bernoulli numbers, drinfeld associators, and the kashiwara ... · theorem (drinfeld) grt = exp(grt)...
TRANSCRIPT
Bernoulli numbers, Drinfeld associators, and the
Kashiwara–Vergne problem
(based on joint works with B. Enriquez, E. Meinrenken,
M. Podkopaeva, P. Severa, C. Torossian)
Anton Alekseev
Department of MathematicsUniversity of Geneva, Switzerland
July 2, 2012
Bernoulli numbers
Jacob Bernoulli
1m+ 2
m+ · · ·+ n
m=
1
m + 1
m�
k=0
�m + 1
k
�Bkn
m+1−k
B0 = 1, B1 = −12 , B2 =
16 , B2k+1 = 0, for k ≥ 1
Generating function:
t
et − 1=
∞�
k=0
Bktk
k!
t
1− e−t= 1 +
t
2+
∞�
k=2
Bktk
k!
A. Alekseev (UniGe) Bernoulli #’s, associators, and KV problem July 2 2 / 23
Campbell–Hausdorff series
x , y , z = generators of a free Lie algebra
ch(x , y) = log(exey) = x +
adx1− e−adx
y + O(y2)
�adx
1− e−adxy = y +
1
2[x , y ] +
∞�
k=2
Bk
k!adkx (y)
�
Theorem
ch(x , y) is the unique Lie series such that
ch(x , y) = x + y +12 [x , y ] + . . .
ch(x , ch(y , z)) = ch(ch(x , y), z)
A. Alekseev (UniGe) Bernoulli #’s, associators, and KV problem July 2 3 / 23
Duflo isomorphism
K = field of characteristic 0,
g = Lie algebra over K, dim g < +∞
Theorem (Duflo, 1977)
Z (Ug) ∼= (Sg)g
Z (Ug) = the center of the universal enveloping algebra
(Sg)g = the ring of invariant polynomials
A. Alekseev (UniGe) Bernoulli #’s, associators, and KV problem July 2 4 / 23
Notation
V = vector space
V ∗= its dual
SV = K[x1, . . . , xn]SV ∗ = K[[p1, . . . , pn]]
Consider elements of SV ∗ as (possibly) infinite order constant coefficient
differential operators
pi �→∂
∂xi
Example: n = 1
A = a0 + a1p + · · ·+ akpk+ . . .
�→ ∂A = a0 + a1d
dx+ · · ·+ ak
dk
dxk+ . . .
A. Alekseev (UniGe) Bernoulli #’s, associators, and KV problem July 2 5 / 23
Duflo isomorphism
The isomorphism (Sg)g ∼= Z (Ug) is a restriction (to g invariants) of the
vector space isomorphism
Duf = Sym ◦ ∂J1/2
where Sym : Sg → Ug is the symmetrization map: xy �→ 12(xy + yx)
and
J12 (x) =
�det
�eadx − 1
adx
�� 12
=
= exp
�1
2Tr adx +
1
2
∞�
k=2
Bk
k · k!Tr(adkx )
�∈ Sg∗
A. Alekseev (UniGe) Bernoulli #’s, associators, and KV problem July 2 6 / 23
Example
g = su(2) = �x , y , z�[x , y ] = z , [y , z ] = x , [z , x ] = y
g∗ = R3, (Sg)g = R[x2 + y2 + z2]
the Casimir element x2 + y2 + z2 ∈ Z (Ug)
Duf : x2 + y2+ z
2 �→ x2+ y
2+ z
2+
1
4
∂J12= 1 +
1
24(∂2
∂x2+
∂2
∂y2+
∂2
∂y2) + . . .
A. Alekseev (UniGe) Bernoulli #’s, associators, and KV problem July 2 7 / 23
Questions
How difficult is the Duflo theorem?
Proofs:
Duflo (1977) ⇐ structure theory
Kontsevich (1997) ⇐ graphical calculus
Torossian, A.A. (2008) ⇐ Drinfeld associators
Why Bernoulli numbers?
A. Alekseev (UniGe) Bernoulli #’s, associators, and KV problem July 2 8 / 23
Kashiwara–Vergne conjecture
1978
∃ A(x , y), B(x , y) Lie series in x and y , such that
1 x + y − log(eyex) = (1− e−adx )A+ (eady − 1)B ,
2 trg(adx ◦ ∂xA+ ady ◦ ∂yB) = 12trg
�adx
eadx−1+
adyeady−1
− adzeadz−1
− 1
�.
Notation: • z = log(exey ),
• ∂xA : g → g, ∂xA(u) =ddtA(x + tu, y)|t=0
A. Alekseev (UniGe) Bernoulli #’s, associators, and KV problem July 2 9 / 23
KV conjecture
Remark: dim g < +∞ =⇒ trg well-defined
Theorem (Kashiwara, Vergne)
KV conjecture =⇒ Duflo isomorphism
Remark: Equation (1) is easy to solve
a =1− e−adx
adxA b =
eady − 1
adyB
x + y − log(eyex) = [x , a] + [y , b]
=⇒ many rational solutions
A. Alekseev (UniGe) Bernoulli #’s, associators, and KV problem July 2 10 / 23
Definition: g is a quadratic Lie algebra if it carries a non-degenerate
symmetric bilinear form:
Q : g× g → K,
Q([x , y ], z) + Q(y , [x , z ]) = 0.
Example: g semisimple, Q Killing form
+ many other examples
Theorem (Torossian, A.A.)
