Square roots of operators associated with perturbed sub-laplacians on Lie groups Lashi Bandara maths.anu.edu.au/~bandara (Joint work with Tom ter Elst, Auckland and Alan McIntosh, ANU) Mathematical Sciences Institute Australian National University November 17, 2011 Monash University Lashi Bandara (ANU) Square roots of operators on Lie groups November 17, 2011 1 / 30
Lie groups Let G be a Lie group of dimension n and g is Lie algebra. We let dµ denote the left invariant Haar measure. Lashi Bandara (ANU) Square roots of operators on Lie groups November 17, 2011 2 / 30
Algebraic basis and vectorfields A set {a 1,..., a k } g is an algebraic basis if we can recover a basis for g by multi-commutation. We assume that the {a i } are linearly independent. Let A i denote the left translation of a i. The vectorfields {A i } are linearly independent and global. Lashi Bandara (ANU) Square roots of operators on Lie groups November 17, 2011 3 / 30
Distance Theorem of Carathéodory-Chow tells us that for any two points x, y G, we can find a curve γ : [0, 1] G such that γ(t) = i γ i (t)a i (γ(t)) span {A i (γ(t))}. The length of such a curve then is given by l(γ) = ˆ 1 0 ( i γ i (t) 2 ) 1 2 dt Define distance d(x, y) as the infimum over the length of all such curves. The measure dµ is Borel-regular with respect to d and we consider (G, d, dµ) as a measure metric space. Lashi Bandara (ANU) Square roots of operators on Lie groups November 17, 2011 4 / 30
Sub-Laplacian Define an associated sub-laplacian by: = i A 2 i. This is a densely-defined, self-adjoint operator on L 2 (G). Lashi Bandara (ANU) Square roots of operators on Lie groups November 17, 2011 5 / 30
We say that a Lie group is nilpotent if g 1 = [g, g], g 2 = [g, g 1 ], g 3 = [g 1, g 2 ],... is eventually 0. That is, there is a k such that g k = 0. On such spaces, we consider the uniformly elliptic second order operator D H = b i,j A i b ij A j where b, b ij L (G). Lashi Bandara (ANU) Square roots of operators on Lie groups November 17, 2011 6 / 30
Theorem (B.-E.-Mc) Let G be nilpotent and suppose there exist κ 1, κ 2 > 0 such that ˆ Re b(x) κ 1 and Re b ij A i ua j u κ 2 A i u 2 for almost all x G and u H 1 (G). Then, (i) D( D H ) = m i=1 D(A i) = H 1 (G), and (ii) D H u m i=1 A iu for all u H 1 (G). G i,j i Lashi Bandara (ANU) Square roots of operators on Lie groups November 17, 2011 7 / 30
Geometric setup Define the bundle W = span {A i } TG and complexify it. Equip W with the inner product h(a i, A j ) = δ ij. Equip G with the sub-connection f = A k fa k where A k = A k W. Equip W with the sub-connection (u i A i ) = ( u i ) A i We have that W = C k and L 2 (G) L 2 (W) = L 2 (C k+1 ). Lashi Bandara (ANU) Square roots of operators on Lie groups November 17, 2011 8 / 30
Operator theory Use the procedure laid out by [AKMc] and consider the following assumptions. Let H be a Hilbert space. (H1) The operator Γ : D(Γ) H H is closed, densely-defined and nilpotent (Γ 2 = 0). (H2) The operators B 1, B 2 L(H ) satisfy Re B 1 u, u κ 1 u whenever u R(Γ ) Re B 2 u, u κ 2 u where κ 1, κ 2 > 0 are constants. whenever u R(Γ) (H3) The operators B 1, B 2 satisfy B 1 B 2 (R(Γ)) N (Γ) and B 2 B 1 (R(Γ )) N (Γ ). Lashi Bandara (ANU) Square roots of operators on Lie groups November 17, 2011 9 / 30
These operators are bi-sectorial and thus the following are bounded operators: R B t = (1 + itπ B ) 1, P B t = (1 + t 2 Π 2 B) 1, Q B t = tπ B (1 + t 2 Π 2 B) 1, Θ B t = tγ B(1 + t 2 Π 2 B) 1 Write R t, P t, Q t, Θ t by setting B 1 = B 2 = I. Lashi Bandara (ANU) Square roots of operators on Lie groups November 17, 2011 10 / 30
Connection to harmonic analysis Proposition Suppose that (Γ, B 1, B 2 ) satisfy (H1)-(H3). Then, Π B satisfies the quadratic estimate ˆ Q B t u 2 dt t u 2 0 for all u R(Π B ) H if and only if Π B has a bounded holomorphic functional calculus. Lashi Bandara (ANU) Square roots of operators on Lie groups November 17, 2011 11 / 30
