C 0 N T E N T S
0. Introduction
1. Cohomology of Monoids and Monoidalgebras
2. Shuffling
3. Harrison Cohomology
4. The Relation between Harrison Cohomology and Algebra Cohomology
5. Harrison Cohomology of Monoid-like ordered Sets
6. Graded Harrison Cohomology
7. Harrison Cohomology of two-dimensional Torus Embeddings
8. An Example
9. References
p A (k,A;M)
Ap{A;M) p
Harr (A ;M)
p Ha (L; Z)
p HA (L;Z)
N 0 T A T I 0 N
algebra cohomology of A with values in M
algebra cohomology of a monoid A with values in M
Harrison cohomology of a monoid A with values in M
(differential ; d)
Homogenous Harrison cohomology of an ordered subset
L of A (differential ; o)
Inhomogenous Harrison cohomology of an orderen
subset L of A
Relative Harrison cohomology of an ordered set L
with respect to the ordered subset L-L 0
0. Introduction
Many authors (among others L&S, Ri, Sch 1, Ch) have tried to
calculate the algebra cohomology groups of singularities of various
special kind. In this paper we study monoid-algebras and give a
method for calculating the algebra cohomology by using the
combinatorial properties of the monoid. We introduce the Harrison
cohomology of commutative monoids as an important tool in the study
of commutative cohomology of commutative algebras. For the two
dimensional torus embeddings over a field of characteristic zero, we
give a formula for computing the cohomology by looking carefully at
some finite subsets of the monoid.
Starting with a monoid-algebra we have different cohomology theories
of which two are very natural, one in the category of commutative
algebras and one in the category of commutative monoids. In the first
chapter a proof of Laudal is simplified to show that the two
cohomology theories coincide. In chapter 2 a number of usefull lemmas
concerning shuffling-theory are stated and proved, mainly as a tool
for working with Harrison cohomology. This cohomology is defined
using the Hochschild complex, dividing out by all shuffle-products.
In (Sch & S) Schlessinger and Stasheff shows that this theory is the
same as the algebra cohomology defined in chapter 1. We give another
proof for this fact, see chapter 3 and 4, using a quite different
method, more specific for monoids. The main point is to show that the
free commutative monoids are models for the theory, i.e. have
vanishing cohomology.
Looking at the graded cohomology groups we are led to the definition
of Harrison cohomology of monoid-like ordered sets. In chapter 5 and
6 we show the close relation between graded cohomology of the monoid
A and Harrison cohomology of monoid-like ordered subsets of A.
In the special case of two-dimensional torus embeddings we have
enough coherence in the monoid and the spectral sequence degenerates.
vve are able to state the results which give the algebra cohomology
groups as a direct sum of Harrison cohomology groups of finite
monoid-like subsets of the monoid. The well-known formulas
dimkA1 (A,k[A])=2(r-1) and dimkA2 (A,k[A])=r(r-2) are easily
verified. In addition we can compute the dimensions in the higher
dimensional cases.
- 1 -
1. Cohomology of monoids and monoidalgebras.
Let k-alg be the category of commutative k-algebras, and let k-free
be the full subcategory of free commutative k-algebras, i.e. polyno
mial rings over k. Let A be an object of k-alg and denote by
& k-free/A the category where the objects are morphisms r + A of the
polynomial ring r into A, and the morphisms are co~tative dia-
grams
(jl r + r
1 . 2
6\ /&2 A
If M is any A-module we define the functor
r by the equality Derk(~6,M) = Derk(r,M) where M is considered as a
A
r-module via 6.
Definition 1 .1. With the notation as above we define the algebra
cohomology groups of A with values in the A-module M by
p A (k,A: M) =
(P) lim Derk(-,M) +--
(k-free/A)0
6 Let F be a free k-algebra and F + A a surjection. Consider the
semi-simplicial k-algebra
where
F . 0
F : A+-----•
•••• x F0 , the fibered product of A
p+1 copies of
- 2 -
Proposition 1.2. There exist a Leray spectral sequence with
• converging to the cohomology A (k,A:M) where F. is as above and
H is considered as a F -module via 6 • •
Proof. Follows immediately from the Leray spectral sequence of
[ La 1 ( 2 . 1. 3 ) ] .
Let ~ be the category of commutative monoids and let free ~ be
the full subcategory of free monoids, i.e monoids isomorphic to
for some n. Let A be an object of ~ and consider the category
6 free ~/A whre the objects are morphisms r + A, with r free.
Define the functor
Der(-,M): free ~/A + Ab
where M is a k[A]-module (and therefore a A-module) and
r Der(~6,M) = Der(r,M)
A
M is considered as a r-module via 6.
Definition 1.3. We define the cohomology groups of the monoid A
with values in M by
1 . ( p) ~m Der(-,M) p)Q.
~
( free mon/ A ) O
6 Let r + A be surjective, with r a free abelian monoid. Consider
the semi-simplicial monoid
0
- 3 -
r : A • r = r x r 0 1 0 .A
Proposition 1.4. There exists a Leray spectral sequence with
• converging to the cohomology A (A: M).
Proof. Follows from [La 1 (2.1 .3)].
D
Now observe that for an abelian monoid A "' Z + and the associated
monoid k-algebra k[A] we have the equality
Der(A: M) = Derk(k(A]: M)
Consider the semi-simplicial monoid •
defined above. It is easily r
seen that the associated k-algebra k[r.] is a semi-simplicial
object of the category k-alg. Moreover, there is a natural morphism
of semi-simplicial k-algebras
where
q,: k[r ] ~ k[r]
4> : n
.
k[r xr X•••Xr ] ~ k[r ] X k[r ] X ••• X k[r ] 0A 0A A O O k(A] O k[A] k[A] O
is defined by <r 1 ,···,y ) r 1 Yn
t n ~ (t ,•••,t ).
Suppose this map is surjective. Then it follows from the proof of
(2.1.3) in [La 1] that we may replace k[r]. by k[r.] in
proposition 1 .2. The reason is that if we replace the complex
c = { ( z, k[ r] ) , ~ } , 0 of lemma 2. 1 . 1 in [La 1 ] by • p p p ...
C' = {C(Z,k[r ]),o } , 0 it is still a resolution of C(Z,k[A)) in • p p P'
(k-free) 0 the category Ab ----
- 4 -
So we must show that ~n is surjective.
Let us assume A is finitely generated. k[r 0 ] has a natural
A-grading so we may work on the A-homogeneous parts. Pick AEA. Since
A is finitely generated there is a finite number of monomials
Y 1 Y m t ,•••,t such that
yi A o(t ) = t .
Let w be a homogeneous element of k[r0 ]p of degree A. He can
write
m y, w = ( l ai t ~
i=1
m y. m y. l b.t ~, ••• , l c.t ~
i=1 ~ i=l ~
m y, l d.t ~)
i=l ~
where a,, b,, c. and d. are elements of the ground field k. We ~ ~ ~ ~
shall prove the surjectivity of ~ by constructing an element
w E k[r ] such that ~ (W) = w. p p
It is easily seen that the element
m-1 (yi,y 1 , ... ,y 1 ) m-1 (yn,y 1 , ... ,y 1 ) m (y ,y,,y 1 , .. ,y1 ) H= L a.t +(b1- L a.)t + l b.t n ~
i=l ~ i=1 ~ i=l ~
m-1 ( Y I • • • 1 Y I Y 1 ) m-1 ( Y I • • • 1 Y I Y • ) + ••• + (d- l c )t n n + l d.t n n ~
1 i=1 1 i=1 ~
has this property.
