ScalingStacks

Orientation signs on bordism currents [04HH]

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Orientation signs on bordism currents

In section 3.1, 3.1.2 we encountered the (n−1)(n-1)-dimensional moduli spaces such as ℳ⁡(b,…,b,α,b′,…,β)\mathcal{M}(b,\ldots,b,\alpha,b^{\prime},\ldots,\beta) and ℳ⁡(b,…,b,γ)\mathcal{M}(b,\ldots,b,\gamma). The special case of holmorphic strips was already mentioned in Example 6.2.

We now consider the moduli ℳ⁡(p1,…​pk)\mathcal{M}(p_{1},\ldots p_{k}) of polygons with at least 3 corners p1,…​pkp_{1},\ldots p_{k}, all regarded as inputs, arranged in clockwise order on ∂Σ\partial\Sigma, each carrying the local system factors Hom⁡(E+,E−)|pi\Hom(E_{+},E_{-})|_{p_{i}} and the orientation factors |opi||o_{p_{i}}|. The clockwise composition of the local system hom factors and the parallel transport along ∂Σ\partial\Sigma, produces a holonomy factor around ∂Σ\partial\Sigma, which is a number in ℚ,ℝ,ℤ\mathbb{Q},\mathbb{R},\mathbb{Z} depending on the coefficient ring choice. Using (69) and Remark 6.9, as well as the clockwise orientation convention on the Stasheff associahedron, we acquire a (naïve) orientation on T​ℳ​(p1,…​pk)T\mathcal{M}(p_{1},\ldots p_{k}). To assign orientation and weighting factors to ℳ⁡(p1,…​pk)\mathcal{M}(p_{1},\ldots p_{k}), we take the product of the holonomy factor, the naïve orientation on T​ℳ​(p1,…​pk)T\mathcal{M}(p_{1},\ldots p_{k}), and another universal sign factor

(−1)deg⁡p1+2​deg⁡p2+…+k​deg⁡pk​(−1)deg⁡pk.(-1)^{\deg p_{1}+2\deg p_{2}+\ldots+k\deg p_{k}}(-1)^{\deg p_{k}}.

The appearance of this universal sign adjustment is a familiar convention in the open-closed map, cf. [2, eqn 5.24]. The notation ℳ\mathcal{M} is a shorthand for the weighted sum of all the (n−1)(n-1)-dimensional moduli spaces involved in the construction of the bordism current.

We equip the domain Σ\Sigma with the complex orientation, and together with an extra minus sign, the orientation on ℳ\mathcal{M} induces the orientation on 𝒞\mathcal{C}. This minus sign arises for the same reason as in Example 6.2, namely the discrepancy between our clockwise convention on ∂Σ\partial\Sigma, with the standard complex orientation on Σ\Sigma.

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