ScalingStacks

5. Gromov-Hausdorff convergence [031J]

Original official author HTML, exact retained edition. Historical TeX conversion verdicts remain unchanged. Cited-edition alignment and mathematical self-containment are not assessed.

Complete original source context · Original author HTML

5. Gromov-Hausdorff convergence

In this section we study the collapsed Gromov-Hausdorff limits of the Ricci–flat metrics ω~t\tilde{\omega}_{t} and prove Theorem 1.2.

Lemma 5.1.

There is an open subset X0⊂XX_{0}\subset X such that (X0,dX)(X_{0},d_{X}) is locally isometric to (N0,ω)(N_{0},\omega) where N0=N\f⁡(S)N_{0}=N\backslash f(S), i.e. there is a homeomorphism ϕ:N0⟶X0\phi:N_{0}\longrightarrow X_{0} such that, for any y∈N0y\in N_{0}, there is a neighborhood By⊂N0B_{y}\subset N_{0} of yy satisfying that, if y1y_{1} and y2∈Byy_{2}\in B_{y},

dω​(y1,y2)=dX​(ϕ⁡(y1),ϕ⁡(y2)).d_{\omega}(y_{1},y_{2})=d_{X}(\phi(y_{1}),\phi(y_{2})).

Furthermore, for any y∈N0y\in N_{0}, there is a compact neighborhood B⊂N0B\subset N_{0} and a holomorphic section s:B→f−1​(B)s:B\rightarrow f^{-1}(B), i.e., f∘s=idf\circ s={\rm id}, such that s⁡(y)→ϕ⁡(y)s(y)\rightarrow\phi(y) under the Gromov-Hausdorff convergence of (M,ω~tk)(M,\tilde{\omega}_{t_{k}}) to (X,dX)(X,d_{X}).

Proof.

Let AA be a countable dense subset of N0N_{0}, and K⊂N0K\subset N_{0} be a compact subset with the interior int⁡K\operatorname{int}K non-empty. Let {Bi}\{B_{i}\} be a finite covering of KK with small Euclidean balls such that each the concentric balls Bi′B_{i}^{\prime} of half radius still cover KK. Let si:Bi→f−1​(Bi)s_{i}:B_{i}\rightarrow f^{-1}(B_{i}) be sections on BiB_{i}, i.e., holomorphic maps with f∘si=idf\circ s_{i}={\rm id}.

Now, we define a map ϕ\phi from A∩K={a1,a2,⋯}A\cap K=\{a_{1},a_{2},\cdots\} to XX. Suppose that the point a1a_{1} lies inside the ball Bi′B_{i}^{\prime}, and consider the points si​(a1)s_{i}(a_{1}) inside MM. Under the Gromov-Hausdorff convergence of (M,ω~tk)(M,\tilde{\omega}_{t_{k}}) to (X,dX)(X,d_{X}), a subsequence of these points converges to a point b1b_{1} in XX, because the diameter of (M,ω~tk)(M,\tilde{\omega}_{t_{k}}) is uniformly bounded. If a1a_{1} also lies inside another ball Bj′B_{j}^{\prime}, then (1.3) (or also [38, (2.10)]) shows that dω~tk​(si​(a1),sj​(a1))→0d_{\tilde{\omega}_{t_{k}}}(s_{i}(a_{1}),s_{j}(a_{1}))\rightarrow 0 when tk→0t_{k}\rightarrow 0. Thus, by passing to subsequences, both si​(a1)s_{i}(a_{1}) and sj​(a1)s_{j}(a_{1}) converge to the same point b1∈Xb_{1}\in X under the Gromov-Hausdorff convergence of (M,ω~tk)(M,\tilde{\omega}_{t_{k}}) to (X,dX)(X,d_{X}). We then define ϕ⁡(a1)=b1\phi(a_{1})=b_{1}. For a2a_{2}, by repeating the above procedure, we obtain that a subsequence sij​(aj)s_{i_{j}}(a_{j}), j=1,2j=1,2, converges to bj∈Xb_{j}\in X, j=1,2j=1,2, respectively. Define ϕ⁡(a2)=b2\phi(a_{2})=b_{2}. By repeating this procedure and with a diagonal argument, we can find a subsequence of (M,ω~tk)(M,\tilde{\omega}_{t_{k}}), denoted by (M,ω~tk)(M,\tilde{\omega}_{t_{k}}) also, such that sij​(aj)s_{i_{j}}(a_{j}) converges to bj∈Xb_{j}\in X along the Gromov-Hausdorff convergence. For any aj∈A∩Ka_{j}\in A\cap K, define ϕ⁡(aj)=bj\phi(a_{j})=b_{j}.

