<p>We prove two boundary Schwarz lemmas for disks. The first one concerns holomorphic maps from the unit disk into the unit ball of <InlineEquation ID="IEq1"> <EquationSource Format="TEX">\(\mathbb {C}^m\)</EquationSource> <EquationSource Format="MATHML"><math> <msup> <mrow> <mi mathvariant="double-struck">C</mi> </mrow> <mi>m</mi> </msup> </math></EquationSource> </InlineEquation>. If <InlineEquation ID="IEq2"> <EquationSource Format="TEX">\(F:\mathbb {D}\rightarrow \mathbb {B}_m\)</EquationSource> <EquationSource Format="MATHML"><math> <mrow> <mi>F</mi> <mo>:</mo> <mi mathvariant="double-struck">D</mi> <mo stretchy="false">→</mo> <msub> <mi mathvariant="double-struck">B</mi> <mi>m</mi> </msub> </mrow> </math></EquationSource> </InlineEquation> is holomorphic, <InlineEquation ID="IEq3"> <EquationSource Format="TEX">\(F(\zeta )\in \partial \mathbb {B}_m\)</EquationSource> <EquationSource Format="MATHML"><math> <mrow> <mi>F</mi> <mrow> <mo stretchy="false">(</mo> <mi>ζ</mi> <mo stretchy="false">)</mo> </mrow> <mo>∈</mo> <mi>∂</mi> <msub> <mi mathvariant="double-struck">B</mi> <mi>m</mi> </msub> </mrow> </math></EquationSource> </InlineEquation> for some <InlineEquation ID="IEq4"> <EquationSource Format="TEX">\(\zeta \in \mathbb {T}\)</EquationSource> <EquationSource Format="MATHML"><math> <mrow> <mi>ζ</mi> <mo>∈</mo> <mi mathvariant="double-struck">T</mi> </mrow> </math></EquationSource> </InlineEquation>, and a finite angular derivative exists at that point, then <InlineEquation ID="IEq5"> <EquationSource Format="TEX">\(\left\| F'(\zeta )\right\| \)</EquationSource> <EquationSource Format="MATHML"><math> <mfenced close="∥" open="∥"> <msup> <mi>F</mi> <mo>′</mo> </msup> <mrow> <mo stretchy="false">(</mo> <mi>ζ</mi> <mo stretchy="false">)</mo> </mrow> </mfenced> </math></EquationSource> </InlineEquation> admits an explicit lower bound in terms of <InlineEquation ID="IEq6"> <EquationSource Format="TEX">\(F(0)\)</EquationSource> <EquationSource Format="MATHML"><math> <mrow> <mi>F</mi> <mo stretchy="false">(</mo> <mn>0</mn> <mo stretchy="false">)</mo> </mrow> </math></EquationSource> </InlineEquation> and <InlineEquation ID="IEq7"> <EquationSource Format="TEX">\(F'(0)\)</EquationSource> <EquationSource Format="MATHML"><math> <mrow> <msup> <mi>F</mi> <mo>′</mo> </msup> <mrow> <mo stretchy="false">(</mo> <mn>0</mn> <mo stretchy="false">)</mo> </mrow> </mrow> </math></EquationSource> </InlineEquation>. The estimate is sharp, and we identify all equality maps; in particular, the extremals are one-dimensional Blaschke-type disks. The second result is a boundary Schwarz lemma for conformal minimal disks <InlineEquation ID="IEq8"> <EquationSource Format="TEX">\(F:\mathbb {D}\rightarrow \mathbb {B}^n\subset \mathbb {R}^n\)</EquationSource> <EquationSource Format="MATHML"><math> <mrow> <mi>F</mi> <mo>:</mo> <mi mathvariant="double-struck">D</mi> <mo stretchy="false">→</mo> <msup> <mrow> <mi mathvariant="double-struck">B</mi> </mrow> <mi>n</mi> </msup> <mo>⊂</mo> <msup> <mrow> <mi mathvariant="double-struck">R</mi> </mrow> <mi>n</mi> </msup> </mrow> </math></EquationSource> </InlineEquation>, <InlineEquation ID="IEq9"> <EquationSource Format="TEX">\(n\ge 3\)</EquationSource> <EquationSource Format="MATHML"><math> <mrow> <mi>n</mi> <mo>≥</mo> <mn>3</mn> </mrow> </math></EquationSource> </InlineEquation>. It follows from the distance-decreasing theorem of Forstnerič and Kalaj for the Poincare metric on the disk and the Cayley–Klein metric on the ball. We also determine the equality case: equality forces the image to be a totally geodesic planar disk, and in the noncentered case the boundary point is aligned with the radial direction of the center.</p>

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Boundary Schwarz lemmas for holomorphic and conformal minimal disks

  • David Kalaj

摘要

We prove two boundary Schwarz lemmas for disks. The first one concerns holomorphic maps from the unit disk into the unit ball of \(\mathbb {C}^m\) C m . If \(F:\mathbb {D}\rightarrow \mathbb {B}_m\) F : D B m is holomorphic, \(F(\zeta )\in \partial \mathbb {B}_m\) F ( ζ ) B m for some \(\zeta \in \mathbb {T}\) ζ T , and a finite angular derivative exists at that point, then \(\left\| F'(\zeta )\right\| \) F ( ζ ) admits an explicit lower bound in terms of \(F(0)\) F ( 0 ) and \(F'(0)\) F ( 0 ) . The estimate is sharp, and we identify all equality maps; in particular, the extremals are one-dimensional Blaschke-type disks. The second result is a boundary Schwarz lemma for conformal minimal disks \(F:\mathbb {D}\rightarrow \mathbb {B}^n\subset \mathbb {R}^n\) F : D B n R n , \(n\ge 3\) n 3 . It follows from the distance-decreasing theorem of Forstnerič and Kalaj for the Poincare metric on the disk and the Cayley–Klein metric on the ball. We also determine the equality case: equality forces the image to be a totally geodesic planar disk, and in the noncentered case the boundary point is aligned with the radial direction of the center.