In this paper, the finite element method is applied to solve the unsteady elastic equations, C-Bézier basis functions are used to construct the shape function spaces, the semi-discrete scheme of the unsteady elastic equations is obtained by Galerkin finite element method and then the fully discretized Galerkin method is obtained by further discretizing the time variable with θ-scheme finite difference. Furthermore, for several numerical examples, the accuracy of approximate solutions are improved by 1–3 order-of magnitudes compared with the Lagrange basis function in L∞ norm, L2 norm and H1 semi-norm, and the numerical examples show that the method proposed possesses a faster convergence rate. It is fully demonstrated that the C-Bézier basis functions have a better approximation effect in simulating unsteady elastic equations.
Citation: Lanyin Sun, Kunkun Pang. Numerical solution of unsteady elastic equations with C-Bézier basis functions[J]. AIMS Mathematics, 2024, 9(1): 702-722. doi: 10.3934/math.2024036
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In this paper, the finite element method is applied to solve the unsteady elastic equations, C-Bézier basis functions are used to construct the shape function spaces, the semi-discrete scheme of the unsteady elastic equations is obtained by Galerkin finite element method and then the fully discretized Galerkin method is obtained by further discretizing the time variable with θ-scheme finite difference. Furthermore, for several numerical examples, the accuracy of approximate solutions are improved by 1–3 order-of magnitudes compared with the Lagrange basis function in L∞ norm, L2 norm and H1 semi-norm, and the numerical examples show that the method proposed possesses a faster convergence rate. It is fully demonstrated that the C-Bézier basis functions have a better approximation effect in simulating unsteady elastic equations.
In [4], Diniz and Veloso gave the definition of Gaussian curvature for non-horizontal surfaces in sub-Riemannian Heisenberg space H1 and the proof of the Gauss-Bonnet theorem. In [1], intrinsic Gaussian curvature for a Euclidean C2-smooth surface in the Heisenberg group H1 away from characteristic points and intrinsic signed geodesic curvature for Euclidean C2-smooth curves on surfaces are defined by using a Riemannian approximation scheme. These results were then used to prove a Heisenberg version of the Gauss-Bonnet theorem. In [5], Veloso verified that Gaussian curvature of surfaces and normal curvature of curves in surfaces introduced by [4] and by [1] to prove Gauss-Bonnet theorems in Heisenberg space H1 were unequal and he applied the same formalism of [4] to get the curvatures of [1]. With the obtained formulas, the Gauss-Bonnet theorem can be proved as a straightforward application of Stokes theorem in [5].
In [1] and [2], Balogh-Tyson-Vecchi used that the Riemannian approximation scheme may depend upon the choice of the complement to the horizontal distribution in general. In the context of H1 the choice which they have adopted is rather natural. The existence of the limit defining the intrinsic curvature of a surface depends crucially on the cancellation of certain divergent quantities in the limit. Such cancellation stems from the specific choice of the adapted frame bundle on the surface, and on symmetries of the underlying left-invariant group structure on the Heisenberg group. In [1], they proposed an interesting question to understand to what extent similar phenomena hold in other sub-Riemannian geometric structures. In [6], Wang and Wei gave sub-Riemannian limits of Gaussian curvature for a Euclidean C2-smooth surface in the affine group and the group of rigid motions of the Minkowski plane away from characteristic points and signed geodesic curvature for Euclidean C2-smooth curves on surfaces. And they got Gauss-Bonnet theorems in the affine group and the group of rigid motions of the Minkowski plane. In [7], Wang and Wei gave sub-Riemannian limits of Gaussian curvature for a Euclidean C2-smooth surface in the BCV spaces and the twisted Heisenberg group away from characteristic points and signed geodesic curvature for Euclidean C2-smooth curves on surfaces. And they got Gauss-Bonnet theorems in the BCV spaces and the twisted Heisenberg group.
In this paper, we solve this problem for the generalized affine group and the generalized BCV spaces. In the case of the generalized affine group, the cancellation of certain divergent quantities in the limit happens and the limit of the Riemannian Gaussian curvature exists. In the case of the generalized BCV spaces, the result is the same as the generalized affine group. We also get Gauss-Bonnet theorems in the generalized affine group and the generalized BCV spaces.
In Section 2, we compute the sub-Riemannian limit of curvature of curves in the generalized affine group. In Section 3, we compute sub-Riemannian limits of geodesic curvature of curves on surfaces and the Riemannian Gaussian curvature of surfaces in the generalized affine group. In Section 4, we prove the Gauss-Bonnet theorem in the generalized affine group. In Section 5, we compute the sub-Riemannian limit of curvature of curves in the generalized BCV spaces. In Section 6, we compute sub-Riemannian limits of geodesic curvature of curves on surfaces and the Riemannian Gaussian curvature of surfaces in the generalized BCV spaces and get a Gauss-Bonnet theorem in the generalized BCV spaces.
When TM=H⨁H⊥ and gTM=gH⨁gH⊥, we may consider the rescaled metric gL=gH⨁LgH⊥, then we may consider the sub-Riemannian limit of some geometric objects like the Gauss curvature and the mean curvature ⋅⋅⋅, when L goes to the infinity. In this case, we call the (M,gTM) as the manifold with the splitting tangent bundle. In this paper, our main objects: the generalized affine group and the generalized BCV spaces are not sub-Riemannian manifolds (groups) in general. But they are manifolds with the splitting tangent bundle. So we can use the Riemannian approximation scheme to get the Gauss-Bonnet theorems in these spaces.
Firstly we give some notations on the generalized affine group. Let G be the generalized affine group and choose the underlying manifold G={(x1,x2,x3)∈R3∣f(x1,x2,x3)>0}. On G, we let
X1=f∂x1,X2=f∂x2+∂x3,X3=f∂x2. | (2.1) |
where f be a smooth function with respect to x1,x2,x3. Then
∂x1=1fX1,∂x2=1fX3,∂x3=X2−X3, | (2.2) |
and span{X1,X2,X3}=TG. Let H=span{X1,X2} be the horizontal distribution on G. Let ω1=1fdx1,ω2=dx3,ω=1fdx2−dx3. Then H=Kerω. For the constant L>0, let gL=ω1⊗ω1+ω2⊗ω2+Lω⊗ω,g=g1 be the Riemannian metric on G. Then X1,X2,~X3:=L−12X3 are orthonormal basis on TG with respect to gL. We have
[X1,X2]=−(f2+f3f)X1+f1X3,[X1,X3]=−f2X1+f1X3,[X2,X3]=f3fX3. | (2.3) |
where fi=∂f∂xi, for 1≤i≤3.
Let ∇L be the Levi-Civita connection on G with respect to gL. Then we have the following lemma,
Lemma 2.1. Let G be the generalized affine group, then
∇LX1X1=(f2+f3f)X2+f3L,∇LX1X2=−(f2+f3f)X1+f12X3,∇LX2X1=−f12X3,∇LX2X2=0,∇LX1X3=−f2X1−f1L2X2,∇LX3X1=−f1L2X2−f1X3,∇LX2X3=f1L2X1,∇LX3X2=f1L2X1−f3fX3,∇LX3X3=f1LX1+f3LfX2. | (2.4) |
Proof. By the Koszul formula, we have
2⟨∇LXiXj,Xk⟩L=⟨[Xi,Xj],Xk⟩L−⟨[Xj,Xk],Xi⟩L+⟨[Xk,Xi],Xj⟩L, | (2.5) |
where i,j,k=1,2,3. So lemma 2.1 holds.
Definition 2.2. Let γ:[a,b]→(G,gL) be a Euclidean C1-smooth curve. We say that γ is regular if ˙γ≠0 for every t∈[a,b]. Moreover we say that γ(t) is a horizontal point of γ if
ω(˙γ(t))=˙γ2(t)f−˙γ3(t)=0, |
where γ(t)=(γ1(t),γ2(t),γ3(t)) and ˙γi(t)=∂γi(t)∂t.
Definition 2.3. Let γ:[a,b]→(G,gL) be a Euclidean C2-smooth regular curve in the Riemannian manifold (G,gL). The curvature kLγ of γ at γ(t) is defined as
kLγ:=√||∇L˙γ˙γ||2L||˙γ||4L−⟨∇L˙γ˙γ,˙γ⟩2L||˙γ||6L. | (2.6) |
Lemma 2.4. Let γ:[a,b]→(G,gL) be a Euclidean C2-smooth regular curve in the Riemannian manifold (G,gL). Then,
kLγ={{{[¨γ1−f2˙γ1˙γ3f−f′˙γ1+f3˙γ1˙γ3f2]+[f1L˙γ3−f2˙γ1f+f1Lω(˙γ(t))]ω(˙γ(t))}2+{[¨γ3+f2˙γ21f2+f3˙γ21f3]+[f3Lω(˙γ(t))f−f1L˙γ1f]ω(˙γ(t))}2+L{f2˙γ21f2L−[f1˙γ1f+f3˙γ3f]ω(˙γ(t))+ddtω(˙γ(t))}2}⋅[(˙γ1f)2+˙γ23+L(ω(˙γ(t)))2]−2−{[˙γ1¨γ1f2−f′˙γ21f3+˙γ3¨γ3]+Ldω(˙γ(t))dtω(˙γ(t))}2⋅[(˙γ1f)2+˙γ23+L(ω(˙γ(t)))2]−3}12 | (2.7) |
In particular, if γ(t) is a horizontal point of γ,
kLγ={{[¨γ1−f2˙γ1˙γ3f−f′˙γ1+f3˙γ1˙γ3f2]2+[¨γ3+f2˙γ21f2+f3˙γ21f3]2+L[f2˙γ21f2L+ddtω(˙γ(t))]2}⋅[(˙γ1f)2+˙γ23]−2−[˙γ1¨γ1f2−f′˙γ21f3+˙γ3¨γ3]2⋅[(˙γ1f)2+˙γ23]−3}12 | (2.8) |
where f′=˙γ(f)=ddtf(γ(t)).
