BURGHELEA-FRIEDLANDER-KAPPELER S GLUING FORMULA AND ITS APPLICATIONS TO THE ZETA-DETERMINANTS OF DIRAC LAPLACIANS

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1 Trends in Mathematics Information Center for Mathematical Sciences Volume 7, Number 1,June 004, Pages BURGHELEA-FRIEDLANDER-KAPPELER S GLUING FORMULA AND ITS APPLICATIONS TO THE ZETA-DETERMINANTS OF DIRAC LAPLACIANS YOONWEON LEE Abstract. For the last two decades the eta-invariant of a Dirac operator on a compact manifold with cylindrical ends has been studied in many ways. In this note we survey some of these results and the BFK-gluing formula for zeta-determinants. As applications of the BFK-gluing formula we give some partial results for the zeta-determinant of a Dirac Laplacian to the analogous questions as those given in the case of eta-invariant. 1 Introduction In this note we briefly explain the Burghelea-Friedlander-Kappeler s gluing formula (BFK-gluing formula) for zeta-determinants and its applications to the zetadeterminants of Dirac Laplacians on manifolds with cylindrical ends. Originally the BFK-gluing formula contains a constant which is determined by the asymptotic expansion of the zeta-determinant of a one parameter family of elliptic ΨDO s called the Dirichlet-to-Neumann operators. Under the assumption of the product structure near the hypersurface of a given manifold this constant was computed explicitly by the author ([L]) and this formula can be applied to a compact oriented manifold with boundary having the product structure near the boundary. On the other hand, for the last two decades the eta-invariant of a Dirac operator has been studied in various ways and many results have been proved by many authors. Moreover, the zeta-determinant of a Dirac Laplacian and the eta-invariant of a Dirac operator are the modulus and phase, up to the value of the zeta-function for a Dirac Laplacian at zero, of the zeta determinant of a Dirac operator, which plays an important role in mathematical physics (cf. [SW]). Hence it s natural to consider the analogous questions for zeta-determinant as the questions given in the case of eta-invariant. Hence for the motivation of our wors in the zeta-determinant we start from the eta-invariant of a Dirac operator. 000 Mathematics Subject Classification. 58J5, 58J50. Key words and phrases. Dirac operator, Dirac Laplacian, eta-invariant, zeta-determinant, BFK-gluing formula, Atiyah-Patodi-Singer boundary condition, adiabatic limit. c 004 Information Center for Mathematical Sciences 103

2 104 YOONWEON LEE Eta-invariant on a manifold with cylindrical end Let M be a 4-dimensional compact oriented manifold. The intersection form B : H (M; Z) H (M; Z) Z is defined as follows. For any [α], [β] H (M; Z) B([α], [β]) = [α β], [z], where [z] is the fundamental class in H 4 (M; Z). Then B is non-degenerate by the Poincaré duality and the signature of M is defined by Sign(M) = {positive eigenvalues of B} {negative eigenvalues of B}. On the other hand, Hirzebruch signature theorem tells that Sign(M) = L(M), where L(M) is the Hirzebruch L-polynomial consisting of Pontrjagin classes. However, in case of manifolds with boundary the situation is more complicated. Suppose that M is a 4-dimensional compact oriented manifold with boundary Y. We define a bilinear form B : H (M, Y ; Z) H (M, Y ; Z) Z M in a similar way. For any [α], [β] H (M, Y ; Z) B([α], [β]) = [α β], [z], where [z] is the fundamental class in H 4 (M, Y ; Z). In this case, B may be degenerate and there is some topological obstruction for B to be non-degenerate (cf. [GS]). Similarly, we define the signature of M by Sign(M) = {positive eigenvalues of B} {negative eigenvalues of B}. But in this case, Sign(M) L(M) 0 and the correction term on the boundary M Y was given by Atiyah, Patodi, Singer in [APS]. We choose a metric on Y and define a Dirac operator on the space of even forms on Y as follows. D : Ω even (Y ) Ω even (Y )

