Volume Conjecture and Asymptotic Expansion of Q-Series Kazuhiro Hikami
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Volume Conjecture and Asymptotic Expansion of q-Series Kazuhiro Hikami CONTENTS We consider the “volume conjecture,” which states that an 1. Introduction asymptotic limit of Kashaev’s invariant (or, the colored Jones type 2. Kashaev’s Invariant and Volume Conjecture invariant) of knot K gives the hyperbolic volume of the com- 3. Torus Knots and q-Series plement of knot K. In the first part, we analytically study an 4. Hyperbolic Knots asymptotic behavior of the invariant for the torus knot, and pro- Acknowledgments pose identities concerning an asymptotic expansion of q-series q N References which reduces to the invariant with being the -th root of unity. This is a generalization of an identity recently studied by Zagier. In the second part, we show that “volume conjecture” is numerically supported for hyperbolic knots and links (knots up to 6-crossing, Whitehead link, and Borromean rings). 1. INTRODUCTION In [Kashaev 95, Kashaev 97], Kashaev defined an invari- ant KN for knot K using a quantum dilogarithm func- tion at the N-th root of unity, and proposed the stimulat- ing conjecture that for a hyperbolic knot K an asymptotic limit N →∞of the invariant KN gives a hyperbolic volume of a knot complement, 2 π 3 lim log KN = v3 · S \K , (2–8) N→∞ N where v3 is the hyperbolic volume of the regular ideal tetrahedron, and · denotes the Gromov norm. It was later proved [Murakami and Murakami 01] that Kashaev’s invariant coincides with a specific value of the colored Jones polynomial. In several attempts since then, a geometrical aspect to relate Kashaev’s R-matrix with an ideal octahedron in the three-dimensional hyperbolic space has been clarified (see, e.g., [Thurston 99, Yokota 00, Hikami 01]). Furthermore, a relationship with the Chern–Simons invariant was pointed out [Murakami et al. 02]. In this paper, we are interested in an explicit form 2000 AMS Subject Classification: Primary 52M27, 11B65; Secondary 57M50, 11Mxx of Kashaev’s invariant for the knot K. In general, this invariant can be regarded as a reduction of certain q- Keywords: Jones polynomial, Rogers-Ramanujan identity, hyperbolic volume series. In [Zagier 01], Zagier derived a strange identity c A K Peters, Ltd. 1058-6458/2003 $ 0.50 per page Experimental Mathematics 12:3, page 319 320 Experimental Mathematics, Vol. 12 (2003), No. 3 (3,2) ∞ for a q-series, F (q)= n=0(q)n, which was origi- 2. KASHAEV’S INVARIANT nally introduced in [Stoimenow 98] as an upper bound AND VOLUME CONJECTURE of the number of linearly independent Vassiliev invari- The quantum invariant of knot K can be constructed once F (3,2) −t half- ants. He showed that (e ) is related to the we have the enhanced Yang–Baxter operator [Turaev 88], differential η of the Dedekind -function. From our view- (R, µ, α, β), satisfying point, F (3,2)(q) with q → e2πi/N is nothing but Kashaev’s (3,2) 2πi/N invariant for the trefoil, 31N = F (e ). This (R ⊗ 1)(1 ⊗ R)(R ⊗ 1) = (1 ⊗ R)(R ⊗ 1)(1 ⊗ R), (2–1) motivates us to study an asymptotic expansion of the q- µ ⊗ µ R R µ ⊗ µ , series which, when q is the N-th root of unity, reduces = (2–2) to Kashaev’s invariant for the torus knot. We introduce ±1 ±1 Tr2 R (1 ⊗ µ) = α β. (2–3) the q-series F (2m+1,2)(q) as a generalization of Zagier’s q-series and prove an identity, The operators R±1 and µ±1 are usually depicted as fol- lows; F (2m+1,2)(e2πi/N ) 2 2 3 πi − πi (2m−1) j j √ N 2 e 4 N 4(2m+1) i i 2 m +1 m−1 ij −1 ij 2 R R j 2 j +1 −Nπi (2j+1) k = k = × − m − j π 4(2m+1) ( 1) ( )sin m e × × j=0 2 +1 k k ∞ 2 (2m+1,2) n − πi (2m−1) Tn π N 4(2m+1) , +e n m N n=0 ! 4(2 +1) i (3–15) / k k −1 k and then propose a conjecture, µ = µ = k / F (2m+1,2)(e−t) ∞ 2 (2m+1,2) n (2m−1) t Tn t 8(2m+1) . =e n 3 m (3–24) K ξ n=0 ! 