Now, we prove that ϕ:A∩int⁡K→X\phi:A\cap\operatorname{int}K\rightarrow X is injective. If it is not true, there are y1y_{1}, y2∈A∩int⁡Ky_{2}\in A\cap\operatorname{int}K such that y1≠y2y_{1}\neq y_{2}, and ϕ⁡(y1)=ϕ⁡(y2)\phi(y_{1})=\phi(y_{2}), which implies dω~tk​(si1​(y1),si2​(y2))→0d_{\tilde{\omega}_{t_{k}}}(s_{i_{1}}(y_{1}),s_{i_{2}}(y_{2}))\rightarrow 0. If γk\gamma_{k} is a minimal geodesic in (M,ω~tk)(M,\tilde{\omega}_{t_{k}}) connecting si1​(y1)s_{i_{1}}(y_{1}) and si2​(y2)s_{i_{2}}(y_{2}), then

C−1​lengthωN​(f⁡(γk)∩K)⩽lengthω~tk​(γk∩f−1​(K))⩽dω~tk​(si1​(y1),si2​(y2)),C^{-1}{\rm length}_{\omega_{N}}(f(\gamma_{k})\cap K)\leqslant{\rm length}_{\tilde{\omega}_{t_{k}}}(\gamma_{k}\cap f^{-1}(K))\leqslant d_{\tilde{\omega}_{t_{k}}}(s_{i_{1}}(y_{1}),s_{i_{2}}(y_{2})),

by (4.1) for a constant C>0C>0 independent of kk. Thus, if f⁡(γk)⊂Kf(\gamma_{k})\subset K for tk≪1t_{k}\ll 1,

dωN​(y1,y2)⩽C​lengthωN​(f⁡(γk))⟶0,d_{\omega_{N}}(y_{1},y_{2})\leqslant C{\rm length}_{\omega_{N}}(f(\gamma_{k}))\longrightarrow 0,

or, if f⁡(γk)∩N\Kf(\gamma_{k})\cap N\backslash K are not empty by passing to a subsequence,

dωN​(y1,∂K)+dωN​(∂K,y2)⩽C​lengthωN​(f⁡(γk)∩K)⟶0.d_{\omega_{N}}(y_{1},\partial K)+d_{\omega_{N}}(\partial K,y_{2})\leqslant C{\rm length}_{\omega_{N}}(f(\gamma_{k})\cap K)\longrightarrow 0.

In both cases, we obtain contradictions. Thus ϕ:A∩int⁡K→X\phi:A\cap\operatorname{int}K\rightarrow X is injective.

Note that there is a r>0r>0 such that, for any y∈int⁡Ky\in\operatorname{int}K, the metric ball Bω​(y,r)B_{\omega}(y,r) is a geodesically convex set, i.e. for any y1y_{1} and y2∈Bω​(y,r)y_{2}\in B_{\omega}(y,r), there is a minimal geodesic γ⊂Bω​(y,r)\gamma\subset B_{\omega}(y,r) connecting y1y_{1} and y2y_{2}, which implies

dω​(y1,y2)=lengthω​(γ)⩽2​r.d_{\omega}(y_{1},y_{2})={\rm length}_{\omega}(\gamma)\leqslant 2r.

We take r≪1r\ll 1 such that there is a Bi′B_{i}^{\prime} with Bω​(y,2​r)⊂Bi′B_{\omega}(y,2r)\subset B_{i}^{\prime}. If y1,y2∈Ay_{1},y_{2}\in A, by Proposition 4.6,

dX​(ϕ⁡(y1),ϕ⁡(y2))=limtk→0dω~tk​(si​(y1),si​(y2))⩽limtk→0lengthω~tk​(si​(γ))=lengthω​(γ)=dω​(y1,y2).\begin{split}d_{X}(\phi(y_{1}),\phi(y_{2}))&=\lim_{t_{k}\rightarrow 0}d_{\tilde{\omega}_{t_{k}}}(s_{i}(y_{1}),s_{i}(y_{2}))\\ &\leqslant\lim_{t_{k}\rightarrow 0}{\rm length}_{\tilde{\omega}_{t_{k}}}(s_{i}(\gamma))\\ &={\rm length}_{\omega}(\gamma)\\ &=d_{\omega}(y_{1},y_{2}).\end{split}

If γk\gamma_{k} is a minimal geodesic in (M,ω~tk)(M,\tilde{\omega}_{t_{k}}) connecting si​(y1)s_{i}(y_{1}) and si​(y2)s_{i}(y_{2}), then (4.19) implies that

e−ε⁡(tk)2​lengthω​(f⁡(γk)∩Bω​(y,2​r))⩽lengthω~tk​(γk)⟶dX​(ϕ⁡(y1),ϕ⁡(y2)),e^{-\frac{\varepsilon(t_{k})}{2}}{\rm length}_{\omega}(f(\gamma_{k})\cap B_{\omega}(y,2r))\leqslant{\rm length}_{\tilde{\omega}_{t_{k}}}(\gamma_{k})\longrightarrow d_{X}(\phi(y_{1}),\phi(y_{2})),

for some function ε⁡(t)→0\varepsilon(t)\to 0 as t→0t\to 0. If f⁡(γk)⊂Bω​(y,2​r)f(\gamma_{k})\subset B_{\omega}(y,2r) for tk≪1t_{k}\ll 1 by passing to a subsequence,

lengthω​(f⁡(γk))⩾lengthω​(γ),{\rm length}_{\omega}(f(\gamma_{k}))\geqslant{\rm length}_{\omega}(\gamma),

