Invariant Nonholonomic Riemannian Structures on Three-Dimensional Lie Groups Dennis I. Barrett Geometry, Graphs and Control (GGC) Research Group Department of Mathematics (Pure and Applied) Rhodes University, Grahamstown 6140 Workshop on Geometry, Lie Groups and Number Theory University of Ostrava, 24 June 2015 Dennis I. Barrett (Rhodes Univ.) Invariant NH Riemannian Structures Univ. Ostrava 2015 1 / 26
Introduction Nonholonomic Riemannian manifold ( M , g , D ) Model for motion of free particle moving in configuration space M kinetic energy L = 1 2 g ( · , · ) constrained to move in “admissible directions” D Invariant structures on Lie groups are of the most interest Objective classify all left-invariant systems on 3D Lie groups restrict to unimodular groups Dennis I. Barrett (Rhodes Univ.) Invariant NH Riemannian Structures Univ. Ostrava 2015 2 / 26
Outline Invariant nonholonomic Riemannian manifolds 1 Isometries Curvature Unimodular 3D Lie groups 2 Classification of 3D structures 3 Contact structure Case 1 Case 2 Dennis I. Barrett (Rhodes Univ.) Invariant NH Riemannian Structures Univ. Ostrava 2015 3 / 26
Outline Invariant nonholonomic Riemannian manifolds 1 Isometries Curvature Unimodular 3D Lie groups 2 Classification of 3D structures 3 Contact structure Case 1 Case 2 Dennis I. Barrett (Rhodes Univ.) Invariant NH Riemannian Structures Univ. Ostrava 2015 4 / 26
Invariant nonholonomic Riemannian manifold (G , g , D ) Ingredients Configuration space G n -dim connected Lie group with Lie algebra g = T 1 G Constraint distribution D = {D x } x ∈ G left invariant: D x = x d , where d ⊂ g is an r -dim subspace completely nonholonomic: d generates g Riemannian metric g g x : T x G × T x G → R is an inner product left invariant: g x ( xU , xV ) = g 1 ( U , V ) for every U , V ∈ g Orthogonal decomposition T G = D ⊕ D ⊥ projectors: P : T G → D and Q : T G → D ⊥ Dennis I. Barrett (Rhodes Univ.) Invariant NH Riemannian Structures Univ. Ostrava 2015 5 / 26
Nonholonomic geodesics Preliminaries Integral curve of D curve γ in G such that ˙ γ ( t ) ∈ D γ ( t ) for every t Levi-Civita connection � ∇ of g “directional derivative” of one vector field along another D’Alembert Principle An integral curve γ of D is called a nonholonomic geodesic of (G , g , D ) if � γ ( t ) ∈ D ⊥ ∇ ˙ γ ( t ) ˙ γ ( t ) for all t Equivalently: P ( � ∇ ˙ γ ( t ) ˙ γ ( t )) = 0 for every t . Dennis I. Barrett (Rhodes Univ.) Invariant NH Riemannian Structures Univ. Ostrava 2015 6 / 26
Nonholonomic connection NH connection ∇ : Γ( D ) × Γ( D ) → Γ( D ) ∇ X Y = P ( � ∇ X Y ) , X , Y ∈ Γ( D ) affine connection parallel transport only along integral curves of D depends only on D , g | D and a choice of complement to D Characterisation of nonholonomic geodesics integral curve γ of D ∇ ˙ γ ( t ) ˙ γ ( t ) = 0 for every t ⇐ ⇒ is a nonholonomic geodesic Dennis I. Barrett (Rhodes Univ.) Invariant NH Riemannian Structures Univ. Ostrava 2015 7 / 26