We have thus proved the following
Theorem 1 . 5. With the notation as above we have an isomorphism of
cohomology groups
p p A (A; M) =A (k,k(A]; M) p>O.
0
- 5 -
2. Shuffling.
Let I be a totally ordered set with n elements. Devide I into
two blocks I = (I <I ) 1 2
where =I =p. 1
Definition 2.1. A (p,n-p)-shuffling a of I into a totally
ordered set J is a bijective map a: I ~ J such that
a ( i ) < a ( j ) if i < j E I 1 or i < j E I 2 .
This definition is slightly more general than the usual definition,
which is obtained by setting I=J={1,2, ... n}.
Lemma 2.2. Let a be a (p,n-p)-shuffling of {1, •.• ,n} into it-
self. Then we have
i) a-1 (1) = 1 or a- 1 (1) = p+l
ii) -1 -1 a (n) = p or a (n) = n
Proof. Suppose a- 1 (1) = j. Two cases are possible. 1 ( j<p+1 implies
a ( 1 ) -1 if p+1<:1<:j<:n a(p+1) -1
( a a ( 1 ) = and we have ( a a ( 1) = 1 .
ii) is proved in a similar way. 0
Remark 2.3. Using lemma 2.2 we can describe the shufflings recursi-
vely. A (p,n-p)-suffling a: {1, •.. ,n} ~ {1, ... ,n} is determined as
follows:
Either 1) a(l )=1 and a: {2, ••• ,n} ~ {2, ••. ,n} a (p-1 ,n-p)-shuffling
1\ 2) a(p+1)=1 and a:{1, •.. p+1, •.. ,n} ~ {2, ... ,n}
a (p,n-p-1)-shuffling.
- 6 -
Starting in the other end we get an alterntive description:
. " 1} a(p}=n and a:{1, ..• ,p, •.. ,n} + {1, ... ,n-1} a (p-l,n-p}-shuffling.
2} a(n}=n and a:{1, ..• ,n-1} + {1, ... ,n-1} a (p,n-p-1}-shuffling.
The third description is as follows. If i E {1, ... ,n} is different
from the maximal element of {1, .•. ,n}, there is a 1-1 correspondance
between (p,n-p}-shufflings a:{1, ... ,n} + {1, .•. ,n} such that
a(i}+1=a(i+1) and (p-l,n-p}-shufflings (resp.(p,n-p-1)-shufflings)
a:{1, ... ,i+1, ... ,n}+{1, ... ,a(i+1), ... ,n} if i E lower block (resp.
upper block.) Moreover we have the mod 2-equality
I cr I = I 0" I + d ( 0" ( j ) I j ) + l ( mod 2 )
where d(a(j),j) is the number of elements in {1, ... ,n} strictly
between a(j) and j. If equality a(j)=j holds, put d(j,j) = -1.
He also need a lemma to produce new shufflings from given ones .
Lemma 2.4. Let ..... I + {1 n} "" I • 0 • I be a (p,n-p)-shuffling, and let
1(i<n. Suppose a-1 (i} and a-1 (i+1) are not in the same block. Let
~ be the transposition which permutes a-1 (i) and a-1 (i+1). Then
ao~ is a (p,n-p)-shuffling of I into {1, ..• ,n}.
Proof. Let s<t be in the same block of I. If {s,t}n{a-1 (i),
a - 1 ( i + 1 ) } = ¢ we have a o 't ( s) = a ( s ) < a ( t) = a ( t) = a o.,; ( t) . Sup-
pose s = a-l (i). Then a(s)= i < a(t) and a o ,;(s)= aat(i+1)= i+l. -1
Assume a(t) = i+l. then t =a (i+l). But s and t are in the
same block which contradicts the fact tht a- 1 (i) and a-1 (i+1) are
not in the same block. So i+1 < a(t) = ao,;(t) and ao,;(s) < ao,;(t).
If s = a- 1 (i+1) we have ao,;(s)= aa-l (i)= i<i+1 = a(s} < a(t}= ao~(t}.
The two other cases are treated similarly.
0
- 7 -
If I is a totally ordered set, with =I=n., it is possible to de-
fine a bijective, order-preserving map
o:I:{l, ..• ,n} +I
For a shuffling cr:I + J we have a diagram
0: I
{l, ... ,n} I
o:J lcr {l, ... ,n} I
Since all the maps are bijective we obtain a permutation -1
o: J ocr o o: I = p of {l, ... ,n}. This construction is unique and we may
define the sign I cr I of the suffling as the sign of the corresponding
permutation.
3. Harrison cohomology
Let r be a commutative semi-group with unit, i.e. a monoid, and M
a r-module. \Je use the notation
Mor(~r,M) = {~:~r+Mj~(y 1 , ... ,y )=0 if 3i such that y,=O} n n n 1
Let ~EMor(llr,M). We say that ~ vanishes on all shuffle-products if n
for every (y 1 , ... ,y )E~r, and every l<p<n the sum n n
where cr runs through all (p,n-p)-shufflings of {y 1 , ... ,yn}, ordered
by the indicies.
The subset of Mor(~r,M) of morphisms vanishing on all shufflen
products is denoted by Mor8 ~r,M). Notice that this generalizes the
definition of algebra cohomology in [Ha].
- 8 -
There is a differential map d:Mor(.u.r,M)+Mor( ..ll1r,M), n)l n n+
defined
by
n .
d<j> ( y 1' • · • 'Y n+l) =y 14> ( Y 2' · · · 'y n+ 1) + i~l ( -l) 1 4> ( y 1' · • • 'y i +y i +1' · ' · 'y n+ 1 )
n+l + (-l) ell(yl, ... ,yn)·yn+l
Lemma 3.1. The differential satisfies the following two conditions:
i) d 2 = 0
ii) If <j>EMor8 (*r,M), then d4>EMor8 (n~1 r,M).
Proof i) See for instance [c&E].
ii) If we n~ke no distinction between a shuffling of {r 1 , .•. ,yn+1 }
and a shuffling of the index set {1, ..• ,n} we can write
-1 d<j>(a (y1' · · .,yn+l)) = Y -1 <j>(y -1 '· •• ,y -1 )
a (1) a (2) a (n+l) n .
+ 2 ( -1 ) ~ell ( y -1 ' • . . ' y -1 +y -1 ' . . • ' y -1 ) i=l a (1) a (i) a (i+l) a (n+1)
n+l + (-1 ) <j>(y -1 , ... ,y -1 )y -1
a (1) a (n) a (n+l)
where a is a (p,n+l-p)-shuffling of {l, ••. ,n+l}.