since γ\gamma is a minimal geodesic in (N0,ω)(N_{0},\omega). If f⁡(γk)∩N0\Bω​(y,2​r)f(\gamma_{k})\cap N_{0}\backslash B_{\omega}(y,2r) is not empty for tk≪1t_{k}\ll 1, then there is a y¯∈f⁡(γk)∩N0\Bω​(y,2​r)\bar{y}\in f(\gamma_{k})\cap N_{0}\backslash B_{\omega}(y,2r). Since y1y_{1}, y2∈Bω​(y,r)y_{2}\in B_{\omega}(y,r) and f⁡(γk)f(\gamma_{k}) connects y1y_{1} and y2y_{2},

lengthω​(f⁡(γk)∩Bω​(y,2​r))⩾dω​(y1,y¯)+dω​(y2,y¯)⩾2​r⩾lengthω​(γ).{\rm length}_{\omega}(f(\gamma_{k})\cap B_{\omega}(y,2r))\geqslant d_{\omega}(y_{1},\bar{y})+d_{\omega}(y_{2},\bar{y})\geqslant 2r\geqslant{\rm length}_{\omega}(\gamma).

In both cases,

dω​(y1,y2)=lengthω​(γ)⩽limtk→0lengthω​(f⁡(γk)∩Bω​(y,2​r))⩽dX​(ϕ⁡(y1),ϕ⁡(y2)).\begin{split}d_{\omega}(y_{1},y_{2})&={\rm length}_{\omega}(\gamma)\\ &\leqslant\lim_{t_{k}\rightarrow 0}{\rm length}_{\omega}(f(\gamma_{k})\cap B_{\omega}(y,2r))\\ &\leqslant d_{X}(\phi(y_{1}),\phi(y_{2})).\end{split}

Thus

dω​(y1,y2)=dX​(ϕ⁡(y1),ϕ⁡(y2)),d_{\omega}(y_{1},y_{2})=d_{X}(\phi(y_{1}),\phi(y_{2})),

i.e. ϕ:(A∩int⁡K,dω)⟶(X,dX)\phi:(A\cap\operatorname{int}K,d_{\omega})\longrightarrow(X,d_{X}) is a local isometric embedding. If {y1,j}\{y_{1,j}\} and {y2,j}\{y_{2,j}\} are two sequences in A∩int⁡KA\cap\operatorname{int}K such that limj→∞dω​(yi,j,y)=0\lim\limits_{j\rightarrow\infty}d_{\omega}(y_{i,j},y)=0 for i=1,2i=1,2, then limj→∞dω​(y1,j,y2,j)=0\lim\limits_{j\rightarrow\infty}d_{\omega}(y_{1,j},y_{2,j})=0 and {y1,j,y2,j}⊂Bω​(y,r)\{y_{1,j},y_{2,j}\}\subset B_{\omega}(y,r) for j≫1j\gg 1. Hence dω​(y1,j,y2,j)=dX​(ϕ⁡(y1,j),ϕ⁡(y2,j))d_{\omega}(y_{1,j},y_{2,j})=d_{X}(\phi(y_{1,j}),\phi(y_{2,j})) and dω​(yi,j,yi,j+ℓ)=dX​(ϕ⁡(yi,j),ϕ⁡(yi,j+ℓ))d_{\omega}(y_{i,j},y_{i,j+\ell})=d_{X}(\phi(y_{i,j}),\phi(y_{i,j+\ell})) for j≫1j\gg 1 and any ℓ⩾0\ell\geqslant 0, which implies that {ϕ⁡(y1,j)}\{\phi(y_{1,j})\} and {ϕ⁡(y2,j)}\{\phi(y_{2,j})\} are two Cauchy sequences, and converge to a unique point x∈Xx\in X. By defining ϕ⁡(y)=x\phi(y)=x, ϕ\phi extends to a unique map, denoted still by ϕ\phi, from int⁡K\operatorname{int}K to XX which is also a local isometric embedding.

Now we prove that ϕ⁡(int⁡K)\phi(\operatorname{int}K) is an open subset of XX. Let x∈ϕ⁡(int⁡K)x\in\phi(\operatorname{int}K), i.e. there is a y∈int⁡Ky\in\operatorname{int}K such that ϕ⁡(y)=x\phi(y)=x, and let x′∈Xx^{\prime}\in X with dX​(x,x′)<ρd_{X}(x,x^{\prime})<\rho for a constant ρ<18​dω​(y,∂K)\rho<\frac{1}{8}d_{\omega}(y,\partial K). From the above construction, y∈Bi′y\in B_{i}^{\prime} for a Bi′B_{i}^{\prime}, and si​(y)→xs_{i}(y)\rightarrow x under Gromov-Hausdorff convergence. There is a sequence of points pk∈(M,ω~tk)p_{k}\in(M,\tilde{\omega}_{t_{k}}) such that pk→x′p_{k}\rightarrow x^{\prime} under the Gromov-Hausdorff convergence. If γk′\gamma_{k}^{\prime} is a minimal geodesic connecting si​(y)s_{i}(y) and pkp_{k} in (M,ω~tk)(M,\tilde{\omega}_{t_{k}}), then

dω~tk​(si​(y),pk)=lengthω~tk​(γk′)⟶dX​(x,x′).d_{\tilde{\omega}_{t_{k}}}(s_{i}(y),p_{k})={\rm length}_{\tilde{\omega}_{t_{k}}}(\gamma_{k}^{\prime})\longrightarrow d_{X}(x,x^{\prime}).