Proof. By (2.2), we have
˙γ(t)=˙γ1fX1+˙γ3X2+ω(˙γ(t))X3. | (2.9) |
By Lemma 2.1 and (2.9), we have
∇L˙γX1=[ff2˙γ1(t)+f3˙γ1(t)f2−f1Lω(˙γ(t))2]X2+[f2˙γ1(t)fL−f1˙γ3(t)2−f1ω(˙γ(t))]X3,∇L˙γX2=[−ff2˙γ1(t)+f3˙γ1(t)f2+f1Lω(˙γ(t))2]X1+[−f3ω(˙γ(t))f+f1˙γ(t)2f]X3,∇L˙γX3=[−f2˙γ1(t)f+Lf1˙γ3(t)2+f1Lω(˙γ(t))]X1+[−f1L˙γ(t)2f+f3Lω(˙γ(t))f]X2. | (2.10) |
By (2.9) and (2.10), we have
∇L˙γ˙γ={[¨γ1−f2˙γ1˙γ3f−f′˙γ1+f3˙γ1˙γ3f2]+[f1L˙γ3−f2˙γ1f+f1Lω(˙γ(t))]ω(˙γ(t))}X1+{[¨γ3+f2˙γ21f2+f3˙γ21f3]+[f3Lω(˙γ(t))f−f1L˙γ1f]ω(˙γ(t))}X2+{f2˙γ21f2L−(f1˙γ1f+f3˙γ3f)ω(˙γ(t))+dω(˙γ(t))dt}X3 | (2.11) |
By (2.6), (2.9) and (2.11), we get Lemma 2.4.
Definition 2.5. Let γ:[a,b]→(G,gL) be a Euclidean C2-smooth regular curve in the Riemannian manifold (G,gL). We define the intrinsic curvature k∞γ of γ at γ(t) to be
k∞γ:=limL→+∞kLγ, |
if the limit exists.
We introduce the following notation: for continuous functions f1,f2:(0,+∞)→R,
f1(L)∼f2(L),asL→+∞⇔limL→+∞f1(L)f2(L)=1. | (2.12) |
Lemma 2.6. Let γ:[a,b]→(G,gL) be a Euclidean C2-smooth regular curve in the Riemannian manifold (G,gL). Then
k∞γ=√[f1˙γ1−f3˙γ2f+f3˙γ3]2+(f1˙γ2)2|f||ω(˙γ(t))|,ifω(˙γ(t))≠0, | (2.13) |
k∞γ={{[¨γ1−f2˙γ1˙γ3f−f′˙γ1+f3˙γ1˙γ3f2]2+[¨γ3+f2˙γ21f2+f3˙γ21f3]2+(f2˙γ21f2)2}⋅[(˙γ1f)2+˙γ23]−2−[˙γ1¨γ1f2−f′˙γ21f3+˙γ3¨γ3]2⋅[(˙γ1f)2+˙γ23]−3}12ifω(˙γ(t))=0andddt(ω(˙γ(t)))=0, | (2.14) |
limL→+∞kLγ√L=|ddt(ω(˙γ(t)))|(˙γ1f)2+˙γ23,ifω(˙γ(t))=0andddt(ω(˙γ(t)))≠0. | (2.15) |
Proof. Using the notation introduced in (2.12), when ω(˙γ(t))≠0, we have
||∇L˙γ˙γ||2L∼(ω(˙γ(t))f)2{[f1˙γ1−f3˙γ2f+f3˙γ3]2+(f1˙γ2)2}L2,asL→+∞, |
||˙γ||2L∼Lω(˙γ(t))2,asL→+∞, |
⟨∇L˙γ˙γ,˙γ⟩2L∼O(L2)asL→+∞. |
Therefore
||∇L˙γ˙γ||2L||˙γ||4L→{[f1˙γ1−f3˙γ2f+f3˙γ3)]2+(f1˙γ2)2}f2ω(˙γ(t))2,asL→+∞, |
⟨∇L˙γ˙γ,˙γ⟩2L||˙γ||6L→0,asL→+∞. |
So by (2.6), we have (2.13). (2.14) comes from (2.8) and
ddt(ω(˙γ(t)))=0. |
When ω(˙γ(t))=0 and ddt(ω(˙γ(t)))≠0,
we have
||∇L˙γ˙γ||2L∼L[ddt(ω(˙γ(t)))]2,asL→+∞, |
||˙γ||2L=(˙γ1f)2+˙γ23, |
⟨∇L˙γ˙γ,˙γ⟩2L=O(1)asL→+∞. |
By (2.6), we get (2.15).
We will say that a surface Σ⊂(G,gL) is regular if Σ is a Euclidean C2-smooth compact and oriented surface. In particular we will assume that there exists a Euclidean C2-smooth function u:G→R such that
Σ={(x1,x2,x3)∈G:u(x1,x2,x3)=0} |
and ux1∂x1+ux2∂x2+ux3∂x3≠0. Let ∇Hu=X1(u)X1+X2(u)X2. A point x∈Σ is called characteristic if ∇Hu(x)=0. We define the characteristic set C(Σ):={x∈Σ|∇Hu(x)=0}. Our computations will be local and away from characteristic points of Σ. Let us define first
p:=X1u,q:=X2u,andr:=˜X3u. |
We then define
l:=√p2+q2,lL:=√p2+q2+r2,¯p:=pl,¯q:=ql,¯pL:=plL,¯qL:=qlL,¯rL:=rlL. | (3.1) |
In particular, ¯p2+¯q2=1. These functions are well defined at every non-characteristic point. Let
vL=¯pLX1+¯qLX2+¯rL~X3,e1=¯qX1−¯pX2,e2=¯rL¯pX1+¯rL¯qX2−llL~X3, | (3.2) |
then vL is the Riemannian unit normal vector to Σ and e1,e2 are the orthonormal basis of Σ. On TΣ we define a linear transformation JL:TΣ→TΣ such that
JL(e1):=e2;JL(e2):=−e1. | (3.3) |
For every U,V∈TΣ, we define ∇Σ,LUV=π∇LUV where π:TG→TΣ is the projection. Then ∇Σ,L is the Levi-Civita connection on Σ with respect to the metric gL. By (2.11), (3.2) and
∇Σ,L˙γ˙γ=⟨∇L˙γ˙γ,e1⟩Le1+⟨∇L˙γ˙γ,e2⟩Le2, | (3.4) |
we have
∇Σ,L˙γ˙γ={¯q[(¨γ1−f2˙γ1˙γ3f−f′˙γ1+f3˙γ1˙γ3f2)+(f1L˙γ3−f2˙γ1f+f1Lω(˙γ(t)))ω(˙γ(t))]−¯p[(¨γ3+f2˙γ21f2+f3˙γ21f3)−(f1L˙γ1−f3Lω(˙γ(t))f)ω(˙γ(t))]}e1+{¯rL¯p[(¨γ1−f2˙γ1˙γ3f−f′˙γ1+f3˙γ1˙γ3f2)+(f1L˙γ3−f2˙γ1f+f1Lω(˙γ(t)))ω(˙γ(t))]+¯rL¯q[(¨γ3+f2˙γ21f2+f3˙γ21f3)−(f1L˙γ1f−f3Lω(˙γ(t))f)ω(˙γ(t))]−llLL12[f2˙γ21f2L−(f1˙γ1f+f3˙γ3f)ω(˙γ(t))+ddt(ω(˙γ(t)))]}e2. | (3.5) |
Moreover if ω(˙γ(t))=0, then
∇Σ,L˙γ˙γ={¯q[¨γ1−f2˙γ1˙γ3f−f′˙γ1+f3˙γ1˙γ3f2]−¯p[¨γ3+f2˙γ21f2+f3˙γ21f3]}e1+{¯rL¯p[¨γ1−f2˙γ1˙γ3f−f′˙γ1+f3˙γ1˙γ3f2]+¯rL¯q[¨γ3+f2˙γ21f2+f3˙γ21f3]−llLL12[f2˙γ21f2L−ddt(ω(˙γ(t)))]}e2. | (3.6) |
Definition 3.1. Let Σ⊂(G,gL) be a regular surface. Let γ:[a,b]→Σ be a Euclidean C2-smooth regular curve. The geodesic curvature kLγ,Σ of γ at γ(t) is defined as
kLγ,Σ:=√||∇Σ,L˙γ˙γ||2Σ,L||˙γ||4Σ,L−⟨∇Σ,L˙γ˙γ,˙γ⟩2Σ,L||˙γ||6Σ,L. | (3.7) |
Definition 3.2. Let Σ⊂(G,gL) be a regular surface. Let γ:[a,b]→Σ be a Euclidean C2-smooth regular curve. We define the intrinsic geodesic curvature k∞γ,Σ of γ at γ(t) to be
k∞γ,Σ:=limL→+∞kLγ,Σ, |
if the limit exists.
Lemma 3.3. Let Σ⊂(G,gL) be a regular surface. Let γ:[a,b]→Σ be a Euclidean C2-smooth regular curve. Then
k∞γ,Σ=|¯p(f1˙γ1−f3˙γ2f+f3˙γ3)+¯qf1˙γ2||f||ω(˙γ(t))|,ifω(˙γ(t))≠0, | (3.8) |
k∞γ,Σ=0,ifω(˙γ(t))=0,andddt(ω(˙γ(t)))=0, |
limL→+∞kLγ,Σ√L=|ddt(ω(˙γ(t)))|(¯q˙γ1f−¯p˙γ3)2,ifω(˙γ(t))=0andddt(ω(˙γ(t)))≠0. | (3.9) |
Proof. we know ˙γ(t)=˙γ1(t)∂x1+˙γ2(t)∂x2+˙γ3(t)∂x3, then by (2.2), ˙γ(t)=˙γ1(t)γ1(t)X1+γ3(t)X2+ω(˙γ(t))X3.
