term (3T) relation, orientation independence (01) relation, O-vanishing relation and 1-vanishing relation to the space of chord diagrams subjected to the four- ...
Le,T.Q.T., Murakami,H., murakami, J. and Ohtsuki, T. Osaka J. Math. 36 (1999), 365-395
A THREE-MANIFOLD INVARIANT VIA THE KONTSEVICH INTEGRAL THANG T U QUOC
LE, HITOSHI MURAKAMI, JUN MURAKAMI and TOMOTADA OHTSUKI (Received September 01, 1997)
We construct an invariant for closed, oriented three-manifolds from the Kontsevich integral of framed links, and show that it includes Lescop's generalization of the Casson-Walker invariant. Combining this result and a formula for computing the Kontsevich integral in [17], we can compute the Casson-Walker invariant combinatorially in terms of q-tangles (non-associative tangles in [3]). Our invariant is obtained from the Kontsevich integral by imposing the threeterm (3T) relation, orientation independence (01) relation, O-vanishing relation and 1-vanishing relation to the space of chord diagrams subjected to the four-term relation. The 3T relation is given by (3T relation)
)
( +
/'•'. +
_ = 0.
Here, dotted lines present chords and the three chord diagrams are identical except within the region where they are as above. The 01 relation is given as follows. Let D b e a chord diagram and let D' be a chord diagram obtained by changing the orientation of a string s of D. Then (01 relation)
D'= (~ϊ) 1, Λ^ and A[£^ are isomorphic by identifying the corresponding basis. Let Λι be a two dimensional space spanned by e 0 and eλ, and we identify Λι with A\ by identifying Θ U Θ U • U Θ with e 0 and Θ 2 U Θ U U Θ with eλ. The image of Λ'(L) in Λι is also denoted by Λ'(L). We give an algebra structure to A\ by e
oeo —
e
0 el ~ el e0 —
e
i ei —
THREE-MANIFOLD INVARIANT VIA KONTSEVICH INTEGRAL
369
Then, for a split union L\ U L 2 of two framed links L1 and L 2 , we have A;(Li U L2) = Λ'(Li) Λ'(L2) € A
(1.3)
For trivial knots oo±i with ±1 framings, we know (from (6.3) in Section 6) that 1 3 1 3 (1.4) Λ'(oo +1 ) = - e0 + - e 1 ; A'too^) = - - e 0 + - eλ. These elements are invertible in A\ and their inverses are 1
Λ'too+iΓ =2eo--eu
1
Λ'(oθ-i)" = ~2e 0 - - e x .
So we can modify Λ'(L) for the KI moves as in the case of the Jones-Witten invariant. Let σ+(L) (resp. σ_(L)) denote the number of positive (resp. negative) eigenvalues of the linking matrix BL of L, and let (1.5)
Λ(L) = tf-°+M-°ΛL) Λ / (oo + 1 )- σ + ( L ) Λ'(oc-iΓ σ - ( L ) Λ'(L).
For a framed link L and the corresponding three-manifold ML, we have the following. Theorem 1. Λ(L) is a topological invariant of the three-manifold MLLet |£Γi(Λfχ,)| denote the order of the first homology group of ML and 6 ( M L ) the first Betti number of ML. Let X(ML) be Lescop's generalization [22] of the Casson-Walker invariant λ(M^). If b1(ML) = 0, it satisfies 1
λ(ML) =
\H1(ML)\\(ML).
Let AQ(L) and Λi(L) be the coefficients of e 0 and β]^ in Λ(L), i.e. (1.6)
A(L) = A 0 (L)e 0 + A 1 (L)e 1 .
Then Λ0(L) and Λχ(L) satisfy the following. Theorem 2. (1)
Ao(L) =
[θ (2)
ifb1(ML)>0.
Ai(L) = -
Theorem 1 is a direct consequence of our construction of Λi. To prove Theorem 2, we use the fourth author's diagonalizing lemma given in [26, Corollary 2.5] and [25, Lemma 2.2]. According to the diagonalizing lemma, we can reduce the proof to the case of algebraically split links, for which we can prove (1). See Section 5 for detail. To prove (2), adding to the diagonalizing lemma, we use Dehn surgery formula obtained in [10] and [22] which expresses the Casson-Walker invariant in terms of linking numbers and coefficients of the Conway polynomial [22]. For
370
T. LE., H.MURAKAMI, J.MURAKAMI AND T.OHTSUKI
algebraically split links, this formula is rather simple and we can prove similar formula for our invariant. Comparing these formulas, we get (2). For detail, see Sections 6 and 7.
