke
p oduc [16, 22]. The Schu symme ic unc ions a e hen canonically de ined as he
F obenius image o he simple modules.
The e a e many mo e examples o his kind o connec ion (see [5, 12, 15]). He e we
a e in e es ed in he bialgeb a s uc u e o he symme ic unc ions in noncommu a i e
a iables [7, 8, 9, 17, 21] and he goal o his pape is o ealize i as he G o hendieck
bialgeb a o he modules o he pa i ion la ice algeb as.
We deno e by NCSym =Ld≥0NCSymd he algeb a o symme ic unc ions in non-
commu a i e a iables, he p oduc is induced om he conca ena ion o wo ds. This
is a Hop algeb a equipped wi h an in e nal comul iplica ion. The space NCSymdis he
subspace o se ies in he noncommu a i e a iables x1,x
2,... wi h homogeneous deg ee
d ha a e in a ian s by any ini e pe mu a ion o he a iables. The algeb a s uc u e
o NCSym was i s in oduced in [21] whe e i was shown o be a ee noncommu a i e
algeb a. This algeb a was used in [9] o s udy ee powe s o noncommu a i e ings. Mo e
ecen ly, a se ies o new bases was gi en o his space, li ing some o he classical bases
o (commu a i e) symme ic unc ions [17]. The Hop algeb a s uc u e was unco e ed in
[2, 7, 8] along wi h o he undamen al algeb aic and geome ic s uc u es.
The (ex e nal) comul iplica ion ∆: NCSymd→LNCSymk⊗NCSymd−kis g aded and
gi es ise o a s uc u e o a g aded Hop algeb a on NCSym. The algeb a NCSym also
has an in e nal comul iplica ion ∆:NCSymd→NCSymd⊗NCSymdwhich is no g aded.
The algeb a NCSym wi h he comul iplica ion ∆is only a bialgeb a (no g aded) and is
di e en om he p e ious g aded Hop s uc u e.
A e in es iga ing he Hop algeb a s uc u e o NCSym, i is na u al o ask i he e
exis s a owe o algeb as {An}n≥0such ha he Hop algeb a NCSym co esponds o he
G o hendieck bialgeb a (o Hop ) algeb a o he An-modules. This was he 2004-2005
ques ion o ou algeb aic combina o ics wo king semina a Fields Ins i u e whe e he
esea ch o his a icle was done.
Ou answe in ol es he pa i ion la ice algeb as (kΠn,∧)and(kΠn,∨) (as well as
he Solomon-Ti s algeb as [10, 18, 20]). Fo each one, wi h ini e modules we can de ine
a enso p oduc o kΠnmodules and a es ic ion om kΠnmodule o kΠk⊗kΠn−k
modules. This allows us o place on LnG0(kΠn), he G o hendieck ing o he kΠn,
a bialgeb a s uc u e (bu no a Hop algeb a s uc u e). We hen de ine a bialgeb a
isomo phism LnG0(kΠn)→NCSym∗. We call his map he F obenius cha ac e is ic
map o he pa i ion la ice algeb as. This singles ou a unique canonical basis o NCSym
(up o au omo phism) co esponding o he simple modules o he kΠn.
Ou pape is di ided in o 4 sec ions as ollows. In sec ion 1 we ecall he de ini ion
and s uc u e o NCSym. We hen s a e ou i s heo em claiming he exis ence o a basis
xo NCSym de ined by ce ain algeb aic p ope ies. The p oo o i will be pos poned
o sec ion 4. In sec ion 2 we ecall he de ini ion and s uc u e o he pa i ion la ice
algeb as kΠnwi h he p oduc gi en by he la ice ope a ion ∧and de ine hei modules.