Let
˙γ(t)=λ1e1+λ2e2. |
Then
{˙γ1(t)γ1(t)=λ1¯q+λ2¯rL¯p˙γ3(t)=−λ1¯p+λ2¯rL¯qω(˙γ(t))=−λ2llLL−12 | (3.10) |
We have
{λ1=¯q˙γ1(t)γ1(t)−¯p˙γ3(t)λ2=−λ2lLlL12ω(˙γ(t)) | (3.11) |
Thus ˙γ∈TΣ, we have
˙γ=(¯q˙γ1f−¯p˙γ3)e1−lLlL12ω(˙γ(t))e2. | (3.12) |
By (3.6), we have
||∇Σ,L˙γ˙γ||2L,Σ={¯q[(¨γ1−f2˙γ1˙γ3f−f′˙γ1+f3˙γ1˙γ3f2)+(f1L˙γ3−f2˙γ1f+f1Lω(˙γ(t)))ω(˙γ(t))]−¯p[(¨γ3+f2˙γ21f2+f3˙γ21f3)−(f1L˙γ1−f3Lω(˙γ(t))f)ω(˙γ(t))]}2+{¯rL¯p[(¨γ1−f2˙γ1˙γ3f−f′˙γ1+f3˙γ1˙γ3f2)+(f1L˙γ3−f2˙γ1f+f1Lω(˙γ(t)))ω(˙γ(t))]+¯rL¯q[(¨γ3+f2˙γ21f2+f3˙γ21f3)−(f1L˙γ1f−f3Lω(˙γ(t))f)ω(˙γ(t))]−llLL12[f2˙γ21f2L−(f1˙γ1f+f3˙γ3f)ω(˙γ(t))+ddt(ω(˙γ(t)))]}2∼L2[¯p(f1˙γ1−f3˙γ2f+f3˙γ3)+¯qf1˙γ2]2ω(˙γ(t))2f2,asL→+∞. | (3.13) |
Similarly, we have that when ω(˙γ(t))≠0,
||˙γ||Σ,L=√(¯q˙γ1f−¯p˙γ3)2+(lLl)2Lω(˙γ(t))2∼L12|ω(˙γ(t))|,asL→+∞. | (3.14) |
By (3.6) and (3.12), we have
⟨∇Σ,L˙γ˙γ,˙γ⟩Σ,L=(¯q˙γ1f−¯p˙γ3)⋅{¯q[(¨γ1−f2˙γ1˙γ3f−f′˙γ1+f3˙γ1˙γ3f2)+(f1L˙γ3−f2˙γ1f+f1Lω(˙γ(t)))ω(˙γ(t))]−¯p[(¨γ3+f2˙γ21f2+f3˙γ21f3)−(f1L˙γ1−f3Lω(˙γ(t))f)ω(˙γ(t))]}−lLlL12ω(˙γ(t))⋅{¯rL¯p[(¨γ1−f2˙γ1˙γ3f−f′˙γ1+f3˙γ1˙γ3f2)+(f1L˙γ3−f2˙γ1f+f1Lω(˙γ(t)))ω(˙γ(t))]+¯rL¯q[(¨γ3+f2˙γ21f2+f3˙γ21f3)−(f1L˙γ1f−f3Lω(˙γ(t))f)ω(˙γ(t))]−llLL12[f2˙γ21f2L−(f1˙γ1f+f3˙γ3f)ω(˙γ(t))+ddt(ω(˙γ(t)))]}∼M0L, | (3.15) |
where M0 does not depend on L. By (3.7), (3.13)–(3.15), we get (3.8). When ω(˙γ(t))=0 and ddt(ω(˙γ(t)))=0,
we have
||∇Σ,L˙γ˙γ||2L,Σ=[¯q(¨γ1−f2˙γ1˙γ3f−f′˙γ1+f3˙γ1˙γ3f2)−¯p(¨γ3+f2˙γ21f2+f3˙γ21f3)]2+[¯rL¯p(¨γ1−f2˙γ1˙γ3f−f′˙γ1+f3˙γ1˙γ3f2)+¯rL¯q(¨γ3+f2˙γ21f2+f3˙γ21f3)−llLL12f2˙γ21f2L]2∼[¯q(¨γ1−f2˙γ1˙γ3f−f′˙γ1+f3˙γ1˙γ3f2)−¯p(¨γ3+f2˙γ21f2+f3˙γ21f3)]2 | (3.16) |
and
||˙γ||Σ,L=|¯q˙γ1f−¯p˙γ3|, | (3.17) |
⟨∇Σ,L˙γ˙γ,˙γ⟩Σ,L=(¯q˙γ1f−¯p˙γ3)⋅[¯q(¨γ1−f2˙γ1˙γ3f−f′˙γ1+f3˙γ1˙γ3f2)−¯p(¨γ3+f2˙γ21f2+f3˙γ21f3)] | (3.18) |
By (3.16)–(3.18) and (3.7), we get k∞γ,Σ=0. When ω(˙γ(t))=0 and ddt(ω(˙γ(t)))≠0,
we have
||∇Σ,L˙γ˙γ||2L,Σ∼L[ddt(ω(˙γ(t)))]2, |
⟨∇Σ,L˙γ˙γ,˙γ⟩Σ,L=O(1), |
so we get (3.9).
Definition 3.4. Let Σ⊂(G,gL) be a regular surface. Let γ:[a,b]→Σ be a Euclidean C2-smooth regular curve. The signed geodesic curvature kL,sγ,Σ of γ at γ(t) is defined as
kL,sγ,Σ:=⟨∇Σ,L˙γ˙γ,JL(˙γ)⟩Σ,L||˙γ||3Σ,L, | (3.19) |
where JL is defined by (3.3).
Definition 3.5. Let Σ⊂(G,gL) be a regular surface. Let γ:[a,b]→Σ be a Euclidean C2-smooth regular curve. We define the intrinsic geodesic curvature k∞γ,Σ of γ at the non-characteristic point γ(t) to be
k∞,sγ,Σ:=limL→+∞kL,sγ,Σ, |
if the limit exists.
Lemma 3.6. Let Σ⊂(G,gL) be a regular surface. Let γ:[a,b]→Σ be a Euclidean C2-smooth regular curve. Then
k∞,sγ,Σ=¯p(f1˙γ1−f3˙γ2f+f3˙γ3)+¯qf1˙γ2|fω(˙γ(t))|,ifω(˙γ(t))≠0, | (3.20) |
k∞,sγ,Σ=0,ifω(˙γ(t))=0,andddt(ω(˙γ(t)))=0, |
limL→+∞kL,sγ,Σ√L=(−¯q˙γ1f+¯p˙γ3)ddt(ω(˙γ(t)))|¯q˙γ1f−¯p˙γ3|3,ifω(˙γ(t))=0andddt(ω(˙γ(t)))≠0. | (3.21) |
Proof. By (3.3) and (3.12), we have
JL(˙γ)=lLlL12ω(˙γ(t))e1+(¯q˙γ1f−¯p˙γ3)e2. | (3.22) |
By (3.5) and (3.22), we have
⟨∇Σ,L˙γ˙γ,JL(˙γ)⟩L,Σ=lLlL12ω(˙γ(t)){¯q[(¨γ1−f2˙γ1˙γ3f−f′˙γ1+f3˙γ1˙γ3f2)+(f1L˙γ3−f2˙γ1f+f1Lω(˙γ(t)))ω(˙γ(t))]−¯p[(¨γ3+f2˙γ21f2+f3˙γ21f3)−(f1L˙γ1−f3Lω(˙γ(t))f)ω(˙γ(t))]}+(¯q˙γ1f−¯p˙γ3)⋅{¯rL¯p[(¨γ1−f2˙γ1˙γ3f−f′˙γ1+f3˙γ1˙γ3f2)+(f1L˙γ3−f2˙γ1f+f1Lω(˙γ(t)))ω(˙γ(t))]+¯rL¯q[(¨γ3+f2˙γ21f2+f3˙γ21f3)−(f1L˙γ1f−f3Lω(˙γ(t))f)ω(˙γ(t))]−llLL12[f2˙γ21f2L−(f1˙γ1f+f3˙γ3f)ω(˙γ(t))+ddt(ω(˙γ(t)))]},∼L32ω(˙γ(t))2¯p(f1˙γ1−f3˙γ2f+f3˙γ3)+¯qf1˙γ2fasL→+∞. | (3.23) |
So we get (3.20). When ω(˙γ(t))=0 and ddt(ω(˙γ(t)))=0, we get
⟨∇Σ,L˙γ˙γ,JL(˙γ)⟩L,Σ=(¯q˙γ1f−¯p˙γ3)⋅[¯rL¯p(¨γ1−f2˙γ1˙γ3f−f′˙γ1+f3˙γ1˙γ3f2)+¯rL¯q(¨γ3+f2˙γ21f2+f3˙γ21f3)−llLL−12f2˙γ21f2]∼M0L−12asL→+∞. | (3.24) |
So k∞,sγ,Σ=0. When ω(˙γ(t))=0 and ddt(ω(˙γ(t)))≠0,
we have
⟨∇Σ,L˙γ˙γ,JL(˙γ)⟩L,Σ∼L12(−¯q˙γ1f+¯p˙γ3)ddt(ω(˙γ(t)))asL→+∞. | (3.25) |
So we get (3.21).
In the following, we compute the sub-Riemannian limit of the Riemannian Gaussian curvature of surfaces in the generalized affine group. We define the second fundamental form IIL of the embedding of Σ into (G,gL):
IIL=(⟨∇Le1vL,e1⟩L,⟨∇Le1vL,e2⟩L⟨∇Le2vL,e1⟩L,⟨∇Le2vL,e2⟩L). | (3.26) |
Similarly to Theorem 4.3 in [3], we have
Theorem 3.7. The second fundamental form IIL of the embedding of Σ into (G,gL) is given by
IIL=(h11,h12h21,h22). | (3.27) |
where
h11=llL[X1(¯p)+X2(¯q)]−¯qL(f2+f3f)−¯q2f2¯rLL−12, |
h12=h21=−lLl⟨e1,∇H(¯rL)⟩L−f1√L2−¯p¯qf2L−12, |
h22=−l2l2L⟨e2,∇H(rl)⟩L+~X3(¯rL)−f1¯pL−¯qLf3f−¯rL¯P2f2L−12. |
Proof. By ei⟨VL,ej⟩L−⟨∇LeiVL,ej⟩L−⟨∇Leiej,VL⟩L=0 and ei⟨VL,ej⟩L=0, we have ⟨∇LeiVL,ej⟩L=−⟨∇Leiej,VL⟩L, i,j=1,2.