Equation (4.19) implies that, for k≫1k\gg 1,

12​lengthω​(f⁡(γk′)∩K)⩽e−ε⁡(tk)2​lengthω​(f⁡(γk′)∩K)⩽lengthω~tk​(γk′)<2​ρ<14​dω​(y,∂K).\begin{split}\frac{1}{2}{\rm length}_{\omega}(f(\gamma_{k}^{\prime})\cap K)&\leqslant e^{-\frac{\varepsilon(t_{k})}{2}}{\rm length}_{\omega}(f(\gamma_{k}^{\prime})\cap K)\\ &\leqslant{\rm length}_{\tilde{\omega}_{t_{k}}}(\gamma_{k}^{\prime})\\ &<2\rho<\frac{1}{4}d_{\omega}(y,\partial K).\end{split}

Thus f⁡(pk)∈K′⊂int⁡Kf(p_{k})\in K^{\prime}\subset\operatorname{int}K where K′K^{\prime} is a compact subset of int⁡K\operatorname{int}K. By passing to a subsequence, f⁡(pk)→y′f(p_{k})\rightarrow y^{\prime} in (K′,ω)(K^{\prime},\omega). By Proposition 4.6, dω~tk​(pk,sik​(f⁡(pk)))→0d_{\tilde{\omega}_{t_{k}}}(p_{k},s_{i_{k}}(f(p_{k})))\rightarrow 0 when tk→0t_{k}\rightarrow 0, and, thus, sik​(f⁡(pk))→x′s_{i_{k}}(f(p_{k}))\rightarrow x^{\prime} under the Gromov-Hausdorff convergence. The above construction shows that ϕ⁡(y′)=x′\phi(y^{\prime})=x^{\prime}, which implies that {x′|dX​(x,x′)<ρ}⊂ϕ⁡(int⁡K)\{x^{\prime}|d_{X}(x,x^{\prime})<\rho\}\subset\phi(\operatorname{int}K). Hence ϕ⁡(int⁡K)\phi(\operatorname{int}K) is open, and ϕ:int⁡K⟶ϕ⁡(int⁡K)\phi:\operatorname{int}K\longrightarrow\phi(\operatorname{int}K) is a homeomorphism.

Let K0⊂⋯⊂Kj⊂Kj+1⊂⋯⊂N0K_{0}\subset\cdots\subset K_{j}\subset K_{j+1}\subset\cdots\subset N_{0} be a family of compact subsets with N0=⋃jint⁡KjN_{0}=\bigcup\limits_{j}\operatorname{int}K_{j}. Given each KjK_{j}, the above argument constructs a local isometric embedding ϕj:(int⁡Kj,ω)⟶(X,dX)\phi_{j}:(\operatorname{int}K_{j},\omega)\longrightarrow(X,d_{X}), which is a homeomorphism onto the image ϕj​(int⁡Kj)\phi_{j}(\operatorname{int}K_{j}). By the same argument as above, ϕj\phi_{j} extends to a local isometric embedding ϕj+1:(int⁡Kj+1,ω)⟶(X,dX)\phi_{j+1}:(\operatorname{int}K_{j+1},\omega)\longrightarrow(X,d_{X}), i.e. ϕj+1|int⁡Kj=ϕj\phi_{j+1}|_{\operatorname{int}K_{j}}=\phi_{j}, which is a homeomorphism onto the image ϕj+1​(int⁡Kj+1)\phi_{j+1}(\operatorname{int}K_{j+1}). By a diagonal argument, we obtain a local isometry ϕ:(N0,ω)⟶(ϕ⁡(N0),dX)⊂(X,dX)\phi:(N_{0},\omega)\longrightarrow(\phi(N_{0}),d_{X})\subset(X,d_{X}). ∎

The above lemma proves the existence of ϕ\phi in Theorem 1.2, and is an analog of Lemma 4.1 in [30] for the collapsing case. In the rest of this section, we prove that X0=ϕ⁡(N0)X_{0}=\phi(N_{0}) is dense in XX.

Let x¯∈X0\bar{x}\in X_{0} and p¯k∈M\bar{p}_{k}\in M such that p¯k→x¯\bar{p}_{k}\rightarrow\bar{x} under the Gromov-Hausdorff convergence of (M,ω~tk)(M,\tilde{\omega}_{t_{k}}) to (X,dX)(X,d_{X}), and let

V¯k​(p,r)=Volω~tk​(Bω~tk​(p,r))Volω~tk​(Bω~tk​(p¯k,1)),\underline{V}_{k}(p,r)=\frac{{\rm Vol}_{\tilde{\omega}_{t_{k}}}(B_{\tilde{\omega}_{t_{k}}}(p,r))}{{\rm Vol}_{\tilde{\omega}_{t_{k}}}(B_{\tilde{\omega}_{t_{k}}}(\bar{p}_{k},1))},

for any p∈Mp\in M and r>0r>0. By Theorem 1.6 in [5], there is a continuous function V¯0:X×[0,∞)⟶[0,∞)\underline{V}_{0}:X\times[0,\infty)\longrightarrow[0,\infty) such that, if pk→xp_{k}\rightarrow x under the convergence of (M,ω~tk)(M,\tilde{\omega}_{t_{k}}) to (X,dX)(X,d_{X}), then

(5.1) V¯k​(pk,r)⟶V¯0​(x,r).\underline{V}_{k}(p_{k},r)\longrightarrow\underline{V}_{0}(x,r).