he elec onic jou nal o combina o ics 13 (2006), #R75 2
We hen in oduce a s uc u e o a semi- owe o algeb as (i.e. we ha e a non-uni al
embedding ρn,m :kΠn⊗kΠm→kΠn+mo algeb as) on he pa i ion la ice algeb as and
show ha i induces a bialgeb a s uc u e on i s G o hendieck ing. Ou second heo em
s a es ha his G o hendieck bialgeb a is dual o NCSym. The classes o simple modules
co espond hen o he basis x. In iew o he wo k o B own [10] we ema k ha his
can also be done wi h he semi- owe o Solomon-Ti s algeb as. In sec ion 3 we build
he same cons uc ion wi h he la ice algeb as kΠnwi h he p oduc ∨.Wi h his owe
o algeb as (i.e. ρn,m is a uni al mo phism o algeb as) we ind ha he G o hendieck
bialgeb a is again dual o NCSym, bu his ime he classes o simple modules co espond
o he monomial basis o NCSym.
In sec ion 4 we gi e he p oo o ou i s heo em and show he basis canonically
de ined in sec ion 2 co esponds o he simple modules o he kΠn. In ligh o he F obe-
nius cha ac e is ic o sec ion 2, he basis can be in e p e ed as an analogue o he Schu
unc ions o NCSym and p o iding an answe o an open ques ion o [17].
1NCSym and he basis {xA}
We ecall he basic de ini ion and s uc u e o NCSym. Mos o i can be ound in [7, 8].
A se pa i ion Ao mis a se o non-emp y subse s A1,A
2,...,A
k⊆[m]={1,2,...,m}
such ha Ai∩Aj=∅ o i6=jand A1∪A2∪···∪Ak=[m]. The subse s Aia e called
he pa s o he se pa i ion and he numbe o non-emp y pa s he leng h o A, deno ed
by `(A). The e is a na u al mapping om se pa i ions o in ege pa i ions gi en by
λ(A)=(|A1|,|A2|,...,|Ak|), whe e he lis is hen so ed so ha he in ege s a e lis ed
in weakly dec easing o de o o m a pa i ion.
We shall use `(λ) o e e o he leng h ( he numbe o pa s) o he pa i ion and |λ|
is he size o he pa i ion ( he sum o he sizes o he pa s), while ni(λ)shall e e o
he numbe o pa s o he pa i ion o size i.Wedeno ebyΠ
m he se o se pa i ions
o m. The numbe o se pa i ions is gi en by he Bell numbe s. These can be de ined
by he ecu ence B0=1andBn=Pn−1
i=0 n−1
iBi.
Fo a se S={s1,s
2,...,s
k}o in ege s siand an in ege nwe use he no a ion
S+n o ep esen he se {s1+n, s2+n,...,s
k+n}.Fo A∈Πmand B∈Π se
pa i ions wi h pa s Ai,1≤i≤`(A)andBi,1≤i≤`(B) espec i ely, we se
A|B={A1,A
2,...,A
`(A),B
1+m, B2+m,...,B
`(B)+m}, he e o e A|B∈Πm+ and his
ope a ion is noncommu a i e in he sense ha , in gene al, A|B6=B|A.
When w i ing examples o se pa i ions, whene e he con ex allows i , we will
use a mo e compac no a ion. Fo example, {{1,3,5},{2},{4}} will be ep esen ed by
{135.2.4}. Al hough he e is no o de on he pa s o a se pa i ion, we will impose an
implied o de such ha he pa s a e a anged by inc easing alue o he smalles elemen
in he subse . This implied o de will allow us o e e ence he i h pa s o he se pa i ion
wi hou ambigui y.
The e is a na u al la ice s uc u e on he se pa i ions o a gi en n. We de ine o
A, B ∈Πn ha A≤Bi o each Ai∈A he e is a Bj∈Bsuch ha Ai⊆Bj(o he wise
s a ed, ha Ais ine han B). The se o se pa i ions o [n] wi h his o de o ms a
he elec onic jou nal o combina o ics 13 (2006), #R75 3
pose wi h ank unc ion gi en by nminus he leng h o he se pa i ion. This pose has a
unique minimal elemen 0n={1.2. ... .n}and a unique maximal elemen 1n={12 ...n}.
The la ges elemen smalle han bo h Aand Bis deno ed
A∧B={Ai∩Bj:1≤i≤`(A),1≤j≤`(B)}
while he smalles elemen la ge han Aand Bis deno ed A∨B. The la ice (Πn,∧,∨)
is called he pa i ion la ice.