By lemma 2.1 and (3.2),
∇Le1e1=∇L(¯qX1−¯pX2)(¯qX1−¯pX2)=[¯qX1(¯q)−¯pX2(¯p)+¯p¯q(f2+f3f)]X1−[¯qX1(¯p)−¯pX2(¯p)−¯q2(f2+f3f)]X2+¯q2f3L−12~X3. | (3.28) |
Then
h11=−⟨∇Le1e1,VL⟩L=−¯pL[¯qX1(¯q)−¯pX2(¯p)]+¯qL[¯qX1(¯p)−¯pX2(¯p)]−¯qL(F2+F3¯fF)−¯pL(F1+F3fF)+¯rLL−12F3F=llL[X1(¯p)+X2(¯q)]−¯qL(F2+F3¯fF)−¯pL(F1+F3fF)+¯rLL−12F3F. | (3.29) |
Similarly,
∇Le1e2=∇L(¯qX1−¯pX2)(¯rL¯pX1+¯rL¯qX2−llL~X3)=[¯qX1(¯rL¯p)−¯pX2(¯rL¯p)+f1¯pL√L2−¯rL¯q2(f2+f3f)+¯qLf2L−12]X1+[¯qX1(¯q)−¯pX2(¯q)+f1¯qL√L2+¯rL¯pq(f2+f3f)]X2+[¯pX2(llL)−¯qX1(llL)+f1¯rL√L2+¯rL¯pqf3L−12]~X3. | (3.30) |
Then
h12=−⟨∇Le1e2,VL⟩L=−llL[¯qX1(¯rL)−¯pX2(¯rL)]+¯rL[¯qX1(llL)−¯pX2(llL)]−12L12(fF2−F1¯f+¯f′F−Ff′)=−lLl⟨e1,∇H(¯rL)⟩L−12L12(fF2−F1¯f+¯f′F−Ff′). | (3.31) |
Since
⟨∇Le2VL,e1⟩L=−⟨∇Le2e1,VL⟩L=−⟨∇Le1e2+[e2,e1],VL⟩L=−⟨∇Le1e2,VL⟩=⟨∇Le1VL,e2⟩L. | (3.32) |
Then,
h21=h12=−lLl⟨e1,∇H(¯rL)⟩L−12L12(fF2−F1¯f+¯f′F−Ff′). | (3.33) |
Since
∇Le2e2=∇L(¯rL¯pX1+¯rL¯qX2−llL~X3)(¯rL¯pX1+¯rL¯qX2−llL~X3)=[¯qX1(¯rL¯p)−¯pX2(¯rL¯p)+f1¯qL√L−¯rL2¯pq(f2+f3f)+¯rL¯pLf2L−12+(llL)2f1]X1+[¯qX1(¯q)−¯pX2(¯q)+f1¯pL√L+¯rL2¯pq(f2+f3f)+(llL)2f3fX2+[¯pX2(llL)−¯qX1(llL)+¯rL2¯pqf3fL−12+¯rL¯pLf1+¯rL¯qLf3f]~X3. | (3.34) |
Then,
h22=−⟨∇Le2e2,VL⟩L=−¯prlX1(¯rL)−¯qrlX2(¯rL)+~X3(¯rL)−¯pLfF3F−¯qL¯fF3F−¯rLF3FL−12=−l2l2L⟨e2,∇H(rl)⟩L+~X3(¯rL)−¯pLfF3F−¯qL¯fF3F−¯rLF3FL−12. | (3.35) |
The Riemannian mean curvature HL of Σ is defined by
HL:=tr(IIL). |
Let
KΣ,L(e1,e2)=−⟨RΣ,L(e1,e2)e1,e2⟩Σ,L,KL(e1,e2)=−⟨RL(e1,e2)e1,e2⟩L. | (3.36) |
By the Gauss equation, we have
KΣ,L(e1,e2)=KL(e1,e2)+det(IIL). | (3.37) |
Proposition 3.8. Away from characteristic points, the horizontal mean curvature H∞ of Σ⊂G is given by
H∞=limL→+∞HL=X1(¯p)+X2(¯q)−f1¯p−¯qf2−2¯qf3f. | (3.38) |
Proof. By
l2l2L⟨e2,∇H(rl)⟩L=¯prlX1(¯rL)+¯qrlX2(¯rL)=O(L−1) |
llL[X1(¯p)+X2(¯q)]→X1(¯p)+X2(¯q),~X3(¯rL)→0, |
¯q2f2¯rLL−12→O(L−1),¯qL→¯q, |
¯rL¯P2f2L−12→O(L−1),¯pL→¯p, |
we get (3.38).
Define the curvature of a connection ∇ by
R(X,Y)Z=∇X∇YZ−∇Y∇XZ−∇[X,Y]Z. | (3.39) |
Then by Lemma 2.1 and (3.39), we have the following lemma,
Lemma 3.9. Let G be the affine group, then
RL(X1,X2)X1=[3Lf214+(f2+f3f)2]X2+[X1(−f12)−X2(f2L)+f21+f22L+f2f3Lf]X3,RL(X1,X2)X2=[−3Lf214+X2(f2+f3f)−(f2+f3f)2]X1+f1f3fX3,RL(X1,X2)X3=[X1(f1L2)+X2(f2)−f21L−f2(f2+f3f)]X1+[X2(f1Lf)−f1f3Lf]X2,RL(X1,X3)X1=−[X1(f1L2)+X3(f2+f3f)+f2f3f−f21L−f2(f2+f3f)]X2+[−f21L4+f21+f22L−X3(f2L)−X1(f1)+f3f(f2+f3f)]X3,RL(X1,X3)X2=[X1(f1L2)−f21L−f2(f2+f3f)+X3(f2+f3f)+f2f3f]X1+[−X1(f3f)+f1f2+f1f3f−f1(f2+f3f)−X3(f12)]X3,RL(X1,X3)X3=[X1(f1L)−f3Lf(f2+f3f)+X3(f2)+f21L24−f21L−f22]X1+[f1L(f2+f3f)+X1(f3Lf)−f1f2L−f1f3Lf+X3(f1L2)]X2,RL(X2,X3)X1=[−X2(f1L2)+f1f3Lf]X2+[X3(f12)−X2(f1)+f1f3f]X3,RL(X2,X3)X2=[X2(f1L2)−f1f3Lf]X1+[f23f2−f21L4−X2(f3f)]X3,RL(X2,X3)X3=[X2(f1L)−X3(f1L2)−f1f3Lf]X1+[X2(f3Lf)−f23Lf2+f21L24]X2. | (3.40) |
Proposition 3.10. Away from characteristic points, we have
KΣ,L(e1,e2)→A0+O(L−12),asL→+∞, | (3.41) |
where
A0:=−f1⟨e1,∇H(X3u|∇Hu|)⟩−f1f2¯p¯q−¯q2f21−34(X3u)2l2f21+¯p2X2(f3f)−¯p[X1(¯p)+X2(¯q)−¯q(f2+f3f)](f1¯p+¯qf3f)+2¯qX3ulf21+2¯qX3uX1(−f12)−2¯pX3ulf1f3f+¯q2X1(f1)−¯q2(f2+f3f)−2¯q¯pX1(f3f)−2¯q¯pX3(f12)−¯p2f23f2. | (3.42) |
Proof. By (3.2), we have
⟨RL(e1,e2)e1,e2⟩L=¯rL2⟨RL(X1,X2)X1,X2⟩L−2llL¯qL−12¯rL⟨RL(X1,X2)X1,X3⟩L+2llL¯pL−12¯rL⟨RL(X1,X2)X2,X3⟩L+(llL¯q)2L−1⟨RL(X1,X3)X1,X3⟩L−2(llL)2¯p¯qL−1⟨RL(X1,X3)X2,X3⟩L+(¯pllL)2L−1⟨RL(X2,X3)X2,X3⟩L. | (3.43) |
By Lemma 3.9, we have
KL(e1,e2)=14l2l2Lf21L−34Lf21¯rL2+2llL¯qL12¯rLf21−¯q2l2l2Lf21−¯rL2(f2+f3f)2+2llL¯qL12¯rLX1(−f12)−2llL¯qL12¯rLX2(−f2L)+2llL¯qL−12¯rLf22+2llL¯qL−12¯rLf2f3f−2¯pL12¯rLf1f3f−¯q2l2l2Lf22L+¯q2l2l2LX3(f2L)+¯q2l2l2LX1(f1)−¯q2l2l2L(f2+f3f)−2¯qpl2l2LX1(f3f)+2¯qpl2l2Lf1f2+2¯qpl2l2Lf1f3f−2¯qpl2l2Lf1(f2+f3f)−2¯qpl2l2LX3(f12)−¯p2l2l2Lf23f2+¯p2l2l2LX2(f3f). | (3.44) |
By (3.35) and
∇H(¯rL)=L−12∇H(X3u|∇Hu|)+O(L−1)asL→+∞ |
we get
det(IIL)=h11h22−h12h21=−f21L4−f1⟨e1,∇H(X3u|∇Hu|)⟩−f1f2¯p¯q−¯p[X1(¯p)+X2(¯q)−¯q(f2+f3f)](f1¯p+¯qf3f)+O(L−12). | (3.45) |
By (3.38), (3.44), (3.45) we get (3.41).
Let us first consider the case of a regular curve γ:[a,b]→(G,gL). We define the Riemannian length measure
dsL=||˙γ||Ldt. |
Lemma 4.1. Let γ:[a,b]→(G,gL) be a Euclidean C2-smooth and regular curve. Let
ds:=|ω(˙γ(t))|dt,d¯s:=121|ω(˙γ(t))|(˙γ21f2+˙γ23)dt. | (4.1) |
Then
limL→+∞1√L∫γdsL=∫bads. | (4.2) |
When ω(˙γ(t))≠0, we have
1√LdsL=ds+d¯sL−1+O(L−2)asL→+∞. | (4.3) |
When ω(˙γ(t))=0, we have
1√LdsL=1√L√˙γ21f2+˙γ23dt. | (4.4) |
Proof. We know that
||˙γ(t)||L=√(˙γ1f)2+˙γ23+Lω(˙γ(t))2, |
similar to the proof of Lemma 6.1 in [1], we can prove (4.2). When ω(˙γ(t))≠0, we have
1√LdsL=√L−1((˙γ1f)2+˙γ23)+ω(˙γ(t))2dt. |
Using the Taylor expansion, we can prove (4.3). From the definition of dsL and ω(˙γ(t))=0, we get (4.4).
Let Σ⊂(G,gL) be a Euclidean C2-smooth surface and Σ={u=0}. Let dσΣ,L denote the surface measure on Σ with respect to the Riemannian metric gL. Then similai to Proposition 4.2 in [7], we have
limL→+∞1√L∫ΣdσΣ,L=dσΣ:=(¯pω2−¯qω1)∧ω. | (4.5) |
Similar to the proof of Theorem 1.1 in $ [1] $, we have
Theorem 4.2. Let Σ⊂(G,gL) be a regular surface with finitely many boundary components (∂Σ)i, i∈{1,⋯,n}, given by Euclidean C2-smooth regular and closed curves γi:[0,2π]→(∂Σ)i. Let A0 be defined by (3.42) and dσΣ,d¯σΣ be defined by (4.5) and d¯s be defined by (4.1) and k∞,sγi,Σ be the sub-Riemannian signed geodesic curvature of γi relative to Σ. Suppose that the characteristic set C(Σ) satisfies H1(C(Σ))=0 where H1(C(Σ)) denotes the Euclidean 1-dimensional Hausdorff measure of C(Σ) and that ||∇Hu||−1H is locally summable with respect to the Euclidean 2-dimensional Hausdorff measure near the characteristic set C(Σ), then
∫ΣKΣ,∞dσΣ+n∑i=1∫γik∞,sγi,Σds=0. | (4.6) |
Example 4.3. Let f=x21+1, then G=R3. Let u=x21+x22+x23−1 and ∑=S2. ∑ is a regular surface. By (2.1), we get
X1(u)=2(x21+1)x1;X2(u)=2(x21+1)x2+2x3. | (4.7) |
Solve the equations X1(u)=X2(u)=0,
then we get
C(Σ)={(0,√22,−√22),(0,−√22,√22)} |
and H1(C(Σ))=0.