Using remark 2.3 we get for p*l and p*n
y -1 <j>(y -1 , ... ,y -1 ) = a (1) a (2) a (n+l)
y14>(y 1 , ... ,y 1 ) a- (2) a- (n+l)
y p+ 1 ell ( y -1 I 0 0 0 I y -1 ) a (2) a (n+1)
where in the first case a:{2, .•. ,n+l}+{2, ... ,n+l} is a (p-l,n+l-p)-1\
shuffling and in the second case a:{l, ••• ,p+l, •.• ,n+l}+{2, ... ,n+l}
is a (p,n-p)-suffling. Taking the alternating sum of all (p,n+l-p)-
shufflings and using the fact that 4> vanishes on all shuffle-
products we get
IC-1)1aly -1 <j>(y -1 , ••. ,y -1 ) = 0 a a (1) a (2) a (n+l)
- 9 -
when o runs through all (p,n+1-p)-shufflings of {1, ... ,n+1}. In a
similar way we obtain
"(-l)lol 41 (y ) L ,..-1( 1 )•••••Y -1 y -1 o u o ( n) o ( n+ 1 )
= 0
If p=1 we get, using the vanishing of 41,
I ( -1 ) I o I Y -1 41 ( Y -1 ' ... ' Y -1 ) = Y 1 41 ( Y 2 ' ••• ' Y n+ 1 ) o o ( 1 ) o ( 2) o ( n+ 1 )
But we also get
, I o l+n+1 I ( -1 ) 41 ( Y _1 , ... , Y _ 1 ) Y _ 1 = -41 ( Y 2 , · · • , Y n+ 1 ) • Y 1 o o (1) o (n) o (n+1)
And the two sums cancel each other. Similarily for p=n. So it
remains to show
~· ~(- 1 )1ol+i~(y ) = o L L 't' 1 I ••• I y 1 +y 1 I • • • I y 1 o i=l o- (1) o- (i) o- (i+l) o- (n+l)
If o-1 (i) and o- 1 (i+1) belong to different blocks, and if ' is
the transposition which changes o-1 (i) and o-1 (i+1), o and oo'
are shufflings with opposite sign (Lemma 2.4) and they will be
cancelled.
Assume and -1
0 (i+1) are in the same block. Then we have
the equality -1 -1
o (i)+l=o (i+1). Let jE{1, .•. ,n+1} satisfy
and j*n+1 and let X.={x 1 , ... ,x.+x. 1 , •.. ,x 1 } be ordered by the J J J+ n+
indicies such that xj+1 <xj+xj+l<xj+ 2 if xj_ 1 exists. We have an
order-preserving bijection
b A {x1 , ... ,xj+xj+1 , ... ,xn+1 } ~ {1, ... ,j+1, ... ,n+l}
Let j be in the lower block (resp. upper block). Consider all
(p-1, n+1-p)-sufflings (resp. (p,n-p)-shufflings)
A 0 {1, ... ,j+1, ... ,n+1} ~ {l, ... ,n}
- 10 -
F'or each a we have a bijective, order-preserving map
{1, .•. ,n} a A -~ {1, •.. ,a(j)+1, ... ,n+1} = Ia(j)+1
1\ so we can consider the shuffling as a map {1, ... ,j+l, ... ,n+l}~
1\ {l, ... ,o(j)+l, ... ,n+l}. Since ~EMor5 (f,ir,M) we have
\' \' I a I -1 ~ 4 L (-1} Ha (l, ... ,a(J}+l, ..• ,n+l)) = 0 J a:I, ~r
J+l n+l
It is easily seen that this is the sum which remains to show
vanishes.
0
Having proved this lemma we can make the following definition
Definition 3.2. We define the n-th Harrison cohomology of r with
values in M by
n n Harr (r,M} = H (Mor 5 (~r,M),d) n)l
The motivation for this definition is the results of the two next
propositions, which are stated here, and proved in the rest of this
section.
Proposition 3.3. Let M be a r 1- and a r 2-module, and therefore a
r 1xr2-module. Then there is an isomorphism of cohomology groups
for all n> 1 .
Proposition 3.4.
- 11 -
Let M be a Z -module. Then we have +
The main result of this section follows as an immediate corollary.
r Theorem 3.5. If r=z is a free abelian monoid, and M is a
+ r-module, it follows that
n Harr (r,M) = 0 n)l
\Je start by proving proposition 3. 3 •
where
and y.=(y. 1 y:)Er 1 ~r2 . Define maps -~ ~ ~
Mor5 (~r 1 ,M) Mor5 (~r 2 ,M) i Mor5 (~(r 1 xr 2 ),M) ex
13 ( <P 1 I <P 2 ) (!) = y 1 ° 0 0 y n <P 2 ( y i I 0 0 0 I y ~) +y i 0 0 0 y ~ <P 1 ( y 1 I 0 0 0 I y n )
a (<P)l(yl, ... ,yn) = q,((y1 10), ••• 1 (yn10 ))
a(q,) 2 (y~~···~Y~) = <P((O,yl), ••• 1 (0 1 y~))
Lemma 3.6. With the notation as above, we have
i) doj3 = j3od
ii) doa = aod
Proof: Follows easily from the definitions.
We have the equality aoj3=id, but j3oa*id. We shall construct a
homotopy j.J.:Mor5 (-ft(r1xr2 ),M)+ Mor5 (*(r 1 )(r2 )~M) such that
j3oa=id+doiJ.+iJ.od. Thus a and 13 will induce isomorphisms in
cohomology.
0
- 12 -
( 1.1 I o o o I l.n-1 )
<r.2 1 ' ' 0 1 I.n)
( y 1 I o • • I Y n I Y i + o • • +y ~ )
<r.,~···~r.n~A) for some
~ is defined recursively by
Let cr (~)(r.)=do~ 1 (~)(y)+~ 2 od(~)(r.)-y 1' ... y'~(y 1 ~···~Y) n - n- - n- - n n
- 13 -
n n n +~n-lod(cpoSY )(dn(~))-(-1) Ln~n-l(cp)(dn(~))
-n n-1 n-1 n n n-1 n
-(-1) ~n-2(cpoSLn-l+Ln)(dn-ldn(~))-(-l) Ln-l~n-2($oSLn)(dn-ldn(!)) 0 n 0 n
- Yl~n-2(cpoSy 1 +y )(dn-ldn(t)) + Ll~n-2($oSY )(dn-ldn(t)) 1 -n -n
+ $ ( y 1 I o o o I y 1 I y 11 + o o • y I 1 +y ) - y 11 o o o y I 1$ ( y 1 I o o o I y 1 I y ) n- n- -n n- n- -n n 0 0
+ (-1) (~n-lod){cpoSYi)(dn(~))- Ll~n-l<P(dn(X:))
0 n n-1 0 + Yl~n-2($oSY +y 1 )(dn-ldn(~))- YlLn~n-2($oSY 1 )(dn-ldn(!))
~ 1 1 n 0 0
- (-l) YlY2~n-2(cpoSyi+Y2)(dn-ldn(~))
+(-l)nYll2~n-2(cpoSY{)(d~-ld~(t))+(-l)nylcp(y21'''1Ynlyi+ ... +y~)
- ( -1) ny 1 Y 2 ... Y ~ <P ( Y 2 I ••• I Y n' Y i) + ( -1) n-1 d<P( ~~ ( ~) ) -y i ... Y ~ $ ( Y 1, ... I Y n)
n n 0 =a 1 (cpoS )(d (x_)) + (-1) y1o 1 (cpoS 1)(d (x_)) n- Ln n - n- y 1 n -
This recursion can be used to give a closed formula for (J (cp). n
Lemma 3.8. There is a closed formula for a (cp) given by n
0
where A= S( )(~ 1 , ... ,~ ) n-q,q n
is the set of all (n-q,q)-shufflings
CJ : { ~ 1 1 • • • 1 ~ n} + { 1 1 • • • 1 n} and
[Li+q
A. = I
~ Y n+l-i
l<i<n-q
n-q+l<i<n
are ordered by the indices. E(n~q) is given by the formula
E(n 1q) = (-l)nE(n-l,q-1) 1 E(n 10)=-l ~n)O,
Proof. By induction on the index n
Assume the formula is proved for k<n-1.
- 14 -
Using Remark 2.3 we may rewrite the sum
I a I -1 e:(n,q)}:(-1) 4>(a (l, •.• ,n))y 1 ••• y
. a q -1 .
