ScalingStacks

Example 3.11 (A variation) . [04IC]

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Example 3.11 (A variation).

In the previous example, Δ\Delta was a graph with three edges meeting in one vertex. All three edges were straight lines. In the spirit of Example 3.9 we can perturb each edge of Δ\Delta to a smooth curve starting at the vertex. Each straight edge of the previous example is contained in a 22-plane which is an integral plane of the distribution spanned by the vectors which are invariant with respect to the holonomy around that edge. For example, consider the edge E1={x1=x2=0,x3≤0}E_{1}=\{x_{1}=x_{2}=0,x_{3}\leq 0\} of Δ\Delta. Then E1E_{1} is contained inside the half plane, P1={x2=0,x3≤0}P_{1}=\{x_{2}=0,x_{3}\leq 0\}, whose tangent vectors are T1T_{1} invariant, where T1=ρ⁡(g1)T_{1}=\rho(g_{1}) is the holonomy of TB0T_{B_{0}} with respect to E1E_{1}. An analogous thing happens with the other two edges. The union of all three half planes gives RR. The new perturbed edges, Ej′E_{j}^{\prime}, must be curves inside the half planes PjP_{j}. More precisely, let τ\tau be a function on Δ\Delta which is the restriction of a smooth function defined on an open neighborhood of Δ\Delta, such that τ⁡(0)=0\tau(0)=0. If we let RR be as in the previous example, define

Δτ\displaystyle\Delta_{\tau} =\displaystyle= {(τ(q),q)∈ℝ×Δ}\displaystyle\{(\tau(q),q)\in\mathbb{R}\times\Delta\}
R+\displaystyle R^{+} =\displaystyle= {(x1,q)∈ℝ×Δ|x1≥τ⁡(q)}\displaystyle\{(x_{1},q)\in\mathbb{R}\times\Delta\ |x_{1}\geq\tau(q)\}
R−\displaystyle R^{-} =\displaystyle= {(x1,q)∈ℝ×Δ|x1≤τ⁡(q)}\displaystyle\{(x_{1},q)\in\mathbb{R}\times\Delta\ |x_{1}\leq\tau(q)\}

Now charts 𝒜={Ui,ϕi}i=1,2\mathscr{A}=\{U_{i},\phi_{i}\}_{i=1,2} on B−ΔτB-\Delta_{\tau} can be defined like in the previous example, but with these new definitions of R+R^{+} and R−R^{-}. It is clear that (B,Δτ,𝒜)(B,\Delta_{\tau},\mathscr{A}) defines an affine manifold with singularities. Two different choices of functions τ\tau define non-isomorphic integral affine manifolds with singularities, unless their graphs inside RR can be mapped one to the other via an integral affine map.

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