A parametrization of Σ is
x1=cos(ϕ)cos(θ),x2=cos(ϕ)sin(θ),x3=sin(ϕ),forϕ∈(−π2,π2),θ∈[0,2π). | (4.8) |
Then
‖∇Hu‖2H=X1(u)2+X2(u)2=4(x21+1)2x21+4(x21+1)2x22+4x23+8(x21+1)x2x3=4(cos(ϕ)2cos(θ)2+1)2cos(ϕ)2+4sin(ϕ)2+8(cos(ϕ)2cos(θ)2+1)sin(ϕ)cos(ϕ)sin(θ). | (4.9) |
By the definitions of wj for 1≤j≤3 and (4.5), we have
dσΣ=1‖∇Hu‖H[(X1(u))dx3−(x21+1)−1(X2(u))dx1]∧((x21+1)−1dx2−dx3)=−1‖∇Hu‖H2cos(ϕ)λ0dθ∧dϕ. | (4.10) |
where
λ0=cos(ϕ)2+2(cos(ϕ)2cos(θ)2+1)−1cos(ϕ)sin(ϕ)sin(θ)+(cos(ϕ)2cos(θ)2+1)−2sin(ϕ)2 |
is a bouned smooth function on Σ. By (4.9) and (4.10), we have ‖∇Hu‖−1H is locally summable around the isolated characteristic points with respect to the measure dσΣ.
We consider some notation on the generalized BCV spaces. Let f(x2), ¯f(x1), F(x1,x2,x3) be smooth functions. The generalized BCV spaces M is the set
{(x1,x2,x3)∈R3∣F(x1,x2,x3)>0} |
Let
X1=F∂x1+f∂x3,X2=F∂x2+¯f∂x3,X3=∂x3. | (5.1) |
Then
∂x1=1F(X1−fX3),∂x2=1F(X2−¯fX3),∂x3=X3, | (5.2) |
and span{X1,X2,X3}=TM. Let H=span{X1,X2} be the horizontal distribution on M. Let ω1=1Fdx1,ω2=1Fdx2,ω=dx3−(fdx1+¯fdx2)F. Then H=Kerω. The generalized BCV spaces have some well-knowed special case. When F=1+λ4(x21+x22),f=−τx2,¯f=τx1, we get the BCV spaces. When F=1,f=f(x2),¯f=¯f(x1), we can the Heisenberg manifolds. When F=1,f=12x22,¯f=0, we get the Martinet distribution. When F=1x1,f=0,¯f=−2, we get the Welyczko's example (see [5]). For the constant L>0, let gL=ω1⊗ω1+ω2⊗ω2+Lω⊗ω,g=g1 be the Riemannian metric on M. Then X1,X2,~X3:=L−12X3 are orthonormal basis on TM with respect to gL. We have
[X1,X2]=−(F2+¯fF3F)X1+(F1+fF3F)X2+(F2f−F1¯f+F¯f′−Ff′)X3,[X2,X3]=−F3FX2+¯fF3FX3,[X1,X3]=−F3FX1+fF3FX3. | (5.3) |
where Fi=∂F∂xi, for 1≤i≤3, f′=∂f∂x2, ¯f′=∂¯f∂x1. Let ∇L be the Levi-Civita connection on M with respect to gL. Then we have the following lemma
Lemma 5.1. Let M be the generalized BCV spaces, then
∇LX1X1=(F2+F3¯fF)X2+F3LFX3,∇LX1X2=−(F2+F3¯fF)X1+12(fF2−F1¯f+F¯f′−Ff′)X3,∇LX1X3=−F3FX1−L2(fF2−F1¯f+F¯f′−Ff′)X2,∇LX2X1=−(F1+F3fF)X2−12(fF2−F1¯f+F¯f′−Ff′)X3,∇LX2X2=(F1+F3fF)X1+F3FLX3,∇LX2X3=L2(fF2−F1¯f+F¯f′−Ff′)X1−F3FX2,∇LX3X1=−L2(fF2−F1¯f+F¯f′−Ff′)X2−fF3FX3,∇LX3X2=L2(fF2−F1¯f+F¯f′−Ff′)X1−¯fF3FX3,∇LX3X3=LfF3FX1+L¯fF3FX2. | (5.4) |
Proof. By the Koszul formula, we have
2⟨∇LXiXj,Xk⟩L=⟨[Xi,Xj],Xk⟩L−⟨[Xj,Xk],Xi⟩L+⟨[Xk,Xi],Xj⟩L, | (5.5) |
where i,j,k=1,2,3. So lemma 5.1 holds.
Definition 5.2. Let γ:[a,b]→(M,gL) be a Euclidean C1-smooth curve. We say that γ(t) is a horizontal point of γ if
ω(˙γ(t))=−fF˙γ1(t)−¯fF˙γ2(t)+˙γ3(t)=0. |
where γ(t)=(γ1(t),γ2(t),γ3(t)) and ˙γi(t)=∂γi(t)∂t.
Similar to the definition 2.3 and definition 2.5, we can define kLγ and k∞γ for the generalized BCV spaces, we have
Lemma 5.3. Let γ:[a,b]→(M,gL) be a Euclidean C2-smooth regular curve in the Riemannian manifold (M,gL). Then
k∞γ={[−˙γ1F(F2f−F1¯f+F¯f′−Ff′)+F3¯fFω(˙γ(t))]2+[˙γ2F(F2f−F1¯f+F¯f′−Ff′)+F3fFω(˙γ(t))]2}12|ω(˙γ(t))|−1,ifω(˙γ(t))≠0. | (5.6) |
k∞γ={{[F′˙γ1−F¨γ1F2−˙γ1˙γ2F2(F2+F3¯fF)+˙γ22F2(F1+F3fF)]2+[F′˙γ2−F¨γ2F2+˙γ21F2(F2+F3¯fF)−˙γ1˙γ2F2(F1+F3fF)]2}[(˙γ21+˙γ22)F2]−2+{[F′˙γ21−F˙γ1¨γ1F3+F′˙γ22−F˙γ2¨γ2F3]2}[(˙γ21+˙γ22)F2]−3}−12ifω(˙γ(t))=0andddt(ω(˙γ(t)))=0, | (5.7) |
where F′=˙γ(F)=ddtF(γ(t)).
limL→+∞kLγ√L=|ddt(ω(˙γ(t)))|˙γ21F2+˙γ22F2,ifω(˙γ(t))=0andddt(ω(˙γ(t)))≠0. | (5.8) |
Proof. By (5.2), we have
˙γ(t)=˙γ1FX1+˙γ2FX2+ω(˙γ(t))X3. | (5.9) |
By Lemma 5.1 and (5.8), we have
∇L˙γX1=[˙γ1F(F2+F3¯fF)−˙γ2F(F1+F3fF)−L2(F2f−F1¯f+F¯f′−Ff′)ω(˙γ(t))]X2+[F3˙γ1LF2−˙γ22F(F2f−F1¯f+F¯f′−Ff′)−F3fF(ω(˙γ(t))]X3,∇L˙γX2=[−˙γ1F+(F2+F3¯fF)+˙γ2F(F1+F3fF)+L2(F2f−F1¯f+F¯f′−Ff′)ω(˙γ(t))]X1+[F3˙γ2LF2+˙γ12F(F2f−F1¯f+F¯f′−Ff′)−F3¯fF(ω(˙γ(t))]X3,∇L˙γX3=[−˙γ1F3F2+˙γ2L2F(F2f−F1¯f+F¯f′−Ff′)+LF3fFω(˙γ(t))]X1+[−˙γ1L2F(F2f−F1¯f+F¯f′−Ff′)−˙γ2F3F2+LF3¯fFω(˙γ(t))]X2. | (5.10) |
By (5.8) and (5.9), we have
∇L˙γ˙γ={F′˙γ1−F¨γ1F2−˙γ1˙γ2F2(F2+F3¯fF)+˙γ22F2(F1+F3fF)+[−F3˙γ1F2+˙γ2LF(F2f−F1¯f+F¯f′−Ff′)+fLF3Fω(˙γ(t))]ω(˙γ(t))}X1+{F′˙γ2−F¨γ2F2+˙γ21F2(F2+F3¯fF)−˙γ1˙γ2F2(F2+F3fF)+[−F3˙γ2F2−˙γ1LF(F2f−F1¯f+F¯f′−Ff′)+¯fLF3Fω(˙γ(t))]ω(˙γ(t))}X2+{(˙γ22+˙γ21)F3F3L−(F3f˙γ1+F3¯f˙γ2)F2ω(˙γ(t))+ddtω(˙γ(t))}X3. | (5.11) |
By (5.8) and (5.10), when ω(˙γ(t))≠0, we have
||∇L˙γ˙γ||2L∼{[˙γ2F(F2f−F1¯f+F¯f′−Ff′)+F3fFω(˙γ(t))]2+[−˙γ1F(F2f−F1¯f+F¯f′−Ff′)+F3¯fFω(˙γ(t))]2}ω(˙γ(t))2L2,asL→+∞,||˙γ||2L∼Lω(˙γ(t))2,asL→+∞,⟨∇L˙γ˙γ,˙γ⟩2L∼O(L2)asL→+∞. | (5.12) |
Therefore
||∇L˙γ˙γ||2L∼[˙γ2F(F2f−F1¯f+F¯f′−Ff′)+F3fFω(˙γ(t))]2ω(˙γ(t))2+[−˙γ1F(F2f−F1¯f+F¯f′−Ff′)+F3¯fFω(˙γ(t))]2ω(˙γ(t))2,asL→+∞, | (5.13) |
⟨∇L˙γ˙γ,˙γ⟩2L||˙γ||6L→0,asL→+∞. |
So by (2.6), we have (5.5). (5.6) comes from (5.8), (5.10), (2.6) and ω(˙γ(t))=0 and ddt(ω(˙γ(t)))=0. When ω(˙γ(t))=0 and ddt(ω(˙γ(t)))≠0, we have
||∇L˙γ˙γ||2L∼L[ddt(ω(˙γ(t)))]2,asL→+∞, |
||˙γ||2L=[˙γ21F2+˙γ22F2]2, |
⟨∇L˙γ˙γ,˙γ⟩2L=O(1)asL→+∞. |
By (2.6), we get (5.7).
