ScalingStacks

Example 6.13 . [04KJ]

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Example 6.13.

Consider a smooth proper Lagrangian fibration f:X→Bf:X\rightarrow B, with B=ℝ×MB=\mathbb{R}\times M and f=(μ,G)f=(\mu,G), where μ\mu is the moment map of a free S1S^{1} action and GG is S1S^{1} invariant. Assuming BB is contractible and having chosen bases γ±\gamma^{\pm} of H1​(X,ℤ)H_{1}(X,\mathbb{Z}) as in (a)(a) and (b)(b) above, on BB we can apply the admissible change of coordinates α\alpha as in Proposition 6.5. Clearly f′=α∘ff^{\prime}=\alpha\circ f is (tautologically) a fake stitched fibration. Given a Lagrangian section σ\sigma of f′f^{\prime}, it easy to see that the normal form for (X,f′​(B),f′,γ+,σ)(X,f^{\prime}(B),f^{\prime},\gamma^{+},\sigma) is of the type (Y,U,fu,γ0,σ0)(Y,U,f_{u},\gamma_{0},\sigma_{0}) where Y=T∗​U/ΛY=T^{*}U/\Lambda and

u⁡(y1,…,yn,b1,…,bn)=(b1,b2−m2​b1,…,bn−mn​b1),u(y_{1},\ldots,y_{n},b_{1},\ldots,b_{n})=(b_{1},b_{2}-m_{2}b_{1},\ldots,b_{n}-m_{n}b_{1}),

i.e. the projection composed with a linear change of coordinates. In this case the only non-zero invariant is ℓ1\ell_{1} which is given by

ℓ1=∑jmj​d​yj.\ell_{1}=\sum_{j}\,m_{j}dy_{j}.

Clearly ℓ1\ell_{1} is fibrewise constant.

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