By Theorem 1.10 in [5], V¯0\underline{V}_{0} induces a unique Radon measure ν\nu on XX such that

(5.2) ν⁡(BdX​(x,r))=V¯0​(x,r),andν⁡(BdX​(x,r1))ν⁡(BdX​(x,r2))⩾μ⁡(r1,r2)>0,\nu(B_{d_{X}}(x,r))=\underline{V}_{0}(x,r),\ \ \ {\rm and}\ \ \frac{\nu(B_{d_{X}}(x,r_{1}))}{\nu(B_{d_{X}}(x,r_{2}))}\geqslant\mu(r_{1},r_{2})>0,

for any x∈Xx\in X, r1⩽r2r_{1}\leqslant r_{2}, where μ⁡(r1,r2)\mu(r_{1},r_{2}) is a function of r1r_{1} and r2r_{2}. For any compact subset K⊂XK\subset X,

ν⁡(K)=limδ→0νδ​(K)=limδ→0inf{∑iV¯0​(xi,ri)|ri<δ},\nu(K)=\lim_{\delta\rightarrow 0}\nu_{\delta}(K)=\lim_{\delta\rightarrow 0}\inf\left\{\sum_{i}\underline{V}_{0}(x_{i},r_{i})|r_{i}<\delta\right\},

where ⋃iBdX​(xi,ri)⊃K\bigcup\limits_{i}B_{d_{X}}(x_{i},r_{i})\supset K. By scaling ω~t\tilde{\omega}_{t} and ω\omega by one positive number, we assume that Bω​(ϕ−1​(x¯),2)⊂N0B_{\omega}(\phi^{-1}(\bar{x}),2)\subset N_{0} and is a geodesically convex set.

Lemma 5.2.

There is a constant υ>0\upsilon>0 such that

ν⁡(X)=υ​∫MωMn,V¯0​(x,r)=υ​∫f−1​(Bω​(ϕ−1​(x),r))ωMn,\nu(X)=\upsilon\int_{M}\omega_{M}^{n},\ \ \ \ \underline{V}_{0}(x,r)=\upsilon\int_{f^{-1}(B_{\omega}(\phi^{-1}(x),r))}\omega_{M}^{n},

whenever x∈X0x\in X_{0} and r⩽1r\leqslant 1 is such that Bω​(ϕ−1​(x),2​r)B_{\omega}(\phi^{-1}(x),2r) is a geodesically convex subset of (N0,ω)(N_{0},\omega).

Proof.

If pk→xp_{k}\rightarrow x under the convergence of (M,ω~tk)(M,\tilde{\omega}_{t_{k}}) to (X,dX)(X,d_{X}), we claim that a subsequence of pkp_{k} converges to a point p′∈f−1​(ϕ−1​(x))p^{\prime}\in f^{-1}(\phi^{-1}(x)) under the metric ωM\omega_{M} on MM. By Lemma 5.1, there is a compact neighborhood B⊂N0B\subset N_{0} of ϕ−1​(x)\phi^{-1}(x) and a section s:B→f−1​(B)s:B\rightarrow f^{-1}(B) such that s​(ϕ−1​(x))→xs(\phi^{-1}(x))\rightarrow x under the Gromov-Hausdorff convergence of (M,ω~tk)(M,\tilde{\omega}_{t_{k}}) to (X,dX)(X,d_{X}). Thus dω~tk​(pk,s⁡(ϕ−1​(x)))→0d_{\tilde{\omega}_{t_{k}}}(p_{k},s(\phi^{-1}(x)))\rightarrow 0 when tk→0t_{k}\rightarrow 0. By Lemma 4.1, there are curves γk\gamma_{k} connecting pkp_{k} and s​(ϕ−1​(x))s(\phi^{-1}(x)) such that lengthω~tk​(γk)=dω~tk​(pk,s⁡(ϕ−1​(x))){\rm length}_{\tilde{\omega}_{t_{k}}}(\gamma_{k})=d_{\tilde{\omega}_{t_{k}}}(p_{k},s(\phi^{-1}(x))), and

lengthω0​(f⁡(γk)∩B)=lengthf∗​ω0​(γk∩f−1​(B))⩽C12​lengthω~tk​(γk)→0.{\rm length}_{\omega_{0}}(f(\gamma_{k})\cap B)={\rm length}_{f^{*}\omega_{0}}(\gamma_{k}\cap f^{-1}(B))\leqslant C^{\frac{1}{2}}{\rm length}_{\tilde{\omega}_{t_{k}}}(\gamma_{k})\rightarrow 0.