= e:(n,q) L Ha (l, ... ,n-l),Xnyl .•• yq aEA
~ lal+q -1 + e:(n,q) L.. (-1) q,(a (l, ..• ,n-l),X )y 1 ... y aEB n-q q
A where B = S( l )(X 1 , •.. ,X , ... ,X) n- -q,q n-q n
The sign function satisfies the equality
e:(n,q)•(-l)q = (-l)n+(n-l)+ •.. +(n-(q-l))e:(n-q,O)(-l)q
= (-l)n+(n-l)+ •.. +(n-(q-l))e:(n-q-1,0)(-l)q
= (-l)(n+l)+ ••• +(n-q)e:(n-q-1,0)
= e: (n-1, q)
Putting this together, using the induction hypothesis and the
recursion for a (<t>) we obtain the postulated formula. n
0
If 4>EMor (~r,M) <f> vanishes on most of the sums in the expression S n
of Lemma 3.8. The remaining part, for q=O or q=n,
is treated in the next lemma.
Lemma 3.9. Let <f> vanish on all (p,n-p)-shuffle products. Then we
have the equality
- 15 -
Proof. The vanishing hypothesis implies
n-1 I I L (-1)1+2+ .•. +(n-p)};(-1) a l!l(a-1(1, ... ,n)) = 0 p=1 0'
£y. 1 <:i<p where for given p; A,- ~ 1 .
~ Yn+p+1-i p+ <~<:n and a runs through all
(p,n-p)-shufflings of {1, ... ,n}.
Every bit of the (p,n-p)-product must be of one out of two types;
ljl( ••• ,y , ••• ,y 1) p p+
or ljl( ••• ,y , ••• ,y) p+1 p
Since p=O and p=n do not occure in the sum, every n-tuple of the
two given types, except for Ill (y 1 , .•• ,yn) (.:):p=n-1) and ljl{yn' •.. ,y 1 )
(~:p=1), occures twice. The first case also in the (p+1,n-p-1)-pro
duct, and the second case also in the (p-1,n-p+1)-product. An
important remark is that these are the only places they can occure.
Checking the signs it is easily seen that the two bits annihilate
each other. What is left then is the sum
where 6 is the cyclic permutation which brings the first element
into the last position. lol=n-1 and we have proved the lemma.
Thus we have proved
do~n- 1 {ljl)('~) + ~nod(ljl)(~) - ~oa:(ljl){x,) + ljl(~)
=a {l!l)(x.)- r 1 ···Y Hr 1~ •..• ,y~) + lll<r 1 •••• ,l. > n - n n - n
0
( ) ( 1 ) ~n ( n-1 ) + 1 ( I I ) ( 1 I ) { ) 0 = -Ill ~ + - Ill Y n ' • • · ' Y 1 Y 1 · · • Y n -y 1 · • • Y n Ill Y 1 ' · • • ' Y n + Ill ~ . =
- 16 -
and ~ is a homotopy between ~oa and the identity. The conclusion
of the proposition follows immediately.
0
The next step is to prove Proposition 3.4, i.e. calculate Harrison
cohomology of the monoid z+. We shall do this by showing that
where
f ( ) t i or the complex Mor8 ~Z+,M •
z+-module M. The calculation is
=
n-1 . + l: <-1>1.
i=2
+
is a homotopy
is the multiplication of i on the
He have proved the equality doh+hod+id=O and the vanishing of the
cohomology groups follows as an immediate consequence.
0
- 17 -
4. The relation between Harrison cohomology and algebra cohomology
The purpose of this section is to prove the coincidence of Harrison
cohomology and algebra cohomology of a monoid.
\'ve can consider 1 for M a A-module 1
Mar (ll-~M):free man/A+ complex of ab.gr. s • ---- --- - -
as a contravariant functor from the category of free monoids over
A into the category of complexes of abelian groups. If we let • 0
c ((free .!!!2!2/A) ~-) be the resolving complex of the functor
lim -+- 0
(free ~/A)
we get the double complex
• 0 K"" = c ((free man/A) 1Mor8 (~-~M))
and the two spectral sequences
'Eplq = 2
lim (p) 0 Hq(Mor8 ~-I M)) (free ~/A)
= Hq li:m(p) 0 (Mor8 (~-~M)) (free !!EE./A)
both converging to the cohomology of the double complex K"". We have
shown that for a free monoid r
For q=1 we get
1 Harr (r~M) = ker{Mor8 (r~M)+Mor8 (r~r~M)}
= {~EMor(fiM)j~(y1+y2)=y1~(y2)+y2~(y1)}
= Der(f 1M)
- 18 -
and the first spectral sequence
To calculate the other sequence \te need a lemma.
Lemma 4. 1 • With the above assumptions and if char k = 0 we have
i) l.!m 0Mor8 (J!.-, M) = Mor S (J!.A, M) (free ~/A)
ii) (p)
lj;m 0Mor 8 (.Jl-, M). = 0 for q)l and p)l (~~/A) q
Proof, Consider a semi-simple monoid r.tA, and the induced cochain
complex
Mor(A 1M) + Mor(r0 1M) + Mor(r 1 ,M) + ••• (*)
The semi-simplicial monoid is A-graded and A is finitly generated
so it is enough to consider the homogeneous case
d() d M + TI M -+ TI M -l
A}.. A~
where ')..EA -1
and A}..=~ (}..)I a finite set.
Let s:TI M + TI 1M be the map given by A n n-
A ~
A}.. • s { ~ ) { a 1 1 • • • 1 an_ 1 ) =
It is easily seen that this forms a homotopy between zero and the
identity of the complex (TI M1d ). i.e. s6-os=id. So for finitely An n
A
generated A the sequence (*) is exact. Now observe that
ftr =(Hr 0 ) . The sequence q p q p
- 19 -
+ •••
may thus be considered as a subcomplex of the acyclic complex
Mor{ {~r 0 )., M). The homotopy s
S {!; ) ( y 1 I • • • I y n-1 )
extends to Mor{r ,M) n
through
-1 where A=~ {~{yi)) and IAI= A. For !;EMors{(~r0 )n,M) an easy
computation shows that s{!;)EMors{(~r0 )n_ 1 ,M). So the homotopy
restricts to the subcomplex, which is acyclic as well. By [La 1] we
have lim 0Mor{Jl-,M)"'Mor{.J1A,M) {free+~/A) q q
and as a consequence
l!m 0Mors{.ll.-,M)"'Mors(.llA,M) for {free mon/A) q q
q)1. A repeated use of Leray
spectral sequence [La 1] to compute lim{p) Mor{ll- M) + 0 I
(free ~/A) q now
proves the lemma.
Thus we have proved the following
Theorem 4.2. With notation as above, we have an isomorphism
0
Combining this theorem with Definition 3.2 we obtain
Corollary 4.3.
0
5. Harrison cohomology of Monoid-like ordered Sets.
Let A be a cancellative monoid with no non-trivial subgroups. Then
A has the structure of an ordered set given by ~ 1 ~ ~ 2 E A if there
exists ~EA such that ~ 1 + ~ = ~ 2 . Now let L c A be some
sub-ordered set.
- 20 -
Definition 5.1. L c A is said to be a monoid-like ordered set if
for all relations A1< A2E L there exists ~EL such that A1+ ~ = A2 as elements of the monoid.
Now define the set
S (L) = {(A 11 .•. 1A) E Lnjw(A) E L} n n -
n where the weight w(~) of is the element w(~) = L A,E A. The
i=1 ].
permutation group ~ acts on s (L) Ln n by
or = 0' (.A 1 1 ••• 1 An) = (A I • • • I A_ ) 0'-1(1) -1 0' ( n)
Let c (L) be the free abelian group on s (L) . The action of n n
on s (L) induces an action on c (L) by permuting the basis n n
elements.
