ScalingStacks

Example 3.23 . [03PG]

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Example 3.23.

Writing 𝒮m={(x0,…,xm)∈ℝm+1:x02+⋯+xm2}{\mathbin{\cal S}}^{m}=\bigl\{(x_{0},\ldots,x_{m})\in{\mathbin{\mathbb{R}}}^{m+1}:x_{0}^{2}+\cdots+x_{m}^{2}\bigr\}, the Whitney sphere L=ι(𝒮m)L=\iota({\mathbin{\cal S}}^{m}) is the Lagrangian immersion ι:𝒮m→ℂm\iota:{\mathbin{\cal S}}^{m}\rightarrow{\mathbin{\mathbb{C}}}^{m} given by

ι:(x0,x1,…,xn)⟼11+x02​(x1​(1+i​x0),…,xn​(1+i​x0)).\iota:(x_{0},x_{1},\ldots,x_{n})\longmapsto\frac{1}{1+x_{0}^{2}}\,\bigl(x_{1}(1+ix_{0}),\ldots,x_{n}(1+ix_{0})\bigr).

It has the special property of having conformal Maslov form. It has one transverse self-intersection point at p=(0,…,0)=ι⁡(1,0,…,0)=ι⁡(−1,0,…,0)p=(0,\ldots,0)=\iota(1,0,\ldots,0)=\iota(-1,0,\ldots,0), with μL−,L+​(p)=−1\mu_{L_{-},L_{+}}(p)=-1, μL+,L−​(p)=m+1\mu_{L_{+},L_{-}}(p)=m+1. Thus if m>2m>2, Lemma 2.23 shows that LL has H​F∗HF^{*} unobstructed, so as in (B), (L,E,b)≅0(L,E,b)\cong 0 in Dbℱ(ℂm)D^{b}{\mathbin{\mathscr{F}}}({\mathbin{\mathbb{C}}}^{m}).

Ekholm Eliashberg, Murphy and Smith [15, §1] construct Lagrangian immersions ȷ:𝒮m→ℂm\jmath:{\mathbin{\cal S}}^{m}\rightarrow{\mathbin{\mathbb{C}}}^{m} for mm odd, with one transverse self-intersection point pp with μL+,L−​(p)=2\mu_{L_{+},L_{-}}(p)=2. If m⩾3m\geqslant 3 it has H​F∗HF^{*} obstructed, as in (A).

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