3 BURGHELEA-FRIEDLANDER-KAPPELER S GLUING FORMULA 105 D(f p ) = ( 1) +p+1 ( d d )f p, (.1) where f p Ω p (Y ) and is the Hodge -operator on Y. Then D is the 1st order elliptic differential operator having discrete spectrum and the eta function is defined by η D (s) = sign(λ i ) λ i s. 0 λ i Spec(D) It is nown that η D (s) is regular for Res > dimy and admits the meromorphic continuation to the whole complex plane. It is a non-trivial fact that η D (s) has a regular value at 0. We call η D (0) the eta-invariant associated to D. The famous Atiyah-Patodi-Singer theorem ([APS]) is that Sign(M) = L(M) η D (0). M For the motivation of a Dirac operator on a manifold with cylindrical ends let us consider the operator D in (.1). ( 1) +p+1 ( d d ) Ω p = ( 1) +p (d + d ) Ω p. For any ω Ω 4 1 p (Y ), (d + d )ω = = (e n en ω e n en ω) 4 1 n=1 4 1 n=1 e n en ω 4 = e 4 1 e4 1 ω + e n en ω = e 4 1 ( e4 1 + n=1 4 n=1 e n e 4 1 en ) ω, (.) where {e 1, e,, e 4 1 } is a local orthonormal frame for the tangent bundle of Y, is the Levi-Civita connection on Y, is the interior product and is the Clifford multiplication on the space of differential forms identified with the Clifford algebra. Now we are going to discuss the eta-invariant on a manifold with boundary. Suppose that (M, g) is an n-dimensional manifold with boundary Y and E M is a Clifford module bundle. Choose a collar neighborhood N of Y which is diffeomorphic to ( 1, 0] Y. We assume that the metric g is a product one on N and the bundle E M also has the product structure on N in the following

4 106 YOONWEON LEE sense. E N = p (E Y ), where p : ( 1, 0] Y Y is the natural projection. From now on we consider a Dirac operator D acting on smooth sections of E having the following properties on N. D N = G( u + B), (.3) where G : E Y E Y is a bundle automorphism, B is a Dirac operator on Y, u is the outward normal derivative on N and they satisfy the following properties. G = Id, G = G, GB = BG. (.4) Obviously the Dirac operator given in (.) satisfies the form (.3) and (.4). It can be checed that D is a symmetric operator and D (= DD = D D) is called the Dirac Laplacian. On the tubular neighborhood N, D = + B. To obtain a discrete spectrum of D we need a proper boundary condition on Y. The most useful one is given by Atiyah, Patodi and Singer. Since B is a 1st order elliptic differential operator on a closed manifold, the spectrum of B is distributed from to. We denote by P > (P < ) : C (Y ) C (Y ) the orthogonal projection onto the space spanned by the positive (negative) eigensections of B. If erb is non-trivial, this boundary condition is not enough. If Bf = 0, then by (.4) BGf = GBf = 0 and hence erb is always an even dimensional vector space. (In fact, G gives the symplectic structure on erb.) We choose a unitary involution σ : erb er B which anticommutes with G, i.e. σ = Id erb, Gσ = σg and σ is unitary. There are many choices for such σ s. Note that er(id σ) is the (+1)- eigenspace of σ and er(id + σ) is the ( 1)- eigenspace of σ. We put P σ = P < + projection on er(id + σ) and define D Pσ by the Dirac operator D with the domain Dom(D Pσ ) = {φ C (M) P σ (φ Y ) = 0}.

5 Then D Pσ BURGHELEA-FRIEDLANDER-KAPPELER S GLUING FORMULA 107 : Dom(D Pσ ) C (M) is an essential self-adjoint operator having discrete spectrum. We define the eta-function η DP σ (s) = 0 λ i Spec(D P σ ) sign(λ i ) λ i s and obtain the eta-invariant η DPσ (0) associated to D Pσ. Next, we are going to discuss the adiabatic limit of an eta-invariant. We denote by M r := M Y [0, r] Y and similarly M := M Y [0, ) Y. Then from the product structure on N we can extend the Dirac operator D and the Clifford module bundle E to the Dirac operator D r and the Clifford module bundle E r on M r. Now we consider the bundle E r M r and the Dirac operator D r,pσ : Dom(D r,pσ ) C (M r ). Then the basic questions are : (1) How does a spectral invariant change when M is stretched to M r? () What is the limit of a spectral invariant as r? The followings are some nown results concerning these questions. (1) η Dr,P (0) does not depend on the cylinder length r. (Müller in [M]) σ () If σ 1 and σ are two unitary involutions on erb satisfying Gσ i = σ i G (i = 1, ), η DPσ1 (0) η DPσ (0) 1 πi log Det(σ 1σ er(g i) ) (Müller in [M], Lesch and Wojciechowsi in [LW]) (modz). (3) Gluing formula for eta-invariant : Let M be a closed manifold and Y be a hypersurface so that M Y has two components. We denote by M 1 and M the components of M Y. Choose a collar neighborhood N of Y which is diffeomorphic to [ 1, 1] Y. We assume the product structure on N and consider a Dirac operator satisfying (.3) and (.4) on N. We denote by D 1 and D the restriction of D to M 1 and M, respectively. Then η D (0) η D1,P σ 1 (0) + η D,P σ (0) + η(d cyl, σ 1, σ ) (modz), where D cyl = G( u + B) and η(d cyl, σ 1, σ ) is the eta-invariant on [ 1, 1] Y with the boundary conditions Id P σ1 on { 1} Y and Id P σ on {1} Y. (Wojciechowsi in [W], see also [BL]) (4) Bune ([B]), Kir and Lesch ([KL]) showed the gluing formula for eta-invariant in R (not in R/Z). Their formulas contain the concepts of spectral flow and Maslov index.