For g quadratic,
KV 1 =⇒ KV 2
A. Alekseev (UniGe) Bernoulli #’s, associators, and KV problem July 2 11 / 23
Theorem
The KV conjecture holds true for all finite-dimensional Lie algebras.
Meinrenken, A.A 2006, using Kontsevich
graphical calculus
Torossian, A.A 2008, using Drinfeld
associators
A. Alekseev (UniGe) Bernoulli #’s, associators, and KV problem July 2 12 / 23
Drinfeld associators
Lie algebra of infinitesimal pure braids tn
Generators: ti ,j , i , j = 1, . . . , n
Relations:
ti ,i = 0, ti ,j = tj ,i ,
[ti ,j , tk,l ] = 0, i , j , k , l distinct
[ti ,j + ti ,k , tj ,k ] = 0.
In knot theory:
i j
= ti ,j ≈ log
� �
i j
A. Alekseev (UniGe) Bernoulli #’s, associators, and KV problem July 2 13 / 23
Drinfeld associators
Notation: ti ,jk = ti ,j + ti ,k
Definition
Φ ∈ K � x , y � is a Drinfeld associator if
1 Φ is group-like (i.e., Φ = exp(Lie series))
2 Φ = 1 +124 [x , y ] + . . .
3 Pentagon equation:
Φ2,3,4
Φ1,23,4
Φ1,2,3
= Φ1,2,34
Φ12,3,4
Φ1,2,3
= Φ(t1,2, t2,3), Φ12,3,4
= Φ(t12,3, t3,4), etc.
A. Alekseev (UniGe) Bernoulli #’s, associators, and KV problem July 2 14 / 23
Pentagon equation
((1 2) 3) 4 (1 (2 3)) 4
1 ((2 3) 4)
1 (2 (3 4))
(1 2) (3 4)
Φ1,2,3
Φ1,23,4
Φ2,3,4
Φ1,2,34
Φ12,3,4
A. Alekseev (UniGe) Bernoulli #’s, associators, and KV problem July 2 15 / 23
Importance of associators
in knot theory (finite type invariants)
in number theory (multiple zeta values)
in quantization (Tamarkin’s approach)
in Lie theory (Etingof–Kazhdan quantization of Lie bialgebras)
A. Alekseev (UniGe) Bernoulli #’s, associators, and KV problem July 2 16 / 23
Theorem (Drinfeld, Le–Murakami)
The pentagon equation admits an explicit solution over C:
Φ(x , y) =
�
k,m
�i
2π
�m1+···+mk
ζ(m1, . . . ,mk)xm1−1
yxm2−1
y . . . xmk−1y
+ regularized terms
where
ζ(m1, . . . ,mk) =�
n1>···>nk>0
1
nm11 . . . nmk
k
Note: ζ(2m) = (−1)m+1 (2π)2m
2(2m)!B2m
A. Alekseev (UniGe) Bernoulli #’s, associators, and KV problem July 2 17 / 23
Theorem (Drinfeld)
The pentagon equation admits solutions over Q
No explicit formulas available.
Definition
The (homogeneous) Grothendieck–Teichmuller Lie algebra grt consists ofall φ ∈ free Lie (x , y), deg(φ) ≥ 3, satisfying
φ1,2,3+ φ1,23,4
+ φ2,3,4= φ12,3,4
+ φ1,2,34.
Theorem (Drinfeld)
GRT = exp(grt) acts freely and transitively on the set of Drinfeld
associators.
A. Alekseev (UniGe) Bernoulli #’s, associators, and KV problem July 2 18 / 23
Associators =⇒ KV
Let
ψ(ΦxΦ−1, y) =
�d
dτΦ(τx , τy)
�
τ=1
Φ(x , y)−1.
ψ is an element of the (inhomogeneous) Grothendieck–Teichmuller Lie
algebra gt.
Recall: ch(x , y) = ln(exey ).
Theorem (Enriquez, Torossian, Podkopaeva, Severa, A.A.)
A(x , y) = ψ(−ch(x , y), x)
B(x , y) = ψ(−ch(x , y), y)− 12ch(x , y)
solves KV.
A. Alekseev (UniGe) Bernoulli #’s, associators, and KV problem July 2 19 / 23
Uniqueness problem
Definition
The Kashiwara–Vergne Lie algebra krv consists of all pairs a, b ∈ free Lie
(x , y), such that
[x , a] + [y , b] = 0
trg(adx ◦ ∂xa+ ady ◦ ∂yb) = 0 for all g
Theorem (Torossian, A.A.)
φ �→ (φ(−x − y , x), φ(−x − y , y))
is an injection of Lie algebras grt � krv
A. Alekseev (UniGe) Bernoulli #’s, associators, and KV problem July 2 20 / 23
Conjectures
Conjecture: grt ∼= krv
numerical evidence up to degree 16.
deg 1 2 3 4 5 6 7 8 9 10 11 . . .dim 0 0 0 0 0 0 0 1 1 0 1 . . .
A. Alekseev (UniGe) Bernoulli #’s, associators, and KV problem July 2 21 / 23
Conjectures
Number theory properties of associators =⇒ Lie algebra dmr0 (Racinet)
Theorem (Furusho)
grt � dmr0
Conjecture: grt ∼= dmr0
numerical evidence up to degree 19
Theorem (Schneps)
dmr0 � krv
Conjecture: dmr0 ∼= krv
A. Alekseev (UniGe) Bernoulli #’s, associators, and KV problem July 2 22 / 23
Thank you!
A. Alekseev (UniGe) Bernoulli #’s, associators, and KV problem July 2 23 / 23