Now, let E ± B = χ± (Π B ), where χ + (ζ) = 1 when Re(ζ) > 0 and 0 otherwise, and similarly, χ (ζ) = 1 when Re(ζ) < 0 and 0 otherwise. Lashi Bandara (ANU) Square roots of operators on Lie groups November 17, 2011 12 / 30
Theorem (Kato square root type estimate) Suppose (Γ, B 1, B 2 ) satisfy (H1)-(H3) and Π B has a bounded holomorphic functional calculus. Then, (i) There is a spectral decomposition H = N (Π B ) E + B E B (where the sum is in general non-orthogonal), and (ii) D(Γ) D(Γ B ) = D(Π B) = D( Π 2 B ) with for all u D(Π B ). Γu + Γ B u Π B u Π 2 B u Lashi Bandara (ANU) Square roots of operators on Lie groups November 17, 2011 13 / 30
Solution to the homogeneous problem (H4) Let X be a complete, connected metric space and µ a Borel-regular measure on X that is doubling. Then set H = L 2 (X, C N ; dµ). (H5) The operators B i are matrix-valued pointwise multiplication operators such that the function x B i (x) are L (X, L(C N )). (H6) For every bounded Lipschitz function ξ : X C, multiplication by ξ preserves D(Γ) and M ξ = [Γ, ξi] is a multiplication operator. Furthermore, there exists a constant m > 0 such that M ξ (x) m Lip ξ(x) for almost all x X. (H7) For each open ball B, we have ˆ Γu dµ = 0 B and ˆ B Γ v dµ = 0 for all u D(Γ) with spt u B and for all v D(Γ ) with spt v B. Lashi Bandara (ANU) Square roots of operators on Lie groups November 17, 2011 14 / 30
We change (H8) in [Bandara] in the following way. (H8) -1 (Poincaré hypothesis) There exists C > 0, c > 0 and an operator Ξ : D(Ξ) L 2 (X, C N ) L 2 (X, C M ) such that D(Π) R(Π) D(Ξ) and ˆ ˆ u u B 2 dµ C r 2 Ξu 2 dµ B for all balls B = B(x, r) and u D(Π) R(Π). -2 (Coercivity hypothesis) There exists C > 0 such that for all u D(Π) R(Π), B Ξu C Πu. Lashi Bandara (ANU) Square roots of operators on Lie groups November 17, 2011 15 / 30
Theorem (B.) Let X, (Γ, B 1, B 2 ) satisfy (H1)-(H8). Then, Π B satisfies the quadratic estimate ˆ Q B t u 2 dt t u 2 0 for all u R(Π B ) L 2 (X, C N ) and hence has a bounded holomorphic functional calculus. Lashi Bandara (ANU) Square roots of operators on Lie groups November 17, 2011 16 / 30
Operator setup Define: Γ : D(Γ) L 2 (G) L 2 (W ) L 2 (G) L 2 (W ) by ( ) 0 0 Γ =. 0 Then, Γ = ( ) 0 div 0 0 and Π = ( ) 0 div, 0 where we define div =. Let B = (b ij ). Then, define B 1 = ( ) b 0 0 0 and B 2 = ( ) 0 0. 0 B Lashi Bandara (ANU) Square roots of operators on Lie groups November 17, 2011 17 / 30
Proof of the homogeneous problem Set X = G and H = L 2 (G) L 2 (W). (H1) The sub-connection is densely-defined and closed and so is Γ. Nilpotency is by construction. (H2) By accretivity assumptions. (H3) By construction. Lashi Bandara (ANU) Square roots of operators on Lie groups November 17, 2011 18 / 30
Proof (cont.) (H4) The measure dµ is Borel-regular and the nilpotency of G implies that it is doubling. (H5) By assumption. (H6) It is an easy fact that for all bounded Lipschitz ξ : G C, for for almost all x G. M ξ (x) = [Γ, ξ(x)i] = ξ(x) k Lip ξ(x) (H7) By the left invariance of the measure dµ. Lashi Bandara (ANU) Square roots of operators on Lie groups November 17, 2011 19 / 30
Proof (cont.) (H8) -1 The nilpotency of G implies the following Poincaré inequality ˆ ˆ f f B 2 dµ r 2 f 2 dµ B B for all balls B, and f C (B). See [SC, (P.1), p118]. Define Ξu = ( u 1, u 2 ). -2 The crucial fact needed here is the regularity result [ERS, Lemma 4.2] which gives A i A j f f for f H 2 (G) = D( ). Lashi Bandara (ANU) Square roots of operators on Lie groups November 17, 2011 20 / 30
Inhomogeneous problem For general Lie groups, we need to consider operators with lower order terms. Let b, b ij, c k, d k, e L (G). Define the following uniformly elliptic second order operator m m m D I = b A i b ij A j u b A i c i u b d i A i u beu. ij=1 i=1 i=1 Lashi Bandara (ANU) Square roots of operators on Lie groups November 17, 2011 21 / 30