6 108 YOONWEON LEE 3 Zeta-determinant of an elliptic operator and the BFK-gluing formula for zeta-determinants In this section we are going to explain briefly the zeta-determinant of an elliptic operator and the BFK-gluing formula for zeta-determinants. For the motivation of zeta-determinant we start from the ordinary determinant of a linear operator acting on a finite dimensional vector space. Suppose that V is an n-dimensional vector space and T : V V is a linear operator with eigenvalues λ 1, λ,, λ n. We define ζ T (s) by ζ T (s) = λ s λ s n = Then from simple computation we have ζ T (0) = n i=1 λ s i. n log λ i = log(λ 1 λ n ) i=1 and e ζ T (0) = λ 1 λ n = dett. (3.1) The formula (3.1) can be generalized to an operator having infinitely many eigenvlaues. Let M be a compact closed manifold and E M be a vector bundle. Suppose that A : C (M) C (M) is a positive definite elliptic pseudo-differential operator with eigenvalues 0 < λ 1 λ. We define ζ A (s) := i=1 λ s i. Then it was shown by Seeley ([S]) that ζ A (s) is regular for Res > dimm ord(a) and admits the meromorphic continuation to the whole complex plane having a regular value at s = 0. We define the zeta-determinant of A by Det ζ A = e ζ A (0) or log Det ζ A = ζ A(0). The zeta-determinant was introduced by Ray and Singer ([RS]) in defining the analytic torsion, which is the analytic counterpart of the Reidemeister torsion. i.e., Example : Suppose that A is the Laplacian acting on smooth functions on S 1, A = θ : C (S 1 ) C (S 1 ).

7 BURGHELEA-FRIEDLANDER-KAPPELER S GLUING FORMULA 109 The the spectrum of A is Spec(A) = {0, 1, ( 1),, ( ), } and ζ A (s) = 1 s + ( 1) s + s + ( ) s + = ζ R (s), where ζ R (s) = n=1 1 n s, the Riemann zeta function. It is a well-nown fact that ζ R (0) = 1 log π. Hence ζ A (0) = 4 ( 1 ) log π and Det ζa = e ζ A (0) = 4π. Next, we are going to explain the BFK-gluing formula for zeta-determinants. Let (M, g) be a compact oriented manifold with boundary Z (Z may be empty) and Y be a hypersurface of M such that Y Z = and M Y has two components. Here we don t assume the product structure near Y. We denote by M 1, M the closure of each component, i.e. M = M 1 Y M. Without loss of generality we asume that Z M. Let M be a Laplacian on M and M1, M be the restriction of M to M 1, M, respectively. Choose an elliptic boundary condition P 0 on Z and we assume that M,P0 is an invertible operator. We denote by P Dir the Dirichlet boundary condition on Y and denote by Mi,P Dir with the domain (i = 1, ) the Laplacian Mi Dom( M1 ) = {φ C (M 1 ) φ Y = 0}, Dom( M ) = {φ C (M ) φ Y = 0, P 0 (φ Z ) = 0}. Now we define the correction operator R : C (Y ) C (Y ) called the Dirichletto-Neumann operator as follows. For any f C (Y ) choose φ 1 C (M 1 ) and φ C (M ) satisfying M1 (φ 1 ) = 0, M (φ ) = 0, φ 1 Y = φ Y = f and P 0 (φ Z ) = 0. (3.) Both φ 1 and φ satisfying (3.) exist uniquely from the Dirichlet boundary condition and the boundary condition P 0. Then we define Q 1 (f) and Q (f) by Q 1 (f) := ( u φ 1 ) Y, Q (f) := ( u φ ) Y and R(f) := Q 1 (f) Q (f) = ( u φ 1 ) Y ( u φ ) Y, where u is the unit outward normal derivative to M 1. R is nown to be a positive definite, invertible, elliptic pseudo-differential operator of order 1 ([L1]) and the following theorem is called the BFK-gluing formula ([BFK], [L1]).

