ScalingStacks

Example 3.10 (Positive vertex) . [04IB]

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Example 3.10 (Positive vertex).

Take B=ℝ×ℝ2B=\mathbb{R}\times\mathbb{R}^{2}, with coordinates (x1,x2,x3)(x_{1},x_{2},x_{3}) and identify ℝ2\mathbb{R}^{2} with {0}×ℝ2\{0\}\times\mathbb{R}^{2}. Inside ℝ2\mathbb{R}^{2} consider the cone over three points:

Δ={x2=0,x3≤0}∪{x3=0,x2≤0}∪{x2=x3,x3≥0}.\Delta=\{x_{2}=0,\,x_{3}\leq 0\}\cup\{x_{3}=0,\,x_{2}\leq 0\}\cup\{x_{2}=x_{3},\,x_{3}\geq 0\}.

Now define closed sets in BB

R\displaystyle R =\displaystyle= ℝ×Δ,\displaystyle\mathbb{R}\times\Delta,
R+\displaystyle R^{+} =\displaystyle= ℝ≥0×Δ,\displaystyle\mathbb{R}_{\geq 0}\times\Delta,
R−\displaystyle R^{-} =\displaystyle= ℝ≤0×Δ,\displaystyle\mathbb{R}_{\leq 0}\times\Delta,

and consider the following cover {Ui}\{U_{i}\} of ℝ3−Δ\mathbb{R}^{3}-\Delta:

U1\displaystyle U_{1} =\displaystyle= ℝ3−R+,\displaystyle\mathbb{R}^{3}-R^{+},
U2\displaystyle U_{2} =\displaystyle= ℝ3−R−.\displaystyle\mathbb{R}^{3}-R^{-}.

It is clear that U1∩U2U_{1}\cap U_{2} has the following three connected components

V1\displaystyle V_{1} =\displaystyle= {x2<0,x3<0},\displaystyle\{x_{2}<0,\ x_{3}<0\},
V2\displaystyle V_{2} =\displaystyle= {x2>0,x2>x3},\displaystyle\{x_{2}>0,\ x_{2}>x_{3}\},
V3\displaystyle V_{3} =\displaystyle= {x3>0,x3>x2}.\displaystyle\{x_{3}>0,\ x_{3}>x_{2}\}.

Take two matrices

T1=(110010001),T2=(10−1010001).T_{1}=\left(\begin{array}[]{ccc}1&1&0\\ 0&1&0\\ 0&0&1\end{array}\right),\ \ \ T_{2}=\left(\begin{array}[]{ccc}1&0&-1\\ 0&1&0\\ 0&0&1\end{array}\right). (9)

Now on U1,U2U_{1},U_{2} we define coordinate maps ϕ1\phi_{1}, ϕ2\phi_{2} as follows

ϕ1\displaystyle\phi_{1} =\displaystyle= Id,\displaystyle\I,
ϕ2\displaystyle\phi_{2} =\displaystyle= {Idon​V¯1∩U2,T1−1on​V¯2∩U2T2on​V¯3∩U2\displaystyle\left\{\begin{array}[]{ll}\I&\text{on}\ \bar{V}_{1}\cap U_{2},\\ T_{1}^{-1}&\text{on}\ \bar{V}_{2}\cap U_{2}\\ T_{2}&\text{on}\ \bar{V}_{3}\cap U_{2}\end{array}\right.

Again we see that 𝒜={Ui,ϕi}i=1,2\mathscr{A}=\{U_{i},\phi_{i}\}_{i=1,2} gives an affine structure on B0=ℝ3−ΔB_{0}=\mathbb{R}^{3}-\Delta. One can compute that given a point b∈B0b\in B_{0} and closed paths g1g_{1}, g2g_{2} and g3g_{3} as in Figure 3, we can choose a basis of Tb∗​B0T^{\ast}_{b}B_{0} with respect to which the holonomy matrices satisfy ρ∗​(gj)=(Tj−1)t\rho^{\ast}(g_{j})=(T_{j}^{-1})^{t} for j=1,2,3j=1,2,3.

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