Example 1.1 Le A={138.24.5.67}and B={1.238.4567}.Aand Ba e no compa able
in he inclusion o de on se pa i ions. We calcula e ha A∧B={1.2.38.4.5.67}and
A∨B={12345678}.
When a collec ion o disjoin se s o posi i e in ege s is no a se pa i ion because he
union o he pa s is no [n] o some n, we may lowe he alues in he se s so ha hey
keep hei ela i e alues so ha he esul ing collec ion is a se pa i ion (o an m<n).
This ope a ion is e e ed o as he ‘s anda diza ion’ o a se o disjoin se s Aand he
esul ing se pa i ion is deno ed s (A).
Now o A∈Πmand S⊆{1,2,...,`(A)}wi h S={s1,s
2,...,s
k}, we de ine AS=
s ({As1,A
s2,...,A
sk}) which is a se pa i ion o |As1|+|As2|+...+|Ask|.Bycon en ion
A{} is he emp y se pa i ion.
Example 1.2 I A={1368.2.4.579}, henA{1,4}={1246.357}.
Fo n≥0, conside a se Xno non-commu ing a iables x1,x
2,...,x
nand he poly-
nomial algeb a RXn=khx1,x
2,...,x
niin hese non-commu ing a iables. The e is a
na u al Snac ion on he basis elemen s de ined by σ(xi1xi2···xik)=xσ(i1)xσ(i2)···xσ(ik).
Le xi1xi2···ximbe a monomial in he space RXn.Wesay ha he ypeo hismonomial
is a se pa i ion A∈Πmwi h he p ope y ha ia=ibi and only i aand ba e in he
same block o he se pa i ion. This se pa i ion is deno ed as ∇(i1,i
2,...,i
m)=A.
No ice ha he leng h o ∇(i1,i
2,...,i
m) is equal o he numbe o di e en alues which
appea in (i1,i
2,...,i
m).
The ec o space NCSym(n)is de ined as he linea span o he elemen s
mA[Xn]= X
∇(i1,i2,...,im)=A
xi1xi2···xim
o A∈Πm, whe e he sum is o e all sequences wi h 1 ≤ij≤n.Fo heemp yse
pa i ion, we de ine by con en ion m{}[Xn]=1.I `(A)>nwe mus ha e ha mA[Xn]=
0. Since o any pe mu a ion σ∈Sn,∇(i1,i
2,...,i
m)=∇(σ(i1),σ(i2),...,σ(im)), we
ha e ha σmA[Xn]=mA[Xn]. In ac , mA[Xn] is he sum o all elemen s in he o bi
o a monomial o ype Aunde he ac ion o Sn. The e o e NCSym(n)is he space o Sn-
in a ian s in he noncommu a i e polynomial algeb a RXn. Fo ins ance, m{13.2}[X4]=
x1x2x1+x1x3x1+x1x4x1+x2x1x2+x2x3x2+x2x4x2+x3x1x3+x3x2x3+x3x4x3+x4x1x4+
x4x2x4+x4x3x4.
he elec onic jou nal o combina o ics 13 (2006), #R75 4
As in he classical case, whe e he numbe o a iables is usually i ele an as long as i
is big enough, we wan o conside ha we ha e an in ini e numbe o non-commu ing a i-
ables. Since NCSym(n)inhe i s om khx1,x
2,...,x
nia g aded algeb a s uc u e, we con-
side , o any m≥n, he homomo phism o g aded algeb as khx1,...,x
mi→khx1,...,x
ni
ha sends a iables xn+1,...,x
m o ze o and he emaining ones o hemsel es. This map
es ic s o a su jec i e homomo phism ρm,n :NCSym(m)→NCSym(n), ha sends mA[Xm]
o mA[Xn]. The amily {NCSym(n):n≥1} oge he wi h he homomo phisms ρm,n o ms
an in e se sys em in he ca ego y o g aded algeb as. Le NCSym be i s in e se limi in
his ca ego y. We call NCSym he algeb a o symme ic unc ions in an in ini e numbe
o non-commu ing a iables.