For a k≫1k\gg 1, if there is a yk∈f⁡(γk)\By_{k}\in f(\gamma_{k})\backslash B, then

lengthω0​(f⁡(γk)∩B)⩾dω0​(yk,ϕ−1​(x))⩾ρ,{\rm length}_{\omega_{0}}(f(\gamma_{k})\cap B)\geqslant d_{\omega_{0}}(y_{k},\phi^{-1}(x))\geqslant\rho,

where ρ>0\rho>0 such that Bω0​(ϕ−1​(x),ρ)⊂BB_{\omega_{0}}(\phi^{-1}(x),\rho)\subset B, which is a contradiction. Thus f⁡(γk)⊂Bf(\gamma_{k})\subset B for k≫1k\gg 1, lengthω0​(f⁡(γk))→0{\rm length}_{\omega_{0}}(f(\gamma_{k}))\rightarrow 0 and f⁡(pk)f(p_{k}) converges to ϕ−1​(x)\phi^{-1}(x) under the metric ω0\omega_{0}. By passing to a subsequence, pkp_{k} converges to a point p′p^{\prime} under the metric ωM\omega_{M}. Since f∗​ω0⩽C′​ωMf^{*}\omega_{0}\leqslant C^{\prime}\omega_{M} for a constant C′>0C^{\prime}>0, dω0​(f⁡(pk),f⁡(p′))⩽C′12​dωM​(pk,p′)→0.d_{\omega_{0}}(f(p_{k}),f(p^{\prime}))\leqslant C^{\prime\frac{1}{2}}d_{\omega_{M}}(p_{k},p^{\prime})\rightarrow 0. Hence f⁡(p′)=ϕ−1​(x)f(p^{\prime})=\phi^{-1}(x) and p′∈f−1​(ϕ−1​(x))p^{\prime}\in f^{-1}(\phi^{-1}(x)).

Let rr satisfy r⩽1r\leqslant 1, and Bω​(ϕ−1​(x),2​r)B_{\omega}(\phi^{-1}(x),2r) is a geodesically convex subset of (N0,ω)(N_{0},\omega). If q∈f−1​(Bω​(ϕ−1​(x),2​r))q\in f^{-1}(B_{\omega}(\phi^{-1}(x),2r)), there is a curve γ¯\bar{\gamma} connecting p′p^{\prime} and qq such that f⁡(γ¯)f(\bar{\gamma}) is the unique minimal geodesic connecting ϕ−1​(x)\phi^{-1}(x) and f⁡(q)f(q). Thanks to (4.18) we have

f∗​ω−ε⁡(tk)​ωM⩽ω~tk⩽f∗​ω+ε⁡(tk)​ωMf^{*}\omega-\varepsilon(t_{k})\omega_{M}\leqslant\tilde{\omega}_{t_{k}}\leqslant f^{*}\omega+\varepsilon(t_{k})\omega_{M}

where ε⁡(tk)→0\varepsilon(t_{k})\rightarrow 0 when tk→0t_{k}\rightarrow 0, on f−1​(Bω​(ϕ−1​(x),2​r))f^{-1}(B_{\omega}(\phi^{-1}(x),2r)). We obtain that

dω~tk​(p′,q)⩽lengthω~tk​(γ¯)⩽lengthω​(f⁡(γ¯))+C​ε​(tk)12=dω​(ϕ−1​(x),f⁡(q))+C​ε​(tk)12.\begin{split}d_{\tilde{\omega}_{t_{k}}}(p^{\prime},q)&\leqslant{\rm length}_{\tilde{\omega}_{t_{k}}}(\bar{\gamma})\\ &\leqslant{\rm length}_{\omega}(f(\bar{\gamma}))+C\varepsilon(t_{k})^{\frac{1}{2}}\\ &=d_{\omega}(\phi^{-1}(x),f(q))+C\varepsilon(t_{k})^{\frac{1}{2}}.\end{split}

If γ¯k\bar{\gamma}_{k} is a minimal geodesic of ω~tk\tilde{\omega}_{t_{k}} connecting p′p^{\prime} and qq, then (4.19) gives

dω~tk​(p′,q)=lengthω~tk​(γ¯k)⩾e−ε⁡(tk)2​lengthω​(f⁡(γ¯k)∩Bω​(ϕ−1​(x),2​r)).d_{\tilde{\omega}_{t_{k}}}(p^{\prime},q)={\rm length}_{\tilde{\omega}_{t_{k}}}(\bar{\gamma}_{k})\geqslant e^{-\frac{\varepsilon(t_{k})}{2}}{\rm length}_{\omega}(f(\bar{\gamma}_{k})\cap B_{\omega}(\phi^{-1}(x),2r)).

If f⁡(γ¯k)⊂Bω​(ϕ−1​(x),2​r)f(\bar{\gamma}_{k})\subset B_{\omega}(\phi^{-1}(x),2r), then

lengthω​(f⁡(γ¯k)∩Bω​(ϕ−1​(x),2​r))⩾lengthω​(f⁡(γ¯))=dω​(ϕ−1​(x),f⁡(q)),{\rm length}_{\omega}(f(\bar{\gamma}_{k})\cap B_{\omega}(\phi^{-1}(x),2r))\geqslant{\rm length}_{\omega}(f(\bar{\gamma}))=d_{\omega}(\phi^{-1}(x),f(q)),

and, otherwise,

lengthω​(f⁡(γ¯k)∩Bω​(ϕ−1​(x),2​r))⩾2​r⩾lengthω​(f⁡(γ¯))=dω​(ϕ−1​(x),f⁡(q)),{\rm length}_{\omega}(f(\bar{\gamma}_{k})\cap B_{\omega}(\phi^{-1}(x),2r))\geqslant 2r\geqslant{\rm length}_{\omega}(f(\bar{\gamma}))=d_{\omega}(\phi^{-1}(x),f(q)),