2 (2 +1) When the knot is given as a closure of a braid with n strands, the invariant τ1(K) is computed as for the (2m+1,2) (2m+1,2) K Here F (q), respectively T -number Tn ,are (1,1)-tangle of knot as defined in Equations (3–22), respectively (3–10). τ K α−w(ξ) β−n b ξ ⊗ µ⊗(n−1) . In Section 2, we review the volume conjecture and 1( )= Tr2,...,n R( )(1 ) explain how to construct Kashaev’s invariant from the (2–4) b ξ enhanced Yang–Baxter operator. In Section 3, we study Here we have associated the homomorphism R( )byre- σ±1 ξ R±1 w ξ analytically in detail Kashaev’s invariant for the torus placing the braid group i in with ,and ( ) knot. We consider an asymptotic expansion of the invari- denotes a writhe (a sum of exponents). ant following Kashaev-Tirkkonen [Kashaev and Tirkko- Kashaev’s invariant is originally defined by use of nen 00], and derive an asymptotic formula for q-series the quantum dilogarithm function with a deformation 2πi/N N with q → e . In Section 4, we study numerically an parameter being the -th root of unity [Fadeev and asymptotic behavior of invariants for hyperbolic knots Kashaev 94], ω π /N . and links. We use PARI/GP [PARI 00], and show that =exp(2 i ) (2–5) there is a universal logarithmic correction to invariants. The invariant is then defined as follows: We use the q- We then propose a conjecture as an extension of Equa- product, tion (2–8), n i (ω)n = (1 − ω ), 3 N 3 0 log KN ∼ v3 · S \K · + #(K) · log N + O(N ), i=1 2 π 2 (4–1) n ∗ −i where #(K) is the number of prime factors of a knot (ω)n = (1 − ω ). considered as a connected-sum of prime knots. i=1 Hikami: Volume Conjecture and Asymptotic Expansion of q-Series 321 Theorem 2.1. [Kashaev 95, Kashaev 97] (See also Conjecture 2.3. [Kashaev 97, Murakami and Murakami [Murakami and Murakami 01].) Kashaev’s invariant 01] The asymptotic behavior of Kashaev’s invariant gives KN for the knot K is defined by Equation (2–4) with the hyperbolic volume of the knot complement of knot K; R µ the following and matrices; π 2 K v · S3 \K, 1−(k−j+1)(−i) lim log N = 3 (2–8) ij Nω ij N→∞ N Rk = ∗ ∗ · θ , (ω)[−k−1] (ω) (ω)[i−j] (ω) k [j−] [k−i] where v3 is the hyperbolic volume of the regular ideal (2–6a) tetrahedron, and · denotes the Gromov norm. −1+(−i−1)(k−j) −1 ij Nω ij (R )k = ∗ ∗ · θ , A mathematically rigorous proof of this conjecture has (ω) (ω)[j−] (ω) (ω)[k−i] k [−k−1] [i−j] not been established yet (only thea case of the figure- (2–6b) eight knot was proved (see, e.g., [Murakami 00]). How- k 1 ever, several geometrical studies have been done; the µ = −δk,+1 ω 2 . (2–6c) relationship between Kashaev’s R-matrix and the ideal Here [x] ∈{0, 1,...,N − 1} modulo N,and hyperbolic octahedron has been established [Thurston 99, Yokota 00, Hikami 01], and it was found that the i ≤ k<≤ j, saddle point equation of the invariant coincides with the ij j ≤ i ≤ k<, hyperbolicity consistency condition. θ =1, if and only if k ≤ j ≤ i ≤ k (with <k), As it is well known [Neumann and Zagier 85, Yoshida k<≤ j ≤ i. 85] that the hyperbolic volume is closely related to the Chern–Simons invariant, we also propose a complexifica- tion of Conjecture 2.3. In [Murakami and Murakami 01], it was shown that this invariant coincides with a specific value of the col- Conjecture 2.4. [Murakami et al. 02, Baseilhac and ored Jones polynomial, the invariant of knot K colored Benedetti 01] by the irreducible SU(2)q-module of dimension N with a π q → π /N 2 3 parameter exp(2 i ). lim log KN = v3 · S \K +iCS(K), (2–9) N→∞ N Theorem 2.2. [Murakami and Murakami 01] Kashaev’s where CS denotes the Chern–Simons invariant, invariant KN coincides with the colored Jones polyno- 2 mial at the N-th root of unity, whose R-matrix is given CS(M)=2π cs(M), by 1 2 csM(A)= Tr A ∧ dA + A ∧ A ∧ A . min(N−1−i,j) 8 π2 3 ij i+j+n M Rk = δ,i+n δk,j−n (−1) n=0 ∗ (ω)i+n (ω)j ij+ 1 (i+j−n) × ω 2 , ∗ ∗ 3. TORUS KNOTS AND q-SERIES (ω)i (ω)j−n (ω)n (2–7a) 3.1 Invariant of the Torus Knot m, p min(N−1−j,i) We consider the ( )-torus knot, where we suppose that R−1 ij δ δ − i+j+n m and p are coprime integers. The knot is expressed in k = ,i−n k,j+n ( 1) n=0 terms of generators of Artin’s braid group as ∗ (ω)i (ω)j+n −ij− 1 (i+j−n) × ω 2 , p ∗ ξ = σ1 σ2 ···σm−1 .