Fo each se pa i ion A he e exi s an unique elemen mAwhose p ojec ion o each
NCSym(n)is mA[Xn]. These elemen s a e called monomial symme ic unc ions in an
in ini e numbe o non-commu ing a iables.
I we decompose NCSym as he sum o i s g aded pieces,
NCSym =M
d≥0
NCSymd,
hen he monomial symme ic unc ions mA,wi hA`[d], is a linea basis o NCSymd.
He e we o ge any e e ence o he a iables x1,x
2,...and hink o elemen s in NCSym
as noncommu a i e symme ic unc ions. The deg ee o a basis elemen mAis gi en by
|A|=dand he p oduc map µ:NCSymd⊗NCSymm−→ NCSymd+mis de ined on he
basis elemen s mA⊗mBby
µ(mA⊗mB):= X
C∈Πd+m
C∧1d|1m=A|B
mC.(1)
This is a li o he mul iplica ion in NCSym(n).
The g aded algeb a NCSym is in ac a Hop algeb a wi h he ollowing comul iplica ion
∆:NCSymd−→ Ld
k=0 NCSymk⊗NCSymd−kwhe e
∆(mA)= X
S⊆[`(A)]
mAS⊗mASc(2)
and Sc=[`(A)] −S. The couni is gi en by :NCSym →Qwhe e (m{})=1and
(mA) = 0 o all A∈Πn o n>0. Mo e de ails on his Hop algeb a s uc u e a e
ound in [7, 8].
The algeb a NCSym was o iginally conside ed by Wol [21] in ex ending he unda-
men al heo em o symme ic unc ions o his algeb a and la e by Be gman and Cohn
[9]. Mo e ecen ly Rosas and Sagan [17] conside ed his space o de ine na u al bases
which a e analogous o bases o he (commu a i e) symme ic unc ions. Mo e p og ess
in unde s anding his space was made in [7, 8] whe e i was conside ed as a Hop alge-
b a. In he Hop algeb a Sym o (commu a i e) symme ic unc ions, he comul iplica ion
co esponds o he ple hysm [X]7→ [X+Y]. I was es ablished in [7] ha he comul i-
plica ion in NCSym co esponds o a noncommu a i e ple hysm F[X]7→ F[X+Y], whe e
he elec onic jou nal o combina o ics 13 (2006), #R75 5

X+Yis he alphabe ( o ally o de ed se o non-commu ing a iables) co esponding
o he disjoin union o Xand Y, oge he wi h he o al o de ob ained om Xand Y
placing all Ya e all X.(Tha is,x<y o all xin Xand all yin Y.)
The Hop algeb a Sym has mo e s uc u e. The e is a second comul iplica ion co e-
sponding o he ple hysm [X]7→ [XY] (see [16, 22]). This second ope a ion is o en
e e ed o as he in e nal comul iplica ion o K onecke comul iplica ion. We end his
sec ion desc ibing o NCSym he analog o his in e nal comul iplica ion. This desc ip ion
is also conside ed in [2].
Fo he Hop algeb a NCSym we de ine a second (in e nal) comul iplica ion
∆:NCSymd−→ NCSymd⊗NCSymd
by
∆(mA)= X
B∧C=A
mB⊗mC.(3)
This ope a ion co esponds o a noncommu a i e ple hysm F[X]7→ F[XY]. Mo e p e-
cisely, assume ha we ha e wo coun able alphabe X=x1,x
2,... and Y=y1,y
2,....
Then, XY =x1y1,x
1y2,...,x
iyj,..., o ally o de ed using he lexicog aphic o de . Tha
is, xy < zw i and only i (x<z)o (x=zand y<w) o all x, z in Xand all
y,w in Y. We conclude ha he ans o ma ion F[X]7→ F[XY ] sends F(x1,x
2,...) o
F(x1y1,x
1y2,...,x
2y1,x
2y2,...).