by the same argument as in the proof of Lemma 5.1. Thus

e−ε⁡(tk)2​dω​(ϕ−1​(x),f⁡(q))⩽dω~tk​(p′,q)⩽dω​(ϕ−1​(x),f⁡(q))+C​ε​(tk)12e^{-\frac{\varepsilon(t_{k})}{2}}d_{\omega}(\phi^{-1}(x),f(q))\leqslant d_{\tilde{\omega}_{t_{k}}}(p^{\prime},q)\leqslant d_{\omega}(\phi^{-1}(x),f(q))+C\varepsilon(t_{k})^{\frac{1}{2}}

where CC is a constant independent of tkt_{k}, p′p^{\prime} and qq. Of course if kk is large we will have that

dω​(ϕ−1​(x),f⁡(q))−C​ε​(tk)12⩽e−ε⁡(tk)2​dω​(ϕ−1​(x),f⁡(q)).d_{\omega}(\phi^{-1}(x),f(q))-C\varepsilon(t_{k})^{\frac{1}{2}}\leqslant e^{-\frac{\varepsilon(t_{k})}{2}}d_{\omega}(\phi^{-1}(x),f(q)).

Thanks to (4.1), there is constant C>0C>0 independent of tkt_{k} such that ω~tk⩽C​ωM\tilde{\omega}_{t_{k}}\leqslant C\omega_{M} on f−1​(Bω​(ϕ−1​(x),2​r))f^{-1}(B_{\omega}(\phi^{-1}(x),2r)). Let γk′\gamma^{\prime}_{k} be minimal geodesics of ωM\omega_{M} connecting pkp_{k} and p′p^{\prime}, which satisfy γk′⊂f−1​(Bω​(ϕ−1​(x),2​r))\gamma^{\prime}_{k}\subset f^{-1}(B_{\omega}(\phi^{-1}(x),2r)) for k≫1k\gg 1. Thus

dω~tk​(p′,pk)⩽lengthω~tk​(γk′)⩽C12​lengthωM​(γk′)=C12​dωM​(p′,pk)→0.d_{\tilde{\omega}_{t_{k}}}(p^{\prime},p_{k})\leqslant{\rm length}_{\tilde{\omega}_{t_{k}}}(\gamma^{\prime}_{k})\leqslant C^{\frac{1}{2}}{\rm length}_{\omega_{M}}(\gamma^{\prime}_{k})=C^{\frac{1}{2}}d_{\omega_{M}}(p^{\prime},p_{k})\rightarrow 0.

The triangle inequality shows that

|dω~tk​(pk,q)−dω​(ϕ−1​(x),f⁡(q))|⩽C​ε​(tk)12+C12​dωM​(p′,pk).|d_{\tilde{\omega}_{t_{k}}}(p_{k},q)-d_{\omega}(\phi^{-1}(x),f(q))|\leqslant C\varepsilon(t_{k})^{\frac{1}{2}}+C^{\frac{1}{2}}d_{\omega_{M}}(p^{\prime},p_{k}).

Hence there is a function ρ⁡(tk)\rho(t_{k}) of tkt_{k} such that ρ⁡(tk)→0\rho(t_{k})\rightarrow 0 when tk→0t_{k}\rightarrow 0, and

f−1​(Bω​(ϕ−1​(x),r−ρ⁡(tk)))⊂Bω~tk​(pk,r)⊂f−1​(Bω​(ϕ−1​(x),r+ρ⁡(tk))).f^{-1}(B_{\omega}(\phi^{-1}(x),r-\rho(t_{k})))\subset B_{\tilde{\omega}_{t_{k}}}(p_{k},r)\subset f^{-1}(B_{\omega}(\phi^{-1}(x),r+\rho(t_{k}))).

We obtain that

limtk→0∫Bω~tk​(pk,r)ωMn=∫f−1​(Bω​(ϕ−1​(x),r))ωMn.\lim_{t_{k}\rightarrow 0}\int_{B_{\tilde{\omega}_{t_{k}}}(p_{k},r)}\omega_{M}^{n}=\int_{f^{-1}(B_{\omega}(\phi^{-1}(x),r))}\omega_{M}^{n}.

Note that

ω~tkn=ctk​tkn−m​ωMn.\tilde{\omega}_{t_{k}}^{n}=c_{t_{k}}t_{k}^{n-m}\omega_{M}^{n}.

Hence

V¯k​(pk,r)=Volω~tk​(Bω~tk​(pk,r))Volω~tk​(Bω~tk​(p¯k,1))=∫Bω~tk​(pk,r)ctk​tkn−m​ωMn∫Bω~tk​(p¯k,1)ctk​tkn−m​ωMn→∫f−1​(Bω​(ϕ−1​(x),r))ωMn∫f−1​(Bω​(ϕ−1​(x¯),1))ωMn,\begin{split}\underline{V}_{k}(p_{k},r)&=\frac{{\rm Vol}_{\tilde{\omega}_{t_{k}}}(B_{\tilde{\omega}_{t_{k}}}(p_{k},r))}{{\rm Vol}_{\tilde{\omega}_{t_{k}}}(B_{\tilde{\omega}_{t_{k}}}(\bar{p}_{k},1))}\\ &=\frac{\int_{B_{\tilde{\omega}_{t_{k}}}(p_{k},r)}c_{t_{k}}t_{k}^{n-m}\omega_{M}^{n}}{\int_{B_{\tilde{\omega}_{t_{k}}}(\bar{p}_{k},1)}c_{t_{k}}t_{k}^{n-m}\omega_{M}^{n}}\to\frac{\int_{f^{-1}(B_{\omega}(\phi^{-1}(x),r))}\omega_{M}^{n}}{\int_{f^{-1}(B_{\omega}(\phi^{-1}(\bar{x}),1))}\omega_{M}^{n}},\end{split}