8 110 YOONWEON LEE Theorem 3.1. With the same notation as above we have log Det ζ M,P0 = c + log Det ζ M1,P Dir + log Det ζ M,P Dir,P 0 + log Det ζ R, where c is determined by the asymptotic symbol of the operator R. This theorem holds in a general compact oriented manifold (with boundary). If we assume the product structure near Y so that the Laplacian M has the form u + Y near Y, it was shown by the author ([L]) that c = log (ζ Y (0) + dimer Y ). In the next section we give some results about the zeta-determinant of a Dirac Laplacian as applications of Theorem Applications of the BFK-gluing formula In this section we start from showing the relation between the eta-invariant of a Dirac operator and the zeta-determinant of a Dirac Laplacian (cf. [SW]). Suppose that D is a Dirac operator with positive eigenvalues {λ negative eigenvalues { µ = 1,, 3, }. Then we have η D (s) = =1 λ s =1 ζ D (s) = µ s, ζ D (s) = =1 λ s + =1 ( µ ) s, =1 λ s + = 1,, 3, } and =1 where all three functions are holomorphic for Res > dimm. Then ζ D (s) = = 1 ( λ s + µ s ( ζ D ( s ) + η D(s) + λ s ) µ s + ( 1) s ( λ s ) + 1 ( e iπs ζ D ( s ) ) η D(s). Taing derivative with respect to s in Res > dimm we have + µ s µ s, λ s ) µ s ζ D(s) = 1 ( 1 ζ D ( s ) ) + η D(s) iπ ( e iπs ζ D ( s ) ) η D(s) + 1 ( 1 e iπs ζ D ( s ) ) η D(s) After taing meromorphic continuation and putting s = 0, we have ζ D(0) = 1 ζ D (0) iπ (ζ D (0) η D(0))

9 BURGHELEA-FRIEDLANDER-KAPPELER S GLUING FORMULA 111 and hence Det ζ D = e ζ D (0) = e πi (ζ D (0) η D(0)) e 1 ζ D (0). This fact implies that the zeta determinant of a Dirac Laplacian and the eta invariant of a Dirac operator are the modulus and phase of the zeta-determinant of a Dirac operator up to π ζ D(0), which is manipulated more or less easily (cf. (Appendix in [PW])). Hence it is natural to consider the analogous questions for the zeta-determinant of a Dirac Laplacian as the questions given in the eta-invariant. More precisely we can as the similar questions for zeta-determinant as those given in Section. Let us assume the product structure near Y so that M N = u + Y. The following results, which can be compared with the results given in Section, are partial answers of the zeta-determinants of Dirac Laplacians for these questions. (1) The constant in Theorem 3.1 is c = log (ζ Y (0) + dimer Y ) and hence log Det ζ M,P0 = log (ζ Y (0) + dimer Y ) + log Det ζ M1, Dir + log Det ζ M,P Dir,P 0 + log Det ζ R. (Lee in [L]) () Supose that (M, g) is a compact closed Riemannian manifold and Y be a hypersurface so that M Y has two components, denoted by M 1 and M as above. Choose a collar neighborhood N diffeomorphic to [ 1, 1] Y and assume the product structure on N. We denote by M r the compact manifold without boundary obtained by attaching [ r 1, r + 1] Y on M ( 1, 1 ) Y by identifying [ 1, 1 ] Y with [ r 1, r 1 ] Y and [ 1, 1] Y with [r + 1, r + 1] Y. We also denote by M 1,r, M,r the manifolds with boundary which are obtained by attaching [ r, 0] Y, [0, r] Y on M 1, M by identifying Y with { r} Y and Y with {r} Y, respectively. Then the Dirac operator D and the bundle E can be naturally extended to D r, E r on M r and D r has, on the cylinder part, the form D r = G( u + B). We assume that erb = {0} and there is no extended L - solutions of D on M := M Y [0, ) Y (for definition see [APS] or [BW]). Then we have : (a) } lim {log Det ζ D Mr log Det ζ D M1,r,P< log Det ζ D M,r,P> = log ζ B (0). r