Isometries Isometry between (G , g , D ) and (G ′ , g ′ , D ′ ) diffeomorphism φ : G → G ′ such that g | D = φ ∗ g ′ � φ ∗ D ⊥ = D ′⊥ � φ ∗ D = D ′ D ′ Properties of isometries preserves NH connection: ∇ = φ ∗ ∇ ′ 1-to-1 correspondence between NH geodesics of isometric structures Dennis I. Barrett (Rhodes Univ.) Invariant NH Riemannian Structures Univ. Ostrava 2015 8 / 26
Curvature ∇ is not a connection on the vector bundle D → G hence Riemannian curvature tensor not defined Schouten curvature tensor K : Γ( D ) × Γ( D ) × Γ( D ) → Γ( D ) K ( X , Y ; Z ) = [ ∇ X , ∇ Y ] Z − ∇ P ([ X , Y ]) Z − P ([ Q ([ X , Y ]) , Z ]) define � K ( W , X ; Y , Z ) = g ( K ( W , X ; Y ) , Z ) (S1) � K ( X , X ; Y , Z ) = 0 (S2) � K ( W , X ; Y , Z ) + � K ( X , Y ; W , Z ) + � K ( Y , W ; X , Z ) = 0 also define R := component of � � K that is skew-symmetric in second two args C := � � K − � R � R behaves like Riemannian curvature tensor Dennis I. Barrett (Rhodes Univ.) Invariant NH Riemannian Structures Univ. Ostrava 2015 9 / 26
Ricci-like curvatures Ricci curvature Ric : Γ( D ) × Γ( D ) → C ∞ (G) r � � Ric( X , Y ) = R ( X i , X ; Y , X i ) i =1 ( X i ) r i =1 is an orthonormal frame for D S := � r i =1 Ric( X i , X i ) is the scalar curvature Ricci-type tensors A sym , A skew : Γ( D ) × Γ( D ) → C ∞ (G) r � � A ( X , Y ) = C ( X i , X ; Y , X i ) i =1 A sym := symmetric part of A A skew := skew-symmetric part of A Dennis I. Barrett (Rhodes Univ.) Invariant NH Riemannian Structures Univ. Ostrava 2015 10 / 26
Curvature in 3D Curvature invariants κ , χ 1 , χ 2 � � − det( g | ♯ det( g | ♯ κ = 1 D ◦ A ♭ D ◦ A ♭ 2 S χ 1 = sym ) χ 2 = skew ) preserved by isometries (i.e., isometric invariants) κ , χ 1 , χ 2 determine K left invariant, hence constant for unimodular groups: χ 2 = 0 Dennis I. Barrett (Rhodes Univ.) Invariant NH Riemannian Structures Univ. Ostrava 2015 11 / 26
Outline Invariant nonholonomic Riemannian manifolds 1 Isometries Curvature Unimodular 3D Lie groups 2 Classification of 3D structures 3 Contact structure Case 1 Case 2 Dennis I. Barrett (Rhodes Univ.) Invariant NH Riemannian Structures Univ. Ostrava 2015 12 / 26
The unimodular 3D Lie groups Bianchi-Behr classification (unimodular algebras) Lie algebra Lie group Name Class R 3 R 3 Abelian Abelian h 3 H 3 Heisenberg Nilpotent se (1 , 1) SE(1 , 1) Semi-Euclidean Completely solvable � se (2) SE(2) Euclidean Solvable � sl (2 , R ) SL(2 , R ) Special linear Semisimple su (2) SU(2) Special unitary Semisimple Dennis I. Barrett (Rhodes Univ.) Invariant NH Riemannian Structures Univ. Ostrava 2015 13 / 26
Left-invariant distributions on 3D groups Killing form K : g × g → R , K ( U , V ) = tr[ U , [ V , · ]] K is nondegenerate ⇐ ⇒ g is semisimple Completely nonholonomic distributions on 3D groups no such distributions on R 3 Up to Lie group automorphism: exactly one distribution on H 3 , SE(1 , 1), � SE(2), SU(2) exactly two distributions on � SL(2 , R ): � denote SL(2 , R ) hyp if K indefinite on D � SL(2 , R ) ell definite '' '' '' '' '' Dennis I. Barrett (Rhodes Univ.) Invariant NH Riemannian Structures Univ. Ostrava 2015 14 / 26