when tk→0t_{k}\rightarrow 0. By (5.1),

V¯0​(x,r)=υ​∫f−1​(Bω​(ϕ−1​(x),r))ωMn,whereυ=(∫f−1​(Bω​(ϕ−1​(x¯),1))ωMn)−1.\underline{V}_{0}(x,r)=\upsilon\int_{f^{-1}(B_{\omega}(\phi^{-1}(x),r))}\omega_{M}^{n},\ \ {\rm where}\ \ \upsilon=\left(\int_{f^{-1}(B_{\omega}(\phi^{-1}(\bar{x}),1))}\omega_{M}^{n}\right)^{-1}.

Recall the diameter bound (1.4)

diamω~tk​(M)⩽D{\rm diam}_{\tilde{\omega}_{t_{k}}}(M)\leqslant D

for a constant D>0D>0. Using (5.1), we have

ν⁡(X)=V¯0​(x,D)=limtk→0V¯k​(pk,D)=υ​∫MωMn.\nu(X)=\underline{V}_{0}(x,D)=\lim_{t_{k}\rightarrow 0}\underline{V}_{k}(p_{k},D)=\upsilon\int_{M}\omega_{M}^{n}.

∎

Proof of Theorem 1.2.

We prove that X0⊂XX_{0}\subset X is dense. If this is not true, there is a metric ball BdX​(x′,ρ)⊂X\X0B_{d_{X}}(x^{\prime},\rho)\subset X\backslash X_{0}. Note that

diamdX​(X)=limtk→0diamω~tk​(M)⩽D.{\rm diam}_{d_{X}}(X)=\lim_{t_{k}\rightarrow 0}{\rm diam}_{\tilde{\omega}_{t_{k}}}(M)\leqslant D.

Because of (5.2), we have

ν⁡(BdX​(x′,ρ))⩾μ⁡(ρ,D)​ν​(X)=ϖ>0.\nu(B_{d_{X}}(x^{\prime},\rho))\geqslant\mu(\rho,D)\nu(X)=\varpi>0.

For any compact subset K⊂X0K\subset X_{0},

ν⁡(K)⩽ν⁡(X)−ϖ=υ​∫MωMn−ϖ\nu(K)\leqslant\nu(X)-\varpi=\upsilon\int_{M}\omega_{M}^{n}-\varpi

by Lemma 5.2. If BdX​(xi,ri)B_{d_{X}}(x_{i},r_{i}) is a family of metric balls in (X,dX)(X,d_{X}) such that ri<δ≪1r_{i}<\delta\ll 1, BdX​(xi,2​ri)B_{d_{X}}(x_{i},2r_{i}) is a geodesically convex subset of X0X_{0}, and ⋃iBdX​(xi,ri)⊃K\bigcup\limits_{i}B_{d_{X}}(x_{i},r_{i})\supset K, then

∑iV¯0​(xi,ri)=∑iυ​∫f−1​(ϕ−1​(BdX​(xi,ri)))ωMn⩾υ​∫f−1​(ϕ−1​(K))ωMn\sum_{i}\underline{V}_{0}(x_{i},r_{i})=\sum_{i}\upsilon\int_{f^{-1}(\phi^{-1}(B_{d_{X}}(x_{i},r_{i})))}\omega_{M}^{n}\geqslant\upsilon\int_{f^{-1}(\phi^{-1}(K))}\omega_{M}^{n}

by Lemma 5.2. Thus

υ​∫f−1​(ϕ−1​(K))ωMn⩽limδ→0νδ​(K)=limδ→0inf{∑iV¯0​(xi,ri)|ri<δ}=ν⁡(K).\upsilon\int_{f^{-1}(\phi^{-1}(K))}\omega_{M}^{n}\leqslant\lim_{\delta\rightarrow 0}\nu_{\delta}(K)=\lim_{\delta\rightarrow 0}\inf\left\{\sum_{i}\underline{V}_{0}(x_{i},r_{i})|r_{i}<\delta\right\}=\nu(K).

By taking KK large enough such that

ν⁡(K)⩾υ​∫f−1​(N0)ωMn−ϖ2=υ​∫MωMn−ϖ2,\nu(K)\geqslant\upsilon\int_{f^{-1}(N_{0})}\omega_{M}^{n}-\frac{\varpi}{2}=\upsilon\int_{M}\omega_{M}^{n}-\frac{\varpi}{2},

we obtain a contradiction. ∎

Remark 5.3.

In fact, the same proof shows that ν⁡(X\X0)=0\nu(X\backslash X_{0})=0.

Original mathematics by the credited authors. Source-backed reader collection; mathematical self-containment is not assessed.