10 11 YOONWEON LEE (Par and Wojciechowsi in [PW], Lee in [L3]). (b) If dimy is even, as r log Det ζ D M 1,r,P < ζ B ( 1 ) r+ ( 1 log ζ B (0) + log Det ζdm 1,PDir 1 ) 4 log Det ζb + log Det ζ (Q 1 + B ) If dimy is odd, as r + O( 1 r ). log Det ζd M1,r,P < A 1 r+ ( 1 log ζ B (0) + log Det ζdm 1,PDir 1 ) 4 log Det ζb + log Det ζ (Q 1 + B ) + O( 1 r ), where A 1 = d ds (s ζ B (s 1 )) s=0 +(s ζ B (s 1 )) s=0 ( 1 Γ ( 1 )+γ +) π with γ the Euler constant (Lee in [L4]). (3) Suppose that (M, g) is a compact oriented manifold with boundary Y and σ 1, σ are two unitary involutions acting on erb with Gσ i = σ i G. Assume that D Pσ1 and D Pσ are invertible operators. Then Det ζ D Pσ1 Det ζ D Pσ ( = det I + I + C(0) (σ σ 1 )(C(0) σ ) I + C(0) ) erb. Here C(0) : erb erb is the scattering matrix determined by D on M (see [M] for details). Moreover, C(0) is a unitary operator with C(0) = Id erb, GC(0) = C(0)G and hence C(0) is the natural choice of a unitary involution anticommuting with G on erb (Lee in [L5]). References [APS] M. F. Atiyah, V. K. Patodi and I. M. Singer, Spectral asymmetry and Riemannian geometry I, Math. Proc. Camb. Phil. Soc. 77 (1975), [BW] B. Booβ-Bavnbe, and K. P. Wojciechowsi, Elliptic Boundary Problems for Dirac Operators, Birhäuser, Boston, [BL] J. Brüning and M. Lesch, On the η-invariant of certain nonlocal boundary value problems, Due Math. Jour. 96 (1999), [B] U. Bune, On the gluing problem for the eta-invariant, Jour. of Diff. Geom. 41 (1995),

11 BURGHELEA-FRIEDLANDER-KAPPELER S GLUING FORMULA 113 [BFK] D. Burghelea, L. Friedlander and T. Kappeler, Mayer-Vietoris type formula for determinants of elliptic differential operators, J. of Funct. Anal. 107 (199), [GS] R. E. Gompf and A. I. Stipsicz, 4-Manifolds and Kirby Calculus, Graduate Studies in Mathematics, Vol 0, Amer. Math. Soc., [KL] P. Kir and M. Lesch, The eta-invariant, Maslov index, and spectral flow for Dirac operators on manifolds with boundary, Preprint (000). [L1] Y. Lee, Mayer-Vietoris formula for the determinants of elliptic operators of Laplace- Beltrami type (after Burghelea, Friedlander and Kappeler), Diff. Geom. and Its Appl. 7 (1997), [L] Y. Lee, Burghelea-Friedlander-Kappeler s gluing formula for the zeta determinant and its applications to the adiabatic decompositions of the zeta-determinant and the analytic torsion, Trans. Amer. Math. Soc (003), [L3] Y. Lee, Burghelea-Friedlander-Kappeler s gluing formula and the adiabatic decomposition of the zeta-determinant of a Dirac Laplacian, Manuscript. Math. 111 (003), [L4] Y. Lee, Asymptotic expansion of the zeta-determinant of a Laplacian on a stretched manifold, preprint 003. [L5] Y. Lee, Lesch-Wojciechowsi type formula for the zeta-determinants of Dirac Laplacians, preprint 004. [LW] M. Lesch and K. P. Wojciechowsi, On the η-invariant of generalized Atiyah-Patodi-Singer problems, Illinoise J. Math. 40 (1996), [M] W. Müller, Eta invariant and manifolds with boundary, J. of Diff. Geom. 40 (1994), [PW] P. Par and K. P. Wojciechowsi with Appendix by Y. Lee, Adiabatic decomposition of the ζ-determinant of the Dirac Laplacian I. The case of an invertible tangential operator, Comm. in PDE. 7 (00), [RS] D. B. Ray and I. M. Singer, R-torsion and the Laplacian on Riemannian manifolds, Adv. in Math. 7 (1971), [SW] S. G. Scott and K. P. Wojciechowsi, The ζ-determinant and Quillen determinant for a Dirac operator on a manifold with boundary, Geom. Funct. Anal. (GAFA) 10 (1999), [S] R. Seeley, Complex powers of elliptic operators, Proceedings of Symposia on Singular Integrals 10 (1967), Amer. Math. Soc., [W] K. P. Wojciechowsi, The ζ-determinant and the additivity of the η-invariant on the smooth, self-adjoint Grassmannian, Comm. Math. Phys. 01 (1999), Department of Mathematics Inha University Incheon, , Korea address : ywlee@math.inha.ac.r