I we le he xi’s commu e wi h he yj’s hen we ha e ha F[XY] can be expanded
in he o m F[XY]=PF1,i[X]F2,i[Y]. We can hen de ine he ope a ion
∆(F)=XF1,i ⊗F2,i.
Equa ion (3) gi es he esul o his when F=mA. Clea ly his ope a ion is a mo phism
o he mul iplica ion, hus NCSym wi h ∆and he mul iplica ion ope a ion o equa ion
(1) o ms a bialgeb a. Bu i is no a Hop algeb a as i does no ha e an an ipode. We
a e now in posi ion o s a e ou i s main heo em.
Rema k: In o de o de ine he sum and p oduc o wo alphabe s, X+Yand XY,
on he in e se limi o khx1, ..., xni, i is necessa y o in oduce a o al o de on each o
hem. On he o he hand, when we es ic ou sel es o elemen s o Sym, he esul is
independen o he pa icula choice o o al o de we made.
Theo em 1.3 The e is a basis {xA:A∈Πn,n≥0}o NCSym such ha
(i) xAxB=xA|B.
(ii) ∆(xC)= X
A∨B=C
xA⊗xB.
The p oo o his heo em is echnical and we di e i o Sec ion 4. We a e con inced
ha he basis {xA:A∈Πn,n ≥0}is cen al in he s udy o NCSym and should ha e
many ascina ing p ope ies. We plan o s udy his basis u he in u u e wo k. Fo now,
we p e e o de elop he ep esen a ion heo y ha will mo i a e ou esul .
he elec onic jou nal o combina o ics 13 (2006), #R75 6
2 G o hendieck bialgeb a o he Semi- owe (Π,∧)=
Ln≥0(kΠn,∧).
In his sec ion we conside he pa i ion la ice algeb as. Fo a ixed nconside he ec o
space (kΠn,∧) o mally spanned by he se pa i ions o n. The mul iplica ion is gi en
by he ope a ion ∧on se pa i ions and wi h he uni 1n={1,2,...,n}.We ema k
ha o all d,weha e ha kΠdis isomo phic as a ec o space o NCSymd ia he pai ing
A↔mA. Mo eo e , i is s aigh o wa d o check using equa ion (3) ha ∆is dual o
∧as ope a o s.
I is well known ha (kΠn,∧) is a commu a i e semisimple algeb a (see [19, Theo em
3.9.2]). To see his, one conside s he algeb a kΠn={ :Π
n→k}which is clea ly
commu a i e and semisimple. We hen de ine he map
δ≥:(kΠn,∧)→kΠn
A7→ δA≥,
whe e δA≥(B)=1i A≥Band 0 o he wise. Nex check ha δA∧B≥=δA≥δB≥which
shows ha δ≥is an isomo phism o algeb as.
The p imi i e o hogonal idempo en s o kΠna e gi en by he unc ions δA=de ined
by δA=(B)=1i A=Band 0 o he wise. We ha e ha δA≥=PB≤AδB=. This implies,
using M¨obius in e sion, ha he p imi i e o hogonal idempo en s o (kΠn,∧)a egi en
by
eA=X
B≤A
µ(B,A)B, (4)
whe e µis he M¨obius unc ion o he pa ially o de ed se Πn.Since(kΠn,∧)iscommu-
a i e and semisimple, we ha e ha he simple (kΠn,∧)-modules o his algeb a a e he
one dimensional spaces VA=kΠn∧eA. He e he ac ion is gi en by he le mul iplica ion
C∧eA=eAi C≥A,
0 o he wise. (5)
This ollows om he co esponding iden i y in kΠnconside ing δ≥Cδ=A.
We now le G0(kΠn,∧) deno e he G o hendieck g oup o he ca ego y o ini e di-
mensional (kΠn,∧)-modules. This is he ec o space spanned by he equi alence classes
o simple (kΠn,∧)-modules unde isomo phisms.