2. Proof of Proposition 1 We prepare several lemmas. Suppose X is a one-dimensional oriented manifold whose components are numbered. A chord diagram with support X is a set consisting of a finite number of unordered pairs of distinct non-boundary points on X, regarded up to orientation and component preserving homeomorphisms. We view each pair of points as a chord on X and represent it as a dashed line connecting them. Let Λ(X) be the vector space over C spanned by all chord diagrams with support X, subject to the well-known 4-term relation (see, for example, [2, 17]). The vector space Λ(X) is graded by the number of chords, and, abusing notation, we use the same Λ(X) for the completion of this vector space with respect to the grading. When X is n numbered lines, Λ(X) is denoted by Vn. All the Vn are algebras: the product of two chord diagrams Ό\ and D2 is obtained by placing D\ on top of D2. The algebra Pi is commutative [2, 13]. We recall the associator Φ e V3 in [16, 17], which is equal to Zf( | \ J ), where | \ J presents the trivial q-tangle on three strings with brackets ((**)*) at the top and (*(**)) at the bottom. Let Φ 32 i = Zf( \/\ ), where | / | presents the trivial q-tangle on three strings with brackets (*(**)) at the top and ((**)*) at the bottom. These associators correspond to the associators of quasi-Hopf algebras in [5, 6] and are also studied in [3]. For p = (p 1? , pg), g(p) = g is the length of p, and |p| = Pi + p 2 + ^ Pg For p and r with the same length g, p > r means Pi > ri > P > r means pi > r{, p > 0 means p{ > 0, and p > 0 means pi > 0 for 1 < 2 < g. Let ζ be Zagier's multiple zeta function defined by
(2-1)
and let (2.2)
C(*i,••,**)=
THREE-MANIFOLD INVARIANT VIA KONTSEVICH INTEGRAL
371
Then
Σ
(2.3) k=2
p>0, q>0, |=k,
where A (resp. B) denotes the chord connecting the first and second (resp. the second and third) strings. Another associator Φ321 is obtained from Φ by substituting B to A and A to B. Let
'"Δ where ε{ = 1 (resp. -1) if the i-th string is oriented downward (resp. upward). L e m m a 1.
For any A £ V2, we have J 1 1 L
(2.4) Ί
Γ
D
Proof. This is a special case of Lemma 2.1 in [19].
Let Δ3 be a mapping from V3 to T± applying Δ to the third (right most) string of V3. Let
α' = i {
era, /?"=
β=
Lemma 2.
£Δ3(Φ)=B,
H
,
••••ΓΔΊ.
Δ 3 ( Φ 3 2 i ) F = F.
Proof. Note that
Ea = Ea',
= Eβ',
aF = a'F,
=
β"F.
Let X be an element of P3. By applying Lemma 1 with A — a' or A = β' to each end point on the third string of X, we get
(2.5a)
E β A3(X) = E A3(X) β1.
372
T. LE., H.MURAKAMI, J.MURAKAMI AND T.OHTSUKI
Similarly, we get (2.5b)
Δ 3 (X)aF
= a' Δ 3 (X) F,
Δ 3 (X)βF
= β' Δ 3 (X) F.
Since
we have 1
(2.6)
a β' = β'a' = a' β" = β" a'.
We use the above expression for Φ and Φ32i In Δ 3 (Φ) and Δ 3 ( Φ 3 2 i ) , Δ 3 (A) = a and Δ 3 ( B ) = -β. By (2.4), (2.5) and (2.6), we have Eβ\*\ api-r* βqi~sι . ..ap9~r* βq9-s9 aM = £ α ' | p | α
β'H,
| s | βP1~r1
Hence (2.7a)
-IJ
fc=2
Σ L p>0, q>0,
,)χ
|p|+|q|=k, 9{p)=g(
2
,
q=Γ
g18:
'•mV(2)"Uθ«
^20: "
Q
2 2
:
Π C
1-2
H Π
1-2
'•
Π R
H Γ
"US'"2,
πm Case that the number of end points of chords on s^ and s^ are equal to 3,
Π=θ .(0
n R
Uθ*"3,
(1) X.ς(2)"
.(0
U'
26
UθM,
Π P ,(0 29
Case that the number of end points of chords on s^ and s^ are equal to 2,
Ί
q
nπ
UD\
nπ Ί F
where D\ * is a non-vanishing chord diagram of degree 1 with k components. ι
FIGURE 12. Non-vanishing chord diagrams in Q for an algebraically split link
THREE-MANIFOLD INVARIANT VIA KONTSEVICH INTEGRAL
, E33(D[£~2))
Let E[h3\
be diagrams obtained by inserting \/
-", Q33(D{£~2)),
and let F^iJ\
I I to Q[iJ\
, Q33(D[£-2)).
By using relations in A?\ E[iJ\
F33(D[£'2))
,
= (u^" 1 ©) U Θ2 and Θk = UfcΘ.
3 ^ «(O O 2,
•dJ) -2 _
J- (Ό Λ^2 '
•C/13
(^) 2 '
-C/14
—
JtL/21
^25
~ ^ ^ 2 5
=
&Ϊ3
0
-^22
,
ϋ&>
1
—
5
W
2
1
'
— ~7JW2
6
c\
2 10
θ2
5
^14
= —2Θ2
F17
=
θ2
,
Fig
=
^9
'
22
,
.F3Q (JD-L
^ - 2 ) ) = D[e~2) U θ ,
>
5
3
'
—
2
-^24 —
TTi / Γ^v."^ -ί-/31 \-^l
U
>
β(i) = _ _- 3 0θ( O 2&> ^
^(0 _ -_ ϊ 0θ (?O) E # =
—
7
J_ ί " / 1 " \ cy 2 '
r\
f\3
P2g = — θ
5
/^v(^)
/\ Ί~^iv ^ I I ί~\ / — — 77 1 ^ )
) = 0,
2
/ " ί1\ 2 i
T-I
3
x
>
υ