We will consider a regular surface Σ1⊂(M,gL) and regular curve γ⊂Σ1. We will assume that there exists a Euclidean C2-smooth function u:M→R such that
Σ1={(x1,x2,x3)∈M:u(x1,x2,x3)=0}. |
Similar to Section 3, we define p,q,r,l,lL,¯p,¯q,¯pL,¯qL,¯rL,vL,e1,e2,JL,kLγ,Σ1,k∞γ,Σ1,kL,sγ,Σ1,k∞,sγ,Σ1. By (3.4) and (5.10), we have
∇Σ1,L˙γ˙γ={¯q[F′˙γ1−F¨γ1F2−˙γ1˙γ2F2(F2+F3¯fF)+˙γ22F2(F1+F3fF)]+¯q[−F3˙γ1F2+˙γ2LF(F2f−F1¯f+F¯f′−Ff′)+fLF3Fω(˙γ(t))]ω(˙γ(t))−¯p[F′˙γ2−F¨γ2F2+˙γ21F2(F2+F3¯fF)−˙γ1˙γ2F2(F2+F3fF)]+¯p[−F3˙γ2F2−˙γ1LF(F2f−F1¯f+F¯f′−Ff′)+¯fLF3Fω(˙γ(t))]ω(˙γ(t))}e1+{¯rL¯p[F′˙γ1−F¨γ1F2−˙γ1˙γ2F2(F2+F3¯fF)+˙γ22F2(F1+F3fF)]+¯rL¯p[−F3˙γ1F2+˙γ2LF(F2f−F1¯f+F¯f′−Ff′)+fLF3Fω(˙γ(t))]ω(˙γ(t))+¯rL¯q[F′˙γ2−F¨γ2F2+˙γ21F2(F2+F3¯fF)−˙γ1˙γ2F2(F2+F3fF)]+¯rL¯q[−F3˙γ2F2−˙γ1LF(F2f−F1¯f+F¯f′−Ff′)+¯fLF3Fω(˙γ(t))]ω(˙γ(t))−llLL12[(˙γ22+˙γ21)F3F3L−(f˙γ1+¯f˙γ2)F3F2ω(˙γ(t))+ddt(ω(˙γ(t)))]}e2=B1e1+B2e2. | (6.1) |
By (5.8) and ˙γ(t)∈TΣ1, we have
˙γ(t)=[¯q˙γ1F−¯p˙γ2F]e1−lLlL12ω(˙γ(t))e2. | (6.2) |
We have
Lemma 6.1. Let Σ1⊂(M,gL) be a regular surface. Let γ:[a,b]→Σ1 be a Euclidean C2-smooth regular curve. Then
k∞γ,Σ1={[q˙γ2(F2f−F1¯f+F¯f′−Ff′)+F3f¯qω(˙γ(t))]+[¯P˙γ1(F2f−F1¯f+F¯f′−Ff′)−F3f¯Pω(˙γ(t))]}|Fω(˙γ(t))|−1,ifω(˙γ(t))≠0, | (6.3) |
k∞γ,Σ1=0,ifω(˙γ(t))=0,andddt(ω(˙γ(t)))=0, |
limL→+∞kLγ,Σ1√L=|ddt(ω(˙γ(t)))|[¯q˙γ1F−¯P˙γ2F]2,ifω(˙γ(t))=0andddt(ω(˙γ(t)))≠0. | (6.4) |
Proof. By (6.1), we have
||∇Σ1,L˙γ˙γ||2L,Σ1=B21+B22∼L2ω(˙γ(t))2{[¯q˙γ2F(F2f−F1¯f+F¯f′−Ff′)+F3f¯qFω(˙γ(t))]+[¯P˙γ1F(F2f−F1¯f+F¯f′−Ff′)−F3¯f¯PFω(˙γ(t))]}2,asL→+∞. | (6.5) |
By (6.2), we have that when ω(˙γ(t))≠0,
||˙γ||Σ1,L∼L12|ω(˙γ(t))|,asL→+∞. | (6.6) |
By (6.1) and (6.2), we have
⟨∇Σ1,L˙γ˙γ,˙γ⟩Σ1,L∼M0L, | (6.7) |
where M0 does not depend on L.
By (3.7), (6.5)–(6.7), we get (6.3). When ω(˙γ(t))=0 and ddt(ω(˙γ(t)))=0, we have
||∇Σ1,L˙γ˙γ||2L,Σ1∼C0:={¯q[F′˙γ1−F¨γ1F2−˙γ1˙γ2F2(F2+F3¯fF)+˙γ22F2(F1+F3fF)]−¯p[F′˙γ2−F¨γ2F2+˙γ21F2(F2+F3¯fF)−˙γ1˙γ2F2(F2+F3fF)]}2,asL→+∞. | (6.8) |
and
||˙γ||2Σ1,L=[¯q˙γ1F−¯P˙γ2F]2, | (6.9) |
⟨∇Σ1,L˙γ˙γ,˙γ⟩Σ1,L=[¯q˙γ1F−¯P˙γ2F]C0. | (6.10) |
By (6.8)–(6.10) and (3.7), we get k∞γ,Σ1=0. When ω(˙γ(t))=0 and ddt(ω(˙γ(t)))≠0, we have
||∇Σ1,L˙γ˙γ||2L,Σ1∼L[ddt(ω(˙γ(t)))]2, |
⟨∇Σ1,L˙γ˙γ,˙γ⟩Σ1,L=O(1), |
so we get (6.4).
Lemma 6.2. Let Σ1⊂(M,gL) be a regular surface. Let γ:[a,b]→Σ1 be a Euclidean C2-smooth regular curve. Then
k∞,sγ,Σ1={[q˙γ2(F2f−F1¯f+F¯f′−Ff′)+F3f¯qω(˙γ(t))]+[¯P˙γ1(F2f−F1¯f+F¯f′−Ff′)−F3f¯Pω(˙γ(t))]}|Fω(˙γ(t))|−1,ifω(˙γ(t))≠0, | (6.11) |
k∞,sγ,Σ1=0,ifω(˙γ(t))=0,andddt(ω(˙γ(t)))=0, |
limL→+∞kL,sγ,Σ1√L=|ddt(ω(˙γ(t)))|[¯q˙γ1F−¯P˙γ2F]2,ifω(˙γ(t))=0andddt(ω(˙γ(t)))≠0. | (6.12) |
Proof. By (3.3) and (6.2), we have
JL(˙γ)=lLlL12ω(˙γ(t))e1+[¯q˙γ1F−¯p˙γ2F]e2. | (6.13) |
By (6.1) and (6.13), we have
⟨∇Σ1,L˙γ˙γ,JL(˙γ)⟩L,Σ1=lLLL12ω(˙γ(t)){¯q[F′˙γ1−F¨γ1F2−˙γ1˙γ2F2(F2+F3¯fF)+˙γ22F2(F1+F3fF)]+¯q[−F3˙γ1F2+˙γ2LF(F2f−F1¯f+F¯f′−Ff′)+fLF3Fω(˙γ(t))]ω(˙γ(t))−¯p[F′˙γ2−F¨γ2F2+˙γ21F2(F2+F3¯fF)−˙γ1˙γ2F2(F2+F3fF)]+¯p[−F3˙γ2F2−˙γ1LF(F2f−F1¯f+F¯f′−Ff′)+¯fLF3Fω(˙γ(t))]ω(˙γ(t))}+[¯q˙γ1F−¯p˙γ2F]{¯rL¯p[F′˙γ1−F¨γ1F2−˙γ1˙γ2F2(F2+F3¯fF)+˙γ22F2(F1+F3fF)]+¯rL¯p[−F3˙γ1F2+˙γ2LF(F2f−F1¯f+F¯f′−Ff′)+fLF3Fω(˙γ(t))]ω(˙γ(t))+¯rL¯q[F′˙γ2−F¨γ2F2+˙γ21F2(F2+F3¯fF)−˙γ1˙γ2F2(F2+F3fF)]+¯rL¯q[−F3˙γ2F2−˙γ1LF(F2f−F1¯f+F¯f′−Ff′)+¯fLF3Fω(˙γ(t))]ω(˙γ(t))−llLL12[(˙γ22+˙γ21)F3F3L−(f˙γ1+¯f˙γ2)F3F2ω(˙γ(t))+ddt(ω(˙γ(t)))]} | (6.14) |
So by (3.17), (6.6) and (6.14), we get (6.11). When ω(˙γ(t))=0 and ddt(ω(˙γ(t)))=0, we get
⟨∇Σ1,L˙γ˙γ,JL(˙γ)⟩L,Σ1∼M0L−12asL→+∞. | (6.15) |
So k∞,sγ,Σ1=0. When ω(˙γ(t))=0 and ddt(ω(˙γ(t)))≠0, we have
⟨∇Σ1,L˙γ˙γ,JL(˙γ)⟩L,Σ1∼L12[¯P˙γ2F−¯q˙γ1F]ddt(ω(˙γ(t))),asL→+∞. | (6.16) |
So we get (6.12).