We also conside K0(kΠn,∧) he G o hendieck g oup o he ca ego y o p ojec i e
(kΠn,∧)-modules. Since (kΠn,∧) is semisimple, he space G0(kΠn,∧)andK0(kΠn,∧)
a e equal as ec o spaces as hey a e bo h linea ly spanned by he elemen s VA o A∈Πn.
We hen se K0(Π,∧)=Ln≥0K0(kΠn,∧).
Gi en wo ini e (kΠn,∧) modules Vand W, we can o m he (kΠn,∧)-module V⊗W
wi h he diagonal ac ion (i is an ac ion since a semig oup algeb a is a bialgeb a o he
cop oduc A→A⊗A). We deno e his (kΠn,∧)-module by VW( o a oid con usion
wi h he enso p oduc o a (kΠn,∧)-module and a (kΠm,∧)-module).
he elec onic jou nal o combina o ics 13 (2006), #R75 7
Lemma 2.1 Gi en wo simple (kΠn,∧)-module VAand VB,
VAVB=VA∨B.(6)
p oo : Le C∈Πnac on eA⊗eB. F om equa ion (5) we ge C∧(eA⊗eB)=
(C∧eA)⊗(C∧eB)=eA⊗eBi and only i C≥Aand C≥B, ha isC≥A∨B.I
no , we ge C∧(eA⊗eB) = 0. We conclude ha he map eA⊗eB7→ eA∨Bis he desi ed
isomo phism in equa ion (6). 
We would like o de ine on G0(Π,∧)=Ln≥0G0(kΠn,∧) a g aded mul iplica ion and
a g aded comul iplica ion co esponding o induc ion and es ic ion. Fo his we need a
ew mo e ools.
Lemma 2.2 The linea map ρn,m :(kΠn,∧)⊗(kΠm,∧)→(kΠn+m,∧)de ined by
ρn,m(A⊗B)=A|B
is injec i e and mul iplica i e. Mo eo e , ρk+n,m ◦(ρk,n ⊗Id)=ρk,n+m◦(Id ⊗ρn,m) o
all k, n and m.
p oo : Le A={A1,...,A
},B={B1,...,B
s}be se pa i ions in Πn,andC=
{C1,...,C
}and D={D1,...,D
u}be se pa i ions in Πm. We ema k ha o all i, j,
we ha e Ai∩(Dj+n)=∅and (Ci+n)∩Bj=∅.Since(Ci+n)∩(Dj+n)=(Ci∩Dj)+n,
we ha e
(A|C)∧(B|D)=Ai∩Bj1≤i≤
1≤j≤s
∪(Ci+n)∩(Dj+n)1≤i≤
1≤j≤u
=(A∧B)(C∧D),
and his shows ha ρn,m is mul iplica i e. The injec i i y o his map is clea om he
ac ha ρn,m maps dis inc basis elemen s in o dis inc basis elemen s. The las iden i y
o he lemma ollows om he associa i i y o he ope a ion “|”
We de ine a semi- owe (Ln≥0An,{φn,m}) o be a di ec sum o algeb as along wi h
a amily o injec i e non-uni al homomo phisms o algeb as φn,m :An⊗Am→An+m.
A owe in he sense de ined in he ecen li e a u e [5, 12, 15] is a semi- owe wi h he
addi ional cons ain ha φn,m(1n,1m)=1n+m(i.e. ha φn,m is a uni al embedding o
algeb as).
De ine he pai (Π,∧)=Ln≥0(kΠn,∧),{ρn,m}which is a semi- owe o he al-
geb as (kΠn,∧). We ema k ha (Π,∧) is a g aded algeb a wi h he mul iplica ion
ρn,m(A, B)=A|Bwhich is associa i e (bu non-commu a i e) and has a uni gi en by
he emp yse pa i ion ∅∈Π0. Mo eo e , each o he homogeneous componen s (kΠn,∧)
o Πa e hemsel es algeb as wi h he mul iplica ion ∧, and Lemma 2.2 gi es he ela-
ionship be ween he wo ope a ions.