Outline Invariant nonholonomic Riemannian manifolds 1 Isometries Curvature Unimodular 3D Lie groups 2 Classification of 3D structures 3 Contact structure Case 1 Case 2 Dennis I. Barrett (Rhodes Univ.) Invariant NH Riemannian Structures Univ. Ostrava 2015 15 / 26
Contact structure Contact form ω on G We have D = ker ω , where ω : X (G) → C ∞ (G) is a 1-form such that ω ∧ d ω � = 0 specified up to sign by condition: d ω ( Y 1 , Y 2 ) = ± 1 , { Y 1 , Y 2 } o.n. frame for D Reeb vector field Y 0 ∈ X (G): ω ( Y 0 ) = 1 and d ω ( Y 0 , · ) ≡ 0 Two natural cases (1) Y 0 ∈ D ⊥ ∈ D ⊥ (2) Y 0 / Dennis I. Barrett (Rhodes Univ.) Invariant NH Riemannian Structures Univ. Ostrava 2015 16 / 26
A fourth invariant Extension of g | D depending on D , g | D extend g | D to a Riemannian metric ˜ g such that Y 0 ⊥ ˜ g D and g ( Y 0 , Y 0 ) = 1 . ˜ angle θ between Y 0 and D ⊥ is given by cos θ = | ˜ g ( Y 0 , Y 3 ) | D ⊥ = span { Y 3 } 0 ≤ θ < π � , 2 , g ( Y 3 , Y 3 ) ˜ fourth isometric invariant: ϑ := tan 2 θ ≥ 0 Y 0 ∈ D ⊥ ⇐ ⇒ ϑ = 0 Dennis I. Barrett (Rhodes Univ.) Invariant NH Riemannian Structures Univ. Ostrava 2015 17 / 26
Case 1: ϑ = 0 D ⊥ determined by D , g | D reduces to a sub-Riemannian structure: A. Agrachev and D. Barilari, Sub-Riemannian structures on 3D Lie groups , J. Dyn. Control Syst. 18 (2012), 21–44. Invariants { κ, χ 1 } complete set of invariants (at least for unimodular case) can rescale structures so that κ 2 + χ 2 κ = χ 1 = 0 or 1 = 1 Dennis I. Barrett (Rhodes Univ.) Invariant NH Riemannian Structures Univ. Ostrava 2015 18 / 26
Classification for case 1 Dennis I. Barrett (Rhodes Univ.) Invariant NH Riemannian Structures Univ. Ostrava 2015 19 / 26
Case 2: ϑ > 0 Canonical frame ( X 0 , X 1 , X 2 ) P ( Y 0 ) X 2 unique unit vector s.t. X 0 = Q ( Y 0 ) X 1 = d ω ( X 1 , X 2 ) = 1 � P ( Y 0 ) � D = span { X 1 , X 2 } , D ⊥ = span { X 0 } canonical left-invariant frame (up to sign of X 0 , X 1 ) on G Commutator relations (determine structure uniquely) c 1 10 X 1 + c 2 [ X 1 , X 0 ] = 10 X 2 c 1 10 , c 2 10 , c 1 20 , c 1 21 ∈ R , [ X 2 , X 0 ] = − c 1 21 X 0 + c 1 20 X 1 − c 1 10 X 2 c 1 21 > 0 X 0 + c 1 [ X 2 , X 1 ] = 21 X 1 Dennis I. Barrett (Rhodes Univ.) Invariant NH Riemannian Structures Univ. Ostrava 2015 20 / 26
Isometries are isomorphisms Proposition (G , g , D ) isometric to (G ′ , g ′ , D ′ ) φ is a Lie = ⇒ w.r.t. φ : G → G ′ group isomorphism hence isometries preserve Killing form K Three new invariants ̺ 0 , ̺ 1 , ̺ 2 ̺ i = − 1 2 K ( X i , X i ) , i = 0 , 1 , 2 κ , χ 1 expressible i.t.o. ̺ i ’s and ϑ ̺ 0 , ̺ 1 , ̺ 2 simpler than κ , χ 1 , χ 2 and have more info Dennis I. Barrett (Rhodes Univ.) Invariant NH Riemannian Structures Univ. Ostrava 2015 21 / 26
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