In the following, we compute the sub-Riemannian limit of the Riemannian Gaussian curvature of surfaces in the generalized BCV spaces. Similarly to Theorem 4.3 in [3], we have
Theorem 6.3. The second fundamental form IIL1 of the embedding of Σ1 into (M,gL) is given by
IIL1=(h11,h12h21,h22), | (6.17) |
where
h11=llL[X1(¯p)+X2(¯q)]−¯qL(F2+F3¯fF)−¯pL(F1+F3fF)+¯rLL−12F3F, |
h12=h21=−lLl⟨e1,∇H(¯rL)⟩L−12L12(fF2−F1¯f+¯f′F−Ff′), |
h22=−l2l2L⟨e2,∇H(rl)⟩L+~X3(¯rL)−¯pLfF3F−¯qL¯fF3F−¯rLF3FL−12. |
Proof. By lemma 5.1 and (3.2),
\begin{align} \nabla^L_{e_1}e_1& = \nabla^L_{(\overline{q}X_1-\overline{p}X_2)}(\overline{q}X_1-\overline{p}X_2)\\ & = [\overline{q}X_1(\overline{q})-\overline{p}X_2(\overline{p})+\overline{p}\overline{q}(F_2+\frac{F_3\overline{f}}{F})+\overline{p}^2(F_1+\frac{F_3f}{F})]X_1\\ &-[\overline{q}X_1(\overline{p})-\overline{p}X_2(\overline{p})-\overline{q}^2(F_2+\frac{F_3\overline{f}}{F})-\overline{p}\overline{q}(F_2+\frac{F_3f}{F})]X_2+\frac{F_3}{F}L^{-\frac{1}{2}}\widetilde{X_3}.\\ \end{align} | (6.18) |
Then
\begin{align} h_{11}& = -\langle\nabla^L_{e_1}e_1, V_L\rangle_L\\ & = -\overline{p_L}[\overline{q}X_1(\overline{q})-\overline{p}X_2(\overline{p})]+\overline{q_L}[\overline{q}X_1(\overline{p})-\overline{p}X_2(\overline{p})]\\ &-\overline{q_L}\left(F_2+\frac{F_3\overline{f}}{F}\right)-\overline{p_L}\left(F_1+\frac{F_3f}{F}\right)+\overline{r_L}L^{-\frac{1}{2}}\frac{F_3}{F}\\ & = \frac{l}{l_L}[X_1(\overline{p})+X_2(\overline{q})]-\overline{q_L}\left(F_2+\frac{F_3\overline{f}}{F}\right)-\overline{p_L}\left(F_1+\frac{F_3f}{F}\right)+\overline{r_L}L^{-\frac{1}{2}}\frac{F_3}{F}.\\ \end{align} | (6.19) |
Similarly,
\begin{align} \nabla^L_{e_1}e_2& = \nabla^L_{(\overline{q}X_1-\overline{p}X_2)}(\overline{r_L}\overline{p}X_1+\overline{r_L}\overline{q}X_2-\frac{l}{l_L}\widetilde{X_3})\\ & = [\overline{q}X_1(\overline{r_L}\overline{p})-\overline{p}X_2(\overline{r_L}\overline{p})+\frac{(fF_2-F_1\overline{f}+F\overline{f'}-Ff')\overline{p_L}\sqrt{L}}{2}\\ &-\overline{r_L}\overline{pq}(F_1+\frac{F_3f}{F})-\overline{r_L}\overline{q}^2(F_2+\frac{F_3\overline{f}}{F})+\overline{q_L}\frac{F_3}{F}L^{-\frac{1}{2}}]X_1\\ &+[\overline{q}X_1(\overline{q})-\overline{p}X_2(\overline{q})+\frac{(fF_2-F_1\overline{f}+F\overline{f'}-Ff')\overline{q_L}\sqrt{L}}{2}\\ &-\overline{r_L}\overline{pq}(F_2+\frac{F_3\overline{f}}{F})+\overline{r_L}\overline{p}^2(F_1+\frac{F_3f}{F})+\overline{p_L}\frac{F_3}{F}L^{-\frac{1}{2}}]X_2\\ &+[\overline{p}X_2(\frac{l}{l_L})-\overline{q}X_1(\frac{l}{l_L})+\frac{(fF_2-F_1\overline{f}+F\overline{f'}-Ff')\overline{r_L}\sqrt{L}}{2}]\widetilde{X_3}.\\ \end{align} | (6.20) |
Then
\begin{align} h_{12}& = -\langle\nabla^L_{e_1}e_2, V_L\rangle_L\\ & = -\frac{l}{l_L}[\overline{q}X_1(\overline{r_L})-\overline{p}X_2(\overline{r_L})]+\overline{r_L}[\overline{q}X_1(\frac{l}{l_L})-\overline{p}X_2(\frac{l}{l_L})]-\frac{1}{2}L^\frac{1}{2}(fF_2-F_1\overline{f}+\overline{f}'F-Ff')\\ & = -\frac{l_L}{l}\langle e_1, \nabla_H(\overline{r_L})\rangle_L-\frac{1}{2}L^\frac{1}{2}(fF_2-F_1\overline{f}+\overline{f}'F-Ff').\\ \end{align} | (6.21) |
Since
\begin{align} \langle\nabla^L_{e_2}V_L, e_1\rangle_L& = -\langle\nabla^L_{e_2}e_1, V_L\rangle_L = -\langle\nabla^L_{e_1}e_2+[e_2, e_1], V_L\rangle_L\\ & = -\langle\nabla^L_{e_1}e_2, V_L\rangle = \langle\nabla^L_{e_1}V_L, e_2\rangle_L.\\ \end{align} | (6.22) |
Then,
\begin{align} h_{21} = h_{12} = -\frac{l_L}{l}\langle e_1, \nabla_H(\overline{r_L})\rangle_L-\frac{1}{2}L^\frac{1}{2}(fF_2-F_1\overline{f}+\overline{f}'F-Ff').\\ \end{align} | (6.23) |
Since
\nabla^L_{e_2}e_2 = \nabla^L_{(\overline{r_L}\overline{p}X_1+\overline{r_L}\overline{q}X_2-\frac{l}{l_L}\widetilde{X_3})}(\overline{r_L}\overline{p}X_1+\overline{r_L}\overline{q}X_2-\frac{l}{l_L}\widetilde{X_3})\\ = [\overline{q}X_1(\overline{r_L}\overline{p})-\overline{p}X_2(\overline{r_L}\overline{p})+(fF_2-F_1\overline{f}+\overline{f}'F-Ff')\overline{q_L}\sqrt{L}-\overline{r_L}^2\overline{pq}(F_2+\frac{F_3\overline{f}}{F})\\ +\overline{r_L}\overline{p_L}\frac{F_3}{F}L^{-\frac{1}{2}}+\overline{r_L}^2\overline{q}^2(F_1+\frac{F_3f}{F})+(\frac{l}{l_L})^2\frac{F_3f}{F}]X_1\\ +[\overline{q}X_1(\overline{q})-\overline{p}X_2(\overline{q})+(fF_2-F_1\overline{f}+\overline{f}'F-Ff')\overline{p_L}\sqrt{L}+\overline{r_L}^2\overline{pq}(F_2+\frac{F_3\overline{f}}{F})\\ -\overline{r_L}\overline{q_L}\frac{F_3}{F}L^{-\frac{1}{2}}-\overline{r_L}^2\overline{pq}(F_1+\frac{F_3f}{F})+(\frac{l}{l_L})^2\frac{F_3\overline{f}}{F}X_2\\ +[\overline{p}X_2(\frac{l}{l_L})-\overline{q}X_1(\frac{l}{l_L})+\overline{r_L}^2\overline{pq}\frac{F_3f}{F}L^{-\frac{1}{2}}+\overline{r_L}^2\overline{q}^2\frac{F_3}{F}L^{-\frac{1}{2}}+\overline{r_L}\overline{p_L}\frac{fF_3}{F}+\overline{r_L}\overline{q_L}\frac{\overline{f}F_3}{F}]\widetilde{X_3}. | (6.24) |
Then,
\begin{align} h_{22}& = -\langle\nabla^L_{e_2}e_2, V_L\rangle_L\\ & = -\overline{p}\frac{r}{l}X_1(\overline{r_L})-\overline{q}\frac{r}{l}X_2(\overline{r_L})+\widetilde{X_3}(\overline{r_L})-\overline{p_L}\frac{fF_3}{F}-\overline{q_L}\frac{\overline{f}F_3}{F}-\overline{r_L}\frac{F_3}{F}L^{-\frac{1}{2}}\\ & = -\frac{l^2}{l_L^2}\langle e_2, \nabla_H(\frac{r}{l})\rangle_L+\widetilde{X_3}(\overline{r_L})-\overline{p_L}\frac{fF_3}{F}-\overline{q_L}\frac{\overline{f}F_3}{F}-\overline{r_L}\frac{F_3}{F}L^{-\frac{1}{2}} .\\ \end{align} | (6.25) |
Similar to Proposition 3.8, we have
Proposition 6.4. Away from characteristic points, the horizontal mean curvature \mathcal{H}_{\infty}^1 of \Sigma_1\subset M is given by
\begin{align} \mathcal{H}_{\infty}^1& = -\left(\overline{p}\frac{fF_3}{F}+\overline{q}\frac{\overline{f}F_3}{F}\right)+X_1(\overline{p})+X_2(\overline{q})-\overline{q}\left(F_2+\frac{F_3\overline{f}}{F}\right)-\overline{p}\left(F_1+\frac{F_3f}{F}\right). \end{align} | (6.26) |
By Lemma 5.1, we have
Lemma 6.5. Let M be the the generalized BCV spaces, then
\begin{align} &R^L(X_1, X_2)X_1 = \left[-X_1(A)-X_2(B)+\frac{3L}{4}C^2+\frac{F^3}{LF^2}+A^2+B^2\right]X_2\\ &+\left[-\frac{1}{2}X_1(C)-X_2\left(\frac{F_3}{LF}\right)+\frac{F_3fC}{F}\right]X_3, \\ &R^L(X_1, X_2)X_2 = \left[X_1(A)+X_2(B)-\frac{3L}{4}C^2-\frac{F_3^2}{LF^2}-A^2-B^2\right]X_1\\ &+\left[-\frac{1}{2}X_2(C)+X_1\left(\frac{F_3}{LF}\right)+\frac{F_3\overline{f}C}{F}\right]X_3, \\ &R^L(X_1, X_2)X_3 = \left[\frac{1}{2}X_2(LC)+X_2\left(\frac{F_3}{F}\right)-\frac{F_3fCL}{F}\right]X_1\\ &+\left[\frac{1}{2}X_2(LC)-X_1\left(\frac{F_3}{F}\right)-\frac{F_3\overline{f}CL}{F}\right]X_2, \\ &R^L(X_1, X_3)X_1 = \left[-X_1(\frac{LC}{2})-X_3(B)+\frac{BF_3}{F}-\frac{F_3^2f^2}{F^2}\right]X_2\\ &+\left[-\frac{LC^2}{4}-X_1(\frac{F_3f}{F})-X_3\left(\frac{F_3}{LF}\right)+\frac{F_3\overline{f}B}{F}+\frac{F_3^2}{LF^2}+\frac{F^3f^2}{F^2}\right]X_3, \\ &R^L(X_1, X_3)X_2 = \left[X_1(\frac{LC}{2})+X_3(B)-\frac{CF_3fL}{F}+\frac{F^3\overline{f}^2}{F^2}+\frac{BF_3}{F}\right]X_1\\ &+\left[-X_1(\frac{F_3\overline{f}}{F})-\frac{1}{2}X_3(C)+\frac{F_3C}{F}-\frac{fF_3B}{F}+\frac{F^3f\overline{f}}{F^2}\right]X_3, \\ &R^L(X_1, X_3)X_3 = \left[X_1\left(\frac{LF_3f}{F}\right)+X_3\left(\frac{F_3}{F}\right)-\frac{F_3\overline{f}L}{F}B+\frac{L^2C^2}{4}-\frac{F_3^2}{F^2}-\frac{LF_3^2f^2}{F^2}\right]X_1\\ &+\left[X_1\left(\frac{LF_3\overline{f}}{F}\right)-\frac{LCF_3}{F}+X_3\left(\frac{LC}{2}\right)+\frac{F_3fL}{F}B-\frac{LF_3^2f\overline{f}}{F^2}\right]X_2, \\ &R^L(X_2, X_3)X_1 = \left[-X_2\left(\frac{LC}{2}\right)+X_3(A)+\frac{F_3\overline{f}LC}{F}+\frac{fF_3^2}{F^2}-\frac{AF_3}{F}\right]X_2\\ &+\left[-X_2\left(\frac{fF_3}{F}\right)-\frac{CF_3}{F}+X_3\left(\frac{C}{2}\right)-\frac{F_3\overline{f}A}{F}+\frac{F_3^2f\overline{f}}{F^2}\right]X_3, \\ &R^L(X_2, X_3)X_2 = \left[X_2\left(\frac{LC}{2}\right)-X_3(A)-\frac{F_3\overline{f}LC}{F}-\frac{fF_3^2}{F^2}+\frac{AF_3}{F}\right]X_1\\ &+\left[-X_2\left(\frac{\overline{f}F_3}{F}\right)+\frac{AfF_3}{F}-X_3\left(\frac{F_3}{FL}\right)-\frac{LC^2}{4}+\frac{F_3^2}{F^2L}+\frac{F_3^2\overline{f}^2}{F^2}\right]X_3, \\ &R^L(X_2, X_3)X_3 = \left[X_2\left(\frac{LF_3f}{2}\right)-X_3(\frac{LC}{2})+\frac{AF_3\overline{f}L}{F}-\frac{Lf\overline{f}F_3^2}{F^2}+\frac{LCF_3}{F}\right]X_2\\ &+\left[X_2\left(\frac{LF_3\overline{f}}{2}\right)+X_3(\frac{F_3}{F})-\frac{AF_3fL}{F}+\frac{L^2C^2}{4}+\frac{LF_3^2\overline{f}^2}{F^2}-\frac{F_3^2}{F^2}\right]X_3. \end{align} | (6.27) |
where
\left(F_1+\frac{F_3f}{F}\right) = A, \left(F_2+\frac{F_3\overline{f}}{F}\right) = B, (F_2f-F_1\overline{f}+F\overline{f}'-Ff') = C. |
Proposition 6.6. Away from characteristic points, we have
\begin{equation} \mathcal{K}^{\Sigma_1, \infty}(e_1, e_2) = -C\langle e_1, \nabla_H(\frac{X_3u}{|\nabla_Hu|})\rangle+\overline{N}+O(L^{-\frac{1}{2}}). \end{equation} | (6.28) |
where \overline{N} = N_0+N ,
N = -\left(\overline{p}\frac{fF_3}{F}+\overline{q}\frac{\overline{f}F_3}{F}\right)\left[X_1(\overline{p})+X_2(\overline{q})-\overline{q}\left(F_2+\frac{F_3\overline{f}}{F}\right)-\overline{p}\left(F_1+\frac{F_3f}{F}\right)\right], |
\begin{align*} N_0& = 2\overline{q}\left[-\frac{1}{2}X_1(C)+\frac{F_3Cf}{F}\right]-2\overline{p}\left[X_1(-\frac{1}{2}X_2(C)+\frac{F_3C\overline{f}}{F}\right]+\overline{p}^2\left[X_2(\frac{F_3\overline{f}}{F})-\frac{F_3Af}{F}-\frac{F_3^2\overline{f}^2}{F^2}\right]\\ &-2\overline{p}\overline{q}\left[X_1(\frac{F_3\overline{f}}{F})-\frac{1}{2}X_3(C)+\frac{F_3Bf}{F}-\frac{F_3C}{F}\right]+\overline{q}^2\left[X_1(\frac{F_3f}{F})-\frac{F_3B\overline{f}}{F}-\frac{F_3^2\overline{f}^2}{F^2}\right]. \end{align*} |
Proof. By (3.43) and Lemma 6.5, we have
\begin{align} \mathcal{K}^{M, L}(e_1, e_2) = &\overline{r_L}^2\left[X_1(A)+X_2(B)-\frac{3LC^2}{4}-\frac{F_3^2}{F^2L}-A^2-B^2\right]\\ &+2\frac{l}{l_L}\overline{q}\overline{r_L}L^\frac{1}{2}\left[-\frac{1}{2}X_1(C)-X_2(\frac{F_3}{FL})+\frac{F_3Cf}{F}\right]\\ &-2\frac{l}{l_L}\overline{p}\overline{r_L}L^\frac{1}{2}\left[X_1(\frac{F_3}{FL})-\frac{1}{2}X_2(C)+\frac{F_3C\overline{f}}{F}\right]\\ &-2\frac{l^2}{l_L^2}\overline{p}\overline{q}\left[X_1(\frac{F_3\overline{f}}{F})-\frac{1}{2}X_3(C)+\frac{F_3Bf}{F}-\frac{F_3C}{F}\right]\\ &+\frac{l^2}{l_L^2}\overline{q}^2\left[\frac{LC^2}{4}+X_1(\frac{F_3f}{F})-\frac{F_3B\overline{f}}{F}+X_3(\frac{F_3}{FL})-\frac{F_3^2}{F^2L}-\frac{F_3^2\overline{f}^2}{F^2}\right]\\ &+\frac{l^2}{l_L^2}\overline{p}^2\left[\frac{LC^2}{4}+X_2(\frac{F_3\overline{f}}{F})-\frac{F_3Af}{F}+X_3(\frac{F_3}{FL})-\frac{F_3^2}{F^2L}-\frac{F_3^2\overline{f}^2}{F^2}\right]\\ &\sim \frac{LC^2}{4}+N_0, \; \; \; \; {\rm as} \; \; \; \; L\rightarrow +\infty. \end{align} | (6.29) |
Similar to (3.45), we have
\begin{align} {\rm det}(II^L_1)& = h_{11}h_{22}-h_{12}h_{21}\\ & = -\frac{LC^2}{4}-C\langle e_1, \nabla_H(\frac{X_3u}{|\nabla_Hu|})\rangle+N +O(L^{-\frac{1}{2}})\; \; {\rm as}\; \; L\rightarrow +\infty. \end{align} | (6.30) |
By (6.21) and (6.22), we have (6.20).
Similar to (4.2) and (4.5), for the generalized BCV spaces, we have
\begin{equation} {\rm lim}_{L\rightarrow +\infty}\frac{1}{\sqrt{L}}ds_L = ds, \; \; \; \; {\rm lim}_{L\rightarrow +\infty}\frac{1}{\sqrt{L}}d\sigma_{\Sigma_1, L} = d\sigma_{\Sigma_1}. \end{equation} | (6.31) |
By (6.20), (6.23) and Lemma 6.2, similar to the proof of Theorem 1 in [1], we have
Theorem 6.7. Let \Sigma_1\subset (M, g_L) be a regular surface with finitely many boundary components (\partial\Sigma_1)_i, i\in\{1, \cdots, n\} , given by Euclidean C^2 -smooth regular and closed curves \gamma_i:[0, 2\pi]\rightarrow (\partial\Sigma_1)_i . Suppose that the characteristic set C(\Sigma_1) satisfies \mathcal{H}^1(C(\Sigma_1)) = 0 and that ||\nabla_Hu||_H^{-1} is locally summable with respect to the Euclidean 2 -dimensional Hausdorff measure near the characteristic set C(\Sigma_1) , then
\begin{equation} \int_{\Sigma_1}\mathcal{K}^{\Sigma_1, \infty}d\sigma_{\Sigma_1}+\sum\limits_{i = 1}^n\int_{\gamma_i}k^{\infty, s}_{\gamma_i, \Sigma_1}d{s} = 0. \end{equation} | (6.32) |
Example 6.8. Let F = 1, f = -x_2^2, \overline{f} = x_1^2. Consider M = \{(x_1, x_2, x_3)\in \mathbb{R}^3\mid F > 0\} = \mathbb{R}^3 , let u = x_1^2+x_2^2+x_3^2-1 and \sum_1 = S^2 . \sum_1 is a regular surface. By (4.1) , we get
\begin{equation} X_1(u) = 2x_1-2x_2^2x_3;\; \; \; X_2(u) = 2x_2+2x_1^2x_3. \end{equation} | (6.33) |
Solve the equations X_1(u) = X_2(u) = 0 , then we get C(\Sigma) = \{(0, 0, 1), (0, 0, -1)\} and \mathcal{H}^1(C(\Sigma_1)) = 0 . A parametrization of \Sigma is
\begin{align} &x_1 = cos(\phi)cos(\theta), \; \; x_2 = cos(\phi)sin(\theta), \\ &x_3 = sin(\phi), \; \; for \; \; \phi\in (-\frac{\pi}{2}, \; \; \frac{\pi}{2}), \; \; \theta \in[0, 2\pi). \end{align} | (6.34) |
Then
\begin{align} \|\nabla_Hu\|_H^2& = X_1(u)^2+X_2(u)^2 = 4(x_1^2+x_2^2)+4(x_1^4+x_2^4)x_3^2\\ & = 4cos(\phi)^2+4sin(\phi)^2cos(\phi)^4(cos(\theta)^4+sin(\theta)^4). \end{align} | (6.35) |
By the definitions of w_j for 1\leq j\leq3 and (6.23) , we have
\begin{align} d\sigma_{\Sigma_1}& = \frac{1}{\|\nabla_Hu\|_H}[(X_1(u))dx_2-(X_2(u))dx_1]\wedge(dx_3+x_2^2dx_1-x_1^2dx_2)\\ & = \frac{1}{\|\nabla_Hu\|_H}[2cos(\phi)^3+2sin(\phi)^2cos(\phi)^5(cos(\theta)^4+sin(\theta)^4)\\ &-4cos(\phi)^4sin(\theta)^2sin(\phi)cos(\theta)+4cos(\phi)^4sin(\theta)sin(\phi)cos(\theta)^2]d\theta\wedge d\phi. \end{align} | (6.36) |
By (6.27) and (6.28) , we have \|\nabla_Hu\|_H^{-1} is locally summable around the isolated characteristic points with respect to the measure d\sigma_{\Sigma_1}.
Firstly, We give some basic definitions of two kinds of spaces, such as 2.3, 2.4 and 2.5. By computation, we get sub-Riemannian limits of Gaussian curvature for a Euclidean C^2 -smooth surface in the generalized affine group and the generalized BCV spaces away from characteristic points and signed geodesic curvature for Euclidean C^2 -smooth curves on surfaces, respectively. Then, by the second fundamental form II^L and the Gauss equation \mathcal{K}^{\Sigma, L}(e_1, e_2) = \mathcal{K}^{L}(e_1, e_2)+{\rm det}(II^L) , we find the gauss curvature on the surface is convergent in two cases. Therefore, a good result is obtained. Finally, we give the proof of Gauss-Bonnet theorems in the generalized affine group and the generalized BCV spaces.
The second author was supported in part by NSFC No.11771070. The authors are deeply grateful to the referees for their valuable comments and helpful suggestions.
The authors declare no conflict of interest.
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