We also define the dual groups: Cn(L) = Homz(Cn(L)~z>~ with the
action of In given by
and rJ E L • n
ln
There is a shuffle-product • C (L) p
® C (L) + C (L) defined by q p+q
~ lol -1 = L(-1) 0' (A 11 ... 1A +) 0' p q
where w(A 11 ... 1A ) E L and where 0' runs through all p+q
(p~q)-shufflings of {1121'''1p+q}. Denote by Sh (L) the n
z-submodule of C (L) generated by all shuffle-products. n
- 21 -
Next we define some differential maps
6 , a : C (L) + C ( L) n n n n-1
(resp. 6n,an: Cn-1 (L) + Cn(L))
by the action on basis elements of
n-1 .
C (L) (resp. n
6 (A1, ... ,A) =I (-l)~(A1''''tA.+A, 1'''''A) n n i=1 ~ ~+ n
n an(A1''"'An) = (A2''"'An) + (-1) (Al'"''An-1)
n-1 . (resp. 6nE(A 1 , ••• ,An) =I (-1)~E(A 1 , ... ,A.+A, 1 , ..• ,A )
. 1 ~ ~+ . n ~= .
n n a E(Al'''''An) = ~0.2, ... ,A.n) + (-1) E(A.1'"''"'n-1))
For the case n=1 we put
It is easily seen that the differentials
we have
6~ (~) = E6 (A) n-and
a nE (~) = Ea (A) n-
Lemma 5.2. We have the equalities
i) 0 0 = 0 n-1 n
are dual,
ii) a & + 6 a + a a = 0 n-1 n n-1 n n-1 n
Proof. A simple computation.
i.e. for
Let D = 6 + a • As a consequence of the lemma D D = 0. The n n n n-1 n
relations between the differentials and the shuffle-products are
stated in the next lemma.
0
- 22 -
Lemma 5.3. With the notation as above the following equations hold
for xEC (L), yEC (L) : p q
i}
ii}
iii}
and as a consequence
iv)
x•y = (-l)p•qy•x
&p+q(x•y) = &P(x)•y +
= a (x} •y + (-1 )px•a (y} p q
D (x•y) = D (x)•y + (-l)Px•D (y) p+q p q
Proof. Another simple calculation.
0
Using these lemmas we may define the Harrison (co-}homology of the
set L.
Definition 5.4.
The homogenous Harrison homology Ha (L) (resp. cohomology n
a} n
Ha (L,Z)) of the ordered set L is the (co-}homology of the complex
c:(L) =c. (L)/Sh.{L) {resp.
the homogenous differential
c;(L) ={Ill E C8 (L)!4l(Sh.(L)) = 0}
& (resp. on). n
with
b) The inhomogenous Harrison (co-)homology HA (L) (resp.HAn(L,Z)) n
8 of the ordered set L is the (co-)homology of the complex c.(L)
• n (resp. Ds(L)) with the inhomogenous differential Dn (resp. D).
Remark 5.5 There is also a relative version of Harrison
(co-)homology. Let L0 ~ L c A and suppose L0 is full in L, i.e.
if yEL, y 0EL0 and y">y 0 , then yEL0 • The relative Harrison complex
is given by
s Cn(L-L0 ,L) = Cn(L-Lo~L)/Shn(L)
(resp. c:(L-L0 ,L) = {4>ECn(L-L0 ,L!4l(Shn(L)) = 0}
where c n ( L-L 0 , L) = c n ( L) I { ( ~ 1 1 ••• 1 ~ n) I w ( ~) E 1.-r. 0 }
(resp. Cn(L-L0 ,L) = {4>ECn(L)!4>(~ 11 ••• ,~n)=O for w(~)E L-L0 }
- 23 -
Proposition 5.6. With the notation as above there is a long-exact
sequence
1 1 1 0 ~ HA (L-L0 ,L:k) + HA (L:k) + HA (L-L0 :k)
+ HA2 (L-L0 ,L:k) +
relating Harrrison cohomology of the ordered sets L and L-L . 0
Proof. The relative complex gives rise to a short-exact sequence of
complexes
where L-L 0
is an ordered set since L0 is full in L.
0
The next theorem is the main result of this section. It relates the
"local" cohomology HAP(~:k) for elements A.EL to the "global"
cohomology HA• (L:k).
Theorem 5.7. There exist a spectral sequence given by
= lim(p)HAq(~:k) m
• converging to HA (L:k).
Proof. Using the definition of the complex it is easily seen that
for some ordered set L
• C (L:k) s
• 1\ = lim C (-·k) s I
tE"L 1\
where A. = {A.'Ej~u·<A.}. The shuffle-products are homogenous and the 1\
inhomogenous differential is well-defined on the sets A..
• Now let D (L,-) be the resolving complex for lim. Denote by K -L
the double complex
• •
- 24 -
• • • • 1\
K = D (L,Cs(-~k))
We have the two associated spectral sequences
and
'Ep,q = HpHq(D• (L,C·(~;k}}} 2 s
= Hp(D• (L,HAq(~;k}}}
= lim(p}HAq(~;k} + L
,.--, q p • • 1\
= H H (D (L,Cs(-;k}}}
is surjective and by [La 2] we have
p > 0
and for p = 0
The double complex is situated in the first quadrant and the two
spectral sequences have the same abutment. The second sequence
d€generates to HAq(L;k) and the theorem follows.
0
- 25 -
6. Graded Harrison Cohomology
Suppose A+(-A)=Zr. We shall equipe the complex MorS(qA,k[A]) with
r d' E r . a Z -gra ~ng. For AO Z we def~ne
Ao Mors (~A,k[A]) = {~EMors(~A,k[A])I$:homogenous of degree A0 }
Homogenous means that $(~) is homogenous and that the element
is independent of choice of ~. This element is called the degree of
~- It is easily seen that the differential respects the grading, and
that the degree of the differential is 0.
Definition 6.1. The graded Harrison Cohomology of A with values in
k[Aj is defined by
for n)Q, AEZr.
Put as an abbreviation M~= Mors(*A,k[A]) and M~'A= Mor~(ftA,k[A]).
Proposition 6.2. 0
a) The inclusion Jl M•, A s
AEZr
-+ M s of complexes induces an inclusion
at the cohomology level;
n, A [ ] n [ ] l1. Harr (A,k A ) + Harr (A,k A ) n ) 0.
AEZr
b) The inclusion is an isomorphism whenever Harrn(A,k[A]) is a
r Z -graded group.
Proof.
a) Let ~EMn,A be homogenous and suppose •EMn-l satisfies d•=~. s s Let •A be the A-graded homogenous part of •· Since deg d=O we
must have $=d•A·
- 26 -
b) Suppose Harrn(A,k[A]) r
is Z -graded and let ~EM ,d~ = 0. Then s we may replace (mod im(d)) ~ by some ~O which is sum of
homogenous components.
The graded Harrison cohomology groups are closely related to the
Harrison cohomology of ordered sets, as defined in the previous
chapter.
0
Theorem 6.3. With the notations as above and in chapter 5 there is
an isomorphism in cohomology
where = (-~+A)nA , and +
A = A-{0} +
Proof. Put where ~ 0 (~) E k. The map
is easily seen to induce an isomorphism of vector spaces
--+
It also takes the graded version of the differential d into the
inhomogenous differential D. Recall that in the definition of
Mor~{llA,k[A]) we agreed that ~(~) = 0 if 3i such that s •
~i = 0. This is the reason why we use the positive part. A+ in stead
of A.