A his poin we need o s ess ha ρn,m is no a uni al embedding o algeb as and hence
(Π,∧) is no a owe o algeb as. The algeb a (kΠn,∧)hasauni gi enby1n={12 ...n},
he elec onic jou nal o combina o ics 13 (2006), #R75 8
bu ρn,m(1n⊗1m)6=1n+m. The owe o algeb as conside ed in he ecen li e a u e
[5, 12, 15] all ha e he p ope y ha he co esponding ρn,m a e (uni al) embeddings o
algeb as. This is he eason we call ou cons uc ion a semi- owe a he han a owe .
The mo i a ion o de ining a owe o algeb as is o allow one o induce and es ic
modules o hese algeb as and ul ima ely o de ine on i s G o hendieck ing a Hop algeb a
s uc u e. He e he ac ha we ha e only a semi- owe causes some p oblems in de ining
es ic ion o modules. Ye we can s ill de ine a weake e sion o es ic ion in ou
si ua ion. Le Aand Bbe wo ini e dimensional algeb as and le ρ:A→Bbe a
mul iplica i e injec i e linea map. Gi en a ini e B-module M, we de ine
ResρM={m∈M:ρ(1A)m=m}⊆M.
In he case whe e ρis an embedding o algeb as his de ini ion ag ees wi h he adi ional
one. Mo e on his gene al heo y will be ound in [14] bu he e we ocus ou a en ion on
(Π,∧).
Lemma 2.3 Fo k≤nand a simple (kΠn,∧)-module VA∈G0(kΠn,∧),
Resρk,n−kVA=(VAi A=B|C o B∈Πkand C∈Πn−k
0o he wise.
p oo : We ha e ha ρn,m(1k⊗1n−k)∧eA=(1k|1n−k)∧eA=eAi 1k|1n−k≥A,and0
o he wise. The condi ion 1k|1n−k≥Ais equi alen o A=B|Cwhe e A|1,...,k =Band
A|k+1,...,n+k=C.
We can now de ine a g aded comul iplica ion on G0(Π,∧) using ou de ini ion o
es ic ion. Fo V∈G0(kΠn,∧)le
∆(V)=
n
X
k=0
Resρk,n−kV. (7)
I ollows om Lemmas 2.2 ha his ope a ion is coassocia i e. Fo a simple module
VA∈G0(kΠn,∧), Lemma 2.3 gi es us
∆(VA)= X
A=B|C
VB⊗VC.(8)
Now we ex end  o G0(Π,∧) by se ing VAVB=0i VAand VBa e no o he same
deg ee.
P oposi ion 2.4 (G0(Π,∧),,∆) is a bialgeb a.
p oo : Le A, B ∈Πn. By equa ion (6), i is su icien o p o e ha ∆(VA∨B)=
∆(VA)∆(VB). Using equa ion (2.3) we can easily educe he p oblem o he ollowing
asse ion: he e a e C∈Πk,D∈Πn−ksuch ha A∨B=C|Di and only i he e a e
he elec onic jou nal o combina o ics 13 (2006), #R75 9
Lemma 4.2
(i) xAxB=xA|B
(ii) ∆(xC)= X
A∨B=C
xA⊗xB.
p oo : Using he same a gumen as in Lemma 2.5 we ha e
xAxB=X
C≤AX
D≤B
µ(C, A)µ(D, B)pCpD
=X
C≤AX
D≤B
µ(C, A)µ(D, B)pC|D=X
E≤A|B
µ(E,A|B)pE=xA|B.
This shows he i s iden i y. Fo he second, he le hand side o (ii) is
∆(xC)=X
E≤C
µ(E,C)∆(pE)=X
E≤C
µ(E,C)pE⊗pE,(19)
and he igh hand side is
X
A∨B=C
xA⊗xB=X
A∨B=CX
E≤A
F≤B
µ(E,A)µ(F, B)pE⊗pF.
Le us isola e he coe icien o pE⊗pFin hesumabo ewege
TC
E,F =X
E≤A≤C
F≤B≤CX
A∨B=C
µ(E,A)µ(F, B) (20)
=X
F≤B≤C
X
E≤A≤C
A∨B=C
µ(E,A)
µ(F, B).