Theorem (B.-E.-Mc) Suppose there exists κ 1, κ 2 > 0 such that Re b(x) κ 1, ˆ ( ) m Re eu + d i A i u u + G i=1 m c i u + i=1 for almost all x G and u H 1 (G). Then, m b ij A j u A i u dµ j=1 κ 2 ( u 2 + (i) D( D I ) = m i=1 D(A i) = H 1 (G), and (ii) D I u u + m i=1 A iu for all u H 1 (G). ) m A i u 2 i=1 Lashi Bandara (ANU) Square roots of operators on Lie groups November 17, 2011 22 / 30
Spaces of exponential growth (X, d, µ) an exponentially locally doubling measure metric space. That is: there exist κ, λ 0 and constant C 1 such that for all x X, r > 0 and t 1. 0 < µ(b(x, tr)) Ct κ e λtr µ(b(x, r)) Lashi Bandara (ANU) Square roots of operators on Lie groups November 17, 2011 23 / 30
Changes to (H7) and (H8) The following (H7) from [Morris]: (H7) There exist c > 0 such that for all open balls B X with r 1, ˆ ˆ Γu dµ cµ(b) 1 2 u and Γ v dµ cµ(b) 1 2 v B for all u D(Γ), v D(Γ ) with spt u, spt v B. B Lashi Bandara (ANU) Square roots of operators on Lie groups November 17, 2011 24 / 30
We introduce the following local (H8): (H8) -1 (Local Poincaré hypothesis) There exists C > 0, c > 0 and an operator Ξ : D(Ξ) L 2 (X, C N ) L 2 (X, C M ) such that D(Π) R(Π) D(Ξ) and ˆ ˆ u u B 2 dµ C r 2 ( Ξu 2 + u 2 ) dµ B for all balls B = B(x, r) and for u D(Π) R(Π). -2 (Coercivity hypothesis) There exists C > 0 such that for all u D(Π) R(Π), B Ξu + u C Πu. Lashi Bandara (ANU) Square roots of operators on Lie groups November 17, 2011 25 / 30
Theorem (Morris) Let X, (Γ, B 1, B 2 ) satisfy (H1)-(H8). Then, Π B satisfies the quadratic estimate ˆ Q B t u 2 dt t u 2 0 for all u R(Π B ) L 2 (X, C N ) and hence has a bounded holomorphic functional calculus. Lashi Bandara (ANU) Square roots of operators on Lie groups November 17, 2011 26 / 30
Setup Set X = G and H = L 2 (G) L 2 (G) L 2 (W) = L 2 (C k+2 ). Let S = (I, ), S = [I div]. Let Γ = ( ) 0 0, Γ = S 0 ( 0 S 0 0 ), and Π = ( 0 S S 0 ). Let B 00 = e, B10 = (c 1,..., c m ), B01 = (d 1,..., d m ) tr, B11 = (b ij ), and B = ( B ij ). Then, we can write B 1 = ( ) b 0 0 0 and B 2 = ( ) 0 0. 0 B Lashi Bandara (ANU) Square roots of operators on Lie groups November 17, 2011 27 / 30
Proof The proofs of (H1)-(H6) are similar to the homogeneous situation. (H7) The proof is the same as the homogeneous situation except the lower order term introduces the term µ(b) 1 2 u on the right. (H8) -1 The existence of a local Poincaré inequality is guaranteed by [ER2, Proposition 2.4]: ˆ ˆ f f B 2 dµ r 2 ( f 2 + f 2 ) dµ B for all balls B = B(x, r) and where f C (B). Define Ξu = ( u 1, u 2, u 3 ). -2 The crucial fact needed here is the regularity result in [ER, Theorem 7.2], for u H 2 (G) = D( ). B A i A j u 2 u 2 + u 2 Lashi Bandara (ANU) Square roots of operators on Lie groups November 17, 2011 28 / 30
References I [AKMc] Andreas Axelsson, Stephen Keith, and Alan McIntosh, Quadratic estimates and functional calculi of perturbed Dirac operators, Invent. Math. 163 (2006), no. 3, 455 497. [Bandara] L. Bandara, Quadratic estimates for perturbed Dirac type operators on doubling measure metric spaces, ArXiv e-prints (2011). [Morris] Andrew Morris, Local hardy spaces and quadratic estimates for Dirac type operators on Riemannian manifolds, Ph.D. thesis, Australian National University, 2010. [SC] L. Saloff-Coste and D. W. Stroock, Opérateurs uniformément sous-elliptiques sur les groupes de Lie, J. Funct. Anal. 98 (1991), no. 1, 97 121. MR 1111195 (92k:58264) [ER] A. F. M. ter Elst and Derek W. Robinson, Subelliptic operators on Lie groups: regularity, J. Austral. Math. Soc. Ser. A 57 (1994), no. 2, 179 229. MR 1288673 (95e:22019) Lashi Bandara (ANU) Square roots of operators on Lie groups November 17, 2011 29 / 30
References II [ER2] A. F. M. Ter Elst and Derek W. Robinson, Second-order subelliptic operators on Lie groups. I. Complex uniformly continuous principal coefficients, Acta Appl. Math. 59 (1999), no. 3, 299 331. MR 1744755 (2001a:35043) [ERS] A. F. M. ter Elst, Derek W. Robinson, and Adam Sikora, Heat kernels and Riesz transforms on nilpotent Lie groups, Colloq. Math. 74 (1997), no. 2, 191 218. MR 1477562 (99a:35029) Lashi Bandara (ANU) Square roots of operators on Lie groups November 17, 2011 30 / 30