0
We end this chapter with a couple of results about the graded •
Harrison cohomology. A close study of the complexes Cs(A+-A0 ,A+~k)
for various ~Ezr gives the next proposition.
Proposition 6.3. Fix some n)l. The cohomology groups
Harrn'~(A,k[A]) are equal for all ~EA.
- 27 -
Proof. If A.EA we have A =(-A.+A )nA =A 0 + + + and
means that for every A.EA we study the same complex.
0
Corollary 6.4. Suppose the cohomology group An(A,k[A]) is of
finite dimension over k. Then
n+l,A. Harr (A,k[A]) = 0
for all A.EA.
Proof. k[A] has infinite dimension over k.
0
7. Harrison Cohomology of two-dimensional Torus Embeddin~s.
The simplest, but still maybe the most important family of
monoid-algebras are the two-dimensional torus embeddings k[A] over
a field of caracteristic zero.
2 Let A c z+ be a commutative saturated monoid and let
positive part, i.e. A= A-{o}. For A. E A we define + +
A+ be the
A(A.) = A.+A.
He want to study the "local" and "global" Harrison cohomology of
monoids, that is, the cohomology of monoidlike subsets of the monoid
as well as the monoid itself. Also the submonoids A+-A(A.) for
various A. E A are of great interest, and we start with a closer +
look at these objects.
Let and be the generators for the one-dimensional faces of
A. (See for instance [k] for details), and define
r. = r.(A.) ={A.' E A I ~ t E z, A.'+t•y, t A(A.)} i = 1,2 1 1 + 1
- 28 -
r. ].
is an ordered set with the same ordering as A. Furthermore it is
easily seen that
and
where L'A. is the "strong" link defined as follows: There is a
unique description of A. given by
A. = a • l
where the a.'s are non-negative rational numbers. We make the ].
definition
L'A. ={A.'= by+ byE A lo < b < a,,i = 1,2} 1 1 2 2 + i ].
(Note: For the normal link the definition is 0 < b ~a,, but A.'*A.) i l.
Proposition 7.1. With the notation as above'there is a
Mayer-Vietoris sequence
for all
Proof. Using the functor HAq(~) on the system of inclusions
- 29 -
of ordered sets we get
and for the higher derivatives, a spectral sequence
Since for p:t:O, 1 the spectral sequence degenerates to the
two exact sequences
and
0 -+ El,p -+ 2
E O,p+l -+ 0 2
0 -+ EO,p 2
-+ !im(p)HAq(~) x }im(p)HAq(~) -+
r 1 r 2
-+ El,p -+ 0 2
Putting this together we obtain the long-exact sequence of the
proposition.
0
To state and prove the next proposition we need some notation and
definitions.
For ~ 0 E A we let C(Lq(~ 0 )) be the vector space on the set
and consider the complex (C(L·(~0 )),&) where the differential is
the homogenous differential of definition 5.3.
Denote by
- 30 -
the subset of A where the q-th homology of the given complex
vanish.
Proposition 7.2. The map qA ql\
HA (:>.. 2 ;k) + HA {:>-. 1 ;k), q;>l, induced by 1\ 1\
the inclusion :>-. 1 ~ :>-. 2 is an isomorphism whenever x2-x 1c Uq, where
u is defined as above. q
Proof. It is enough to show the proposition for L c L'c U where q
L'-L = {u} is a one-element set and u is minimally greater than
L, that is if u;>u', then u'E L. This is because we have a
filtration
where = {u.}, u. ~
is minimally greater than L. ~
and u.
belongs to u . q
~
Consider the exact sequences of complexes
0--+ D~(u;k)--+ C~(L';k) ~ C~(L;k)--+ 0
where
~
and the differential is the dual of the homogenous differential given
above.
• We must show that the complex (D (u;k) ,d) s is acyclic. Dualizing the
problem we are led to the study of the short-exact sequence of
complexes
• 0 --+ Sh(L (u);k)
• s • ~ C(L (u);k) --+ C (L' (u);k ~ 0 (*)
where Sh(Lq(u);k) is the subspace of C(Lq(u);k) consisting of all
shuffle-products x•y with x E C(LP(u);k) and y E C(Lq-p(u);k),
and
- 31 -
The differentials are the homogenous ones.
• The question is whether a homotopy for C(L (u):k) will induce a
homotopy for the subcomplex Sh(L.(u):k). We are working over a field
of caracteristic zero and the following lemma gives an answer .
• Lemma 7.3. Let g be a homotopy for C(L (u);k). The map
• • h: Sh(L (u);k) ~ Sh(L (u);k)
defined by
h(x•y) = 1/2(g(x)•y + (-l)px•g(y))
for x E C(Lp(u) ;k), y E C(Lq-p(u) ;k) is a homotopy for the • subcomplex Sh(L (u);k).
Proof.
(dh + hd)(x•y) = d(~ (g(x)•y)) + d(~(-l)P(x•g(y)))
+ h(d(x)•y) + (-1)Ph(x•d(y))
= ~ (d(g(x))•y + (-1 )p-1g(x)•d(y)
+ (-1)Pd(x)•g(y) + (-1) 2Px•d(g(y)))
+ ~ (g(d(x) )•y + (-1 )p-ld(x)•g(y)
+ (-1)Pg(x)•d(y)+(-1) 2Px•g(d(y))
=~((dog+ god)(x)•y + ((-1)p-l+ (-1)P)g(x)•d(y)
+ ((-1)p+ (-1)p-1 )d(x)•g(y) + x•(dog + god)(y))
= ~ (x•y + x•y)
= x•y
The assumption in the proposition ensures that the complex
0
C(L.(u);k) is acyclic and since working over a field, dualizing of
the complex (*) will give an inclusion of acyclic cochain complexes
0
- 32 -
The assumption in Prop 7.2. is that the set sits inside •
So we must study the set Uq, or better, the complex C(L (u)) with
differential &.
A basis for C(Lq(u)) consists of all tuples (n 1 , .•• ,nq) with
In.= u. These elements may also be written as ordered tuples; J
+ n 1 < n1+ ..• + n = u q- q
Observ that all the tuples has u as their maximal element. Removing
this top element we obtain an ordered tuple of the ordered set L(u).
It is easy to see that this sets up a bijection between U Lq(u) and q
the simplicial set associated to the set L(u).
The homogenous differential coincide through the bijection with the
usual differential of the ordered set, the alternating sum of the
face maps.
The homology of the simplicial sets L(u) are studied in [La & Sl]
and we state, without proof, one result from this paper.
Let a be the right-most minimal element of A excepting the +
generator of the face y 1 , and be the left-most minimal element
excepting the generator of the face y 2 . Denote by U the subset of
A+ given by
Lemma 7.4. For all we have the inclusion
Proof. See Lemma 2.5 of [La & Sl].
ucu . q·
0
Corollary 7.5. The morphism HAq(A 1 < A2 ) is an isomorphism for all 1\ 1\
q)l whenever A2 -A 1c u.
- 33 -
Proof. Combine Proposition 7.2 and Lemma 7.4
D
To calculate the graded algebra cohomology groups we have seen that
we need information about invers limits of the pre-sheaves HAq(~) over various ordered subsets of the monoid A. In [La 2] it is shown
that these calculations can be made over even smaller subsets under
the assumption of cofinality.