By symme y (in e changing he ole o Eand Fi needed), we may assume ha F6<E.
In [19], Co olla y 3.9.3 is dual o he ollowing s a emen
X
A≤1n
A∨B=1n
µ(0n,A)=µ(0n,1n)i B=0n,
0 o he wise.
whe e, as usual, 0n={1.2. ... .n}. This implies ha he sum o in b acke in equa ion
(20) is equal o
X
E≤A≤C
A∨B=C
µ(E,A)=µ(E,C)i B=E,
0 o he wise. (21)
he elec onic jou nal o combina o ics 13 (2006), #R75 16

This ollows om he ac ha µis mul iplica i e and in gene al he in e al [E,C]⊆Πnis
isomo phic o a ca esian p oduc o (smalle ) pa i ion la ices (see Example 3.9.4 in [19]).
I we subs i u e his back in equa ion (20) we ha e wo cases o conside . When F6=E,ou
assump ion ha F6<Ep ohibi s he possibili y ha F≤B=E.Thuswemus always
ha e B6=Eand in his case TC
E,F = 0. When F=E, he only alue o Bwhe e equa ion
(21) does no anish is when B=E=Fand we ge TC
E,E =µ(E,C)µ(E,E)=µ(E,C).
I we compa e his o equa ion (19) we conclude ou p oo o (ii). 
No ice ha he cha ac e o he module ( he ace o he ma ix ep esen ing he ac ion
o kΠn)VB om o mula (5) is gi en by he o mula χVB(A)=δB≤(A)whenA∈kΠn
ac s on VB. We obse e ha equa ion (18) o xAyields
pA=X
B≤A
xB=X
B
χVB(A)xB.
This means ha he cha ac e s o he simple modules o (Π,∧) a e encoded in he
change o basis coe icien s be ween he pand xbasis.
Simila ly, he cha ac e o he module WBwhen ac ed on by he elemen A∈kΠn
a e gi en by he o mula χWB(A)=δB≥(A) om equa ion (11). O cou se he de ining
ela ion o he pbasis om equa ion (17) shows ha
pA=X
B
χWB(A)mB.
We obse e in his o mula ha he cha ac e s o he simple modules o (Π,∨) a e encoded
in he change o basis coe icien s be ween he pand mbasis.
Bo h hese o mulas a e in ai ly close analogy wi h he o mula o he expansion o
he powe basis in he Schu basis in he algeb a o he symme ic unc ions. The e he
change o basis coe icien s a e he cha ac e s o he simple modules o he symme ic
g oup. This shows ha he p-basis which was de ined by Rosas and Sagan [17] does
ep esen he analogue o he powe basis in he algeb a o he symme ic unc ions and
he xand he mbases encode in hei coe icien s he cha ac e s o he modules ha hey
ep esen .
Rema k 4.3 One could also de ine a hi d algeb a (kΠn,@) whe e A@B=δA=BAand
cons uc he simple modules as we ha e done he e o (kΠn,∧)and(kΠn,∨). This
same cons uc ion shows ha he simple modules o his algeb a sa is y a enso p oduc ,
induc ion and es ic ion ope a ions which make he G o hendieck ing (o he ca ego y
o he ini e dimensional p ojec i e modules) o his algeb a isomo phic again o NCSym
as a bialgeb a whe e he simple modules beha e as he elemen s pA∈NCSym and pAis
de ined in (17).
he elec onic jou nal o combina o ics 13 (2006), #R75 17
Rema k 4.4 Summa y o bases in NCSym.
The mbasis:
mAmB=X
C∧(1n|1k)=A|B
mC
∆(mA)= X
S⊆[`(A)]
mAS⊗mASc
∆(mA)= X
B∧C=A
mB⊗mC
The pbasis:
pApB=pA|B
∆(pA)= X
S⊆[`(A)]
pAS⊗pASc
∆(pA)=pA⊗pA
The xbasis:
xAxB=xA|B
∆(xA)= X
B∨C=A
xB⊗xC
I would be in e es ing o ind a o mula o ∆(xA).
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