Let we put B (r) = {y•e r h'> y}. y 0 0
Definition 7.6. A subset r c r 0
is called cofinal if the following
two conditions are satisfied:
i) For every yEr, we have BY(r0 ) * !i1 ii) For every finite family y ,y , .•. ,y
1 2 s of elements of a < r > y 0
there exists a y OE BY(r0 ) such that for every i=l,2, ... ,s we
have either or
Using the theory for cofinal subsets it is rather easy to prove the
next proposition.
Proposition 7.7.
a) For a two-dimensional torus embedding A=k[A] we have
b) For the subsets r 1 and r 2 , defined above we have the same
equation
}_.im(p)HAq(~) = 0 for i=1, 2 , p)1, q)l. r. ~
Proof. In all the sets A n U, r n U and r n u there are cofinal + 1 2
subsets isomorphic to z+. Equipe these with the constant presheaf
HAq(~). It is well-known that the higher derivatives vanish. Using
the cofinality of the subset and Th 1 .2.4 of [La 2] the proposition
follows.
D
- 34 -
The following is also true:
Proposition 7.8. For a two-dimensional torus embedding A=k[A] we
have
Proof. An irnrnideate consequence of corollary 6.4 and the facrt that
A is an isolated singularity (see for instance [Pi]) and therefore
has finite dimensional cohomology groups.
0
Combining these results we obtain the next theorem, concerning the
vanishing of the cohomology groups HAq(~;k).
Proposition 7.9.
let A. E U c A,
2 Let A c Z+ be a commutative saturated monoid and
A. >> 0. U is as defined above. Then we have
Proof. The set u has two components, one for each face of the
monoid and we put u = v u 1 v2. Using definition 7.6 it is easily seen
that v', i=l , 2, are cofinal ·~
in A+. Thus we have an isomorphism of
derived functors
A consequence of this last equality is that the spectral sequence
· degenerates and we have an isomorphism
The right side of the equation vanishes (Prop. 7.8.) and so does the
invers limit. But HAq(~) is constant on U as seen in Cor. 7.5. and
the result of the theorem follows.
a
- 35 -
We may use Prop. 7.8 in another context, namely together with the
result of Prop. 5.5 concerning the relation between Harrison
Cohomology and relative Harrison Cohomol09Y·
Remark 7 . 1 0 • If we put L=A +
and L = A -A(A.) 0 + into Proposition
5.5. and use Proposition 7.8. a) we get an exact sequence
0 ~ HA1 (A+-A(A.),A+;k)--+ HA1 (A+;k) ~ HA1 (A+-A(A.);k)
2 --+ HA (A+-A(A.),A+;k) --+ 0
and isomorphisms
\~e will come back to the use of this in the next chapter.
As a consequence of Proposition 7.7.b) the Mayer-Vietoris sequence of
Proposition 7.1. splits up into the following exact sequences
(2)
The isomorphism of (2) proves the next theorem which states that the
algebra Cohomology Groups of a monoid-algebra may be calculated as
the Harrison Cohomology of finite monoid-like subsets of the monoid.
Theorem 7.11. Let A be as above and let A.EA. There is a spectral
sequence
•
.li!!l (p)HAq(~;k) A+=i\TA.)
}-im(p- 1 )HAq(~;k) L'A.
converging to HA (A+-A(A.);k).
p = 0,1
p ) 2
- 36 -
Proof. The theorem is just a reformulation of theorem 5.6. using the
isomorphism (2) above.
8. An Example
• ~ie want to end this paper with the computation of A for the
two-dimensional torus embedding with all multiples e.= 2. ].
0
For this purpose we need a description of the monoid-algebra A and
we give it as the invariant set of the group action of Z/(r+l) on
the free algebra k[x1 ,x2 ] given by
x1 --+ l;•x 1
x2 --+ l;•x 2
where I; is a primitive (r+1)-th root of unity (see [K]). This
gives A as the monoidalgebra k[A] where A consists of all pairs
(i,j) E z! such that i+j = 0 (mod r+1) (see [La & SL]). It has a
natural z+-grading given by
deg(i, j) = (i+j) / (r+l)
He want to use the information from chapter 7, but we need some
specific calculations. The results are listed below, each'With a
short argument.
The values of I; E ker o1 is determined by the values on the
elements (O,r+l) and (r+l,O). An explicit formula is easily
given.
b)
There are at least 2 minimal elements in A -A(A) and for these +
e'lements the value of I; E ker D 1 is zero. But the values on two
minimal elements generates all the values of 1;.
- 37 -
t2 if A is non-minimal or A=(O,r+1) or A=(r+l,O)
c) dimkHA1(A+-A(A):k) = 3 if A= (1,r) or A= (r,1) 4 if A= (a,b) with 1<a<r, b = r+1-a
Just counting of the number of linear independent vectors in 1 ker D .
From chapter 7 (Remark 7.10.) we have the long-exact sequence
0 ~ 1
HA (A+-A(A),A+:k) --+ 2 --+ HA (A+-A(A),A+:k) -4- 0
We divide into different cases depending on A and put the values of
a), b) and c) above into the sequence.
Case 1: A is non-minimal
Counting the dimensions gives
Case 2: A = (O,r+1) or A = ( r+1 , 0)
di~HA2 (A+ -A(A) ,A+:k) = 0
Case 3: A = ( 1 , r) or A = ( r, 1 )
dim HA2 (A -A(A),A :k) k + +
= 1
Case 4: A is minimal, but none of case 2 or 3
dimkHA2 (A+-A(A),A+:k) = 2
Summing up for the four cases we get
Computation 8.1.
0
For the higher cohomology groups the main tool is the spectral
sequence
- 38 -
q A But as we have seen in Proposition 7.9. HA (A;k)=O for A>>O and
AEU. Now for A>>O AEA+-A(A) implies AEU and therefore
for A> >0 and AEA+- A (A). The spectral sequence
degenerates and what is left are the groups with q=1 . Ttn~s we have
the equation
Ep-1,1 = 2
and we can use theorem 7.11. which
strong links L'A and the,groups
suggest a closer look
lim(p-l)HA 1 (~;k). -L'A
at the
To do this we introduce some new subsets of the monoid. Let
2 (a,b) E Z+, and define
1\ (a,b) = {A= (i,j) E Ala<i, b<j}
Lemma 8.2. Let A be as above and let (a,b) E z2 . Suppose +
a+b) 2(r+1) + 1 and ab * 0. Then we have
1 A di~HA ((a, b) ;k) = 2
If a=O or b=O the equality is
1 A di~HA ((a, b) ;k) =
Proof. The second case is the linear ordered set and the cohomology
is easily calculated. In the first case suppose temporarily that
a+b = 2(r+1) + 1. Then (a-1,b) and (a,b-1) are elements of the
monoid A. Let s and t be the number of minimal elements beneath A
the two maximal elements in the set (a,b) with s>t. A carefull
look at the map o 1 and the group c0 ((a,b)",o) shows that ker o1
consist of s+2 variables and (s+t+l)/2 = s (since s=t+l)
independent relations.
- 39 -
1 A 2 It follows that HA ((a,b) :k) = k. A consequence of this is tll9r ~}'e
1 - i i!
value of ~E ker D is determined by the value on two arbitra~X
minimal elements. If we concider a more general subset of the monoid, A 1
say ~. elements ~ E ker D are still determined by the values on
the minimal elements. Looking at subsets of the type described above,
it is easily seen that the value on two minimal elements will
determine everything.
From chapter 7 we have the equality
1 A 1 A ~im HA (-:k) = HA (~;k)
A ~
0
In addition the cofinality of the one-element top set of implies the
vanishing
of the higher derivatives. A repeated use of a degenerated version of
the Mayer-Vietori& sequence of Prop. 7. 1 • (In fact the excision
theorem) together with the fact that lim(p)F = 0 whenever the L
of the ordered set (the length of the longest ordered chain)
than the exponent of derivation of the functor, gives us the
following
Proposition 8.3. Let ~EA+ be of degree p. Then we have
whenever q * p-2.
is
depth
less
0
- 40 -
Corollary 8.4. Let AEA+ be of degree p. The spectral sequence of
Th. 7.11 degenerates completely and we have
Er,q- 0 2 -
whenever r * p-2 or q:l:1
Proof. A direct consequence of prop. 8.3.
We shall use this to compute the dimension of the second algebra
cohomology group of the monoid algebra k[A].
The corollary above tells us that
and from Prop. 7.1. we have the long-exact sequence
0 ~ 1A 1A 1A
~ HA (-:k) --+ tim HA (-:k) x tim HA (-rk) A+:.A\A) r 1 r 2
. 1" . (1) 1" --? };~m HA (-rk) ~ t~( HA (-:k) --+ 0 L'A A+-A A)
Putting this into c) above we can divide into different cases:
(1) 1" (G = J:i{ HA (-rk))
A+ -A A)
A = (2r+2,0) or A = (0,2r+2) 0 + k2 + k2 + 0 + G + 0
f... (2r+1,1) A ( 1 , 2r+1 ) 0 + k2 2 + 0 = or = + k X k + k + G
A (2r,2) f... (2,2r) 0 2 k 2 X k2 + k2 + G + 0 = or = + k +
f... = (a, b) a+b = 2r+2 J(a(r 0 + k2 + k2 X k 2 + ka + G + 0
a=r+1 0 + k2 + k 2 X k2 + kr + G + 0
3<b(r 0 + k2 + k 2 X k2 + kb + G + 0
Summing up this diagram, we get
0
- 41 -
1+2+ ... +(r-1)+(r-2)+(r-2)+(r-2)+(r-1) + •.. +2+1
for the second cohomology group, which makes up to the formula.
Computation B.S.
0
Before we can make further computations we need some more specific
calculations for the monoid A= {(i,j) E z;li+j = 0 (mod r+1)}. We
list them below, each with a short argument.
d) There are r+2 minimal elements of A+' and for A minimal we
have , 1 (A ) du~HA A ;k • 1.
Obvious.
e) There are 2r+3 elements of degree 2 in A+: we denote them by
(a,b) where a+b = 2(r+1) and Q(a,b(2(r+1). Furthermore:
, 1 A d~~HA ((a, b) :k) = min
ra+21 ,b+21 ( -2- I -2- ).
Drawing a picture will be a good idea.
f) 2r~ 2 . ( fa+21 12r+4-a1)
£ m1n ~ , 2 = a=O
An easy computation based on induction on r, together with the
formula
t Q tt t(t+2) ~ a -L "2" -
a=l i (t+1)2
when t is even
when t is odd
g) There are d•r + (d+1) elements of degree d in A+, and
1 A l di~HA (A:k) = 2d•(r+1) deg=d
- 42 -
There are d•r+(d-1) elements with 1 1\
di~HA (A;k)=2, two elements
where the dimension equals one and alltogether it adds up to
2d • ( r+1 ) .
The methode we are going to use for the rest of the computations can
be described as follows:
We consider the resolving complex • 1 1\
C (L'A,HA (-;k)) for
. 1 1\ t J.m HA (- ; k ) ; L'A
with differential as described in section 1 .3. of [La 1 ].
He have seen (Prop. 8.4.) that for A of degree p the complex
is acyclic.
vle have as an abbrivation have put Cq(L'A,HA1 (~;k) = Cq(A)). We
are interested in the total dimension of the algebra cohomology and
we have the equalities .
To calculate this sum we use the additivity property of an exact
sequence and obtain the formula
for p)3.
We just outline how this alternating sum is c.alculated in the case
p=3 0
- 43 -
1. There are 3r+4 elements of degree 3 (see g) above).
If A. = (0, 3r+3) or A. = (3r+3, 0), then di~ ttr HAl (~;k) = 0
If A. = ( 1 , 3r+2) or A. = (3r+2,1), then di~ t~l HA1 (~;k) = 1
1 1\ If none of the above cases, then dimk !im HA (-;k) = 2
L'A. Which adds up the formula
I dimk lim 1 1\
6r+2 HA (-;k) = deg A.=3 L'A.
2. There are r+2 minimal elements in A+. Each minimal element is
contained in 2r+1 ordered sets of type L'A., deg A. = 3. Every
degree-2-elements is contained in r ordered sets of this type. On
the other hand the s~ of the dimensions of HA1 (~:k) with 1 2 deg A.= 2 are in e) and f) above shown to be 2(r +7r+8). Thus we
have
1 2 = (r+2)(2r+l) + r 2<r +7r+8)
3. The number of ordering relations are calculated as follows: There
are r+2 minimal elements, each are related to r+2
degree-2-elements. Each of the degree-2-elements lies in r ordered
sets of type L' A. with deg A. = 3. The conclusion must be
I dimkc1 (A.) = (r+2)(r+2)r deg A.=3
Making an alternating sum out of these numbers we have made
Computation 8.6.
Similar calculations can be made for p)4. Knowing the method in
details we drop the tedious details and just list
0
- 44 -
Computation 8.7.
a)
b)
dlmkA4 (A,k[A]) = ~ r(r-1 ) 3
dimkA5 (A,lc.[A.J) = ~ r 2 (r-1 ) 3
0
- 45 -
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[C&E] CARTAN, H. and ElLENBERG, S., Homological Algebra Princeton Univ. Press (1956) '
[ch] CHRISTOPHERSEN, J., Monomial curves and obstructions on cyclic quotient singularities Proc of Symp, Lambrect 1985. Lecture Notes in Math. 1273 pp. 117-133 Springer-Verlag.
[Ha] HARRISON, D.K., Commutative Algebras and Cohomology. Trans. Amer. Math. Soc. 104 (1962) pp. 191-204
[K] Kempf. G., Knudsen F., Mumford D., Saint-Donat B., Toroidal Embeddings I ,Lecture Notes in Math. 339, Springer Verlag.
[L&S] LAUDAL, O.A. and SLETSJ¢E, A.B., Bettinumbers of Monoid Algebras. Application to 2-dimensional Torus E;mbeddings. Math. Scand. 56 (1985) pp. 145-162
[La 1] LAUDAL, O.A., Formal Moduli of Algebraic Structures
Lecture Notes in Math. 754 Springer Verlag.
[LA 2] LAUDAL, O.A., Sur la theorie des limites projectives et inductives. Theorie homologique des ensembles ordonnes. Annals Sci. de l'Ecole Normal Superreure. 3 serie t. 82 (196S),pp. 241-296.
[Pij PINKHAM , H.C., Deformation of Quotient Surface sigularities
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[Ri] RIEMENSCHNEIDER, 0., Deformationen von Quotientensigularitaten. Mathematische Annalen 209 (1974) pp. 211-248
(Sch 1]SCHLESSINGER, M., Rigidity of Quotient Singularities Iventiones Math. 14 (1971) pp. 17-26
[Sch&S]SCHLESSINGER, M. and STASHEFF, J., The Lie algebra structure of tangent cohomology and deformation theory J. Pure Appl. Algebra 38 (1985) no. 2-3 pp. 313-322.