1) The problem of orienting moduli spaces of pseudo-holomorphic discs with totally real boundary conditions is really a problem in index theory. It was solved Vin de Silva in his (unpublished) D. Phil. thesis, using Atiyah's Real K-theory, and independently by FOOO. There's an excellent account in Seidel's book (section 11, especially Lemma 11.7).

A totally real Cauchy-Riemann problem is by definition a loop in the totally real Grassmannian $\mathrm{Gr}(V)$ of a complex vector space $V$ with a given real structure. The resulting space $L\mathrm{Gr}(V)$ parametrizes a family of Fredholm operators (the Cauchy-Riemann operator for functions on the closed disc, valued in $V$, with boundary conditions specified by the loop). Hence there is a determinant index bundle $$\underline{det} \to L \mathrm{Gr}(V),$$ and the basic orientation problem is to describe $w_1(\underline{det}) \in H^1(L \mathrm{Gr}(V);\mathbb{Z}/2)$. For the component $L_k\mathrm{Gr}(V)$ of Maslov index $k$ loops, $H^1(L_k \mathrm{Gr}(V);\mathbb{Z}/2)$ is 2 dimensional, as one calculates using a homotopy equivalence $\mathrm{Gr}(\mathbb{C}^n) \simeq U(n)/O(n)$, so there are just four possible answers to the orientation question for each $k$.

The answer is simpler to state assuming $k$ is even. It is then as follows (I learned this from de Silva's thesis): Take a loop $\gamma\colon S^1\to L_k\mathrm{Gr}(V)$. Then $\langle w_1(\underline{det}), \gamma\rangle = \langle w_2, T_\gamma \rangle$, where $w_2$ is the second SW class of the universal totally real bundle on $\mathrm{Gr}(V)$, and $T_\gamma\colon S^1\times S^1\to \mathrm{Gr}(V)$ is the torus of boundary values swept out by $\gamma$.

So, in a space of pseudo-holomorphic discs in a symplectic manifold $X$ attached to an orientable Lagrangian $\Lambda$, $w_1$ of the determinant bundle evaluates on a loop $\gamma$ by evaluating the torus of boundary values $T_\gamma$ on $w_2(T\Lambda)$. Essentially for this reason, it's natural to trivialize the determinant bundle by trivializing $w_2(\Lambda)$, i.e. specifying a Pin structure(though this isn't quite right because $TX$ isn't trivialized; . More generally, one actually needs can trivialize $w_2(\Lambda)$ relative to a fixed background class $b\in H^2(\Lambda;\mathbb{Z}/2)$ which restricts to $w_2(\Lambda)$, i.e. specify a relative (or "twisted") Pin structurerelative to . That suffices essentially because the torus of boundary values is the boundary of a 3-chain in $TX$). This X$, and so vanishes if$w_2$is perhaps the only restriction of$b$. Choosing Pin structures relative to a fixed background class$b$gives a uniform way to orient moduli spaces of pseudo-holomorphic discs for relatively Pin Lagrangians. 2) For Cauchy-Riemann operators on other curves, one can degenerate to a nodal union of discs and closed Riemann surfaces, combining the orientations for the space of discs with the complex orientation of the determinant line bundle over the moduli of closed curves. Thus no additional obstructions to orientation appear. To be precise, what goes into this is a gluing theorem for the index bundle, which is part of the linear analysis that underpins Floer theory and Gromov-Witten theory. It implies that the determinant index line over a connected sum is canonically isomorphic to the tensor product of the determinant index lines on the summands. Again, see Seidel's book, section 11 for the argument. For the underlying analysis, I'd recommend Donaldson's Floer homology book, chapter 3. 3) I don't know, but there are further concrete calculations for real loci in the work of Welschinger and also Solomon. 3 Details and references added. 1) The problem of orienting moduli spaces of pseudo-holomorphic discs with totally real boundary conditions is really a problem in index theory. It was solved Vin de Silva in his (unpublished) D. Phil. thesis, using Atiyah's Real K-theory, and independently by FOOO. There's an excellent account in Seidel's book (section 11)11, especially Lemma 11.7). A totally real Cauchy-Riemann problem is by definition a loop in the totally real Grassmannian$\mathrm{Gr}(V)$of a complex vector space$V$with a given real structure. The resulting space$L\mathrm{Gr}(V)$parametrizes a family of Fredholm operators (the Cauchy-Riemann operator for functions on the closed disc, valued in$V$, with boundary conditions specified by the loop). Hence there is a determinant index bundle $$\underline{det} \to L \mathrm{Gr}(V),$$ and the basic orientation problem is to describe$w_1(\underline{det}) \in H^1(L \mathrm{Gr}(V);\mathbb{Z}/2)$. For the component$L_k\mathrm{Gr}(V)$of Maslov index$k$loops,$H^1(L_k \mathrm{Gr}(V);\mathbb{Z}/2)$is 2 dimensional, as one calculates using a homotopy equivalence$\mathrm{Gr}(\mathbb{C}^n) \simeq U(n)/O(n)$, so there are just four possible answers to the orientation question for each$k$. The answer is simpler to state assuming$k$is even. It is then as follows (I learned this from de Silva's thesis): Take a loop$\gamma\colon S^1\to L_k\mathrm{Gr}(V)$. Then$\langle w_1(\underline{det}), \gamma\rangle = \langle w_2, T_\gamma \rangle$, where$w_2$is the second SW class of the universal totally real bundle on$\mathrm{Gr}(V)$, and$T_\gamma\colon S^1\times S^1\to \mathrm{Gr}(V)$is the torus of boundary values swept out by$\gamma$. So, in a space of pseudo-holomorphic discs in a symplectic manifold$X$attached to an orientable Lagrangian$\Lambda$,$w_1$of the determinant bundle evaluates on a loop$\gamma$by evaluating the torus of boundary values$T_\gamma$on$w_2(T\Lambda)$. Essentially for this reason, it's natural to trivialize the determinant bundle by trivializing$w_2(\Lambda)$, i.e. specifying a Pin structure (though this isn't quite right because$TX$isn't trivialized; one actually needs a Pin structure relative to$TX$). This is perhaps the only uniform way to orient moduli spaces of pseudo-holomorphic discs. 2) For Cauchy-Riemann operators on other curves, one can degenerate to a nodal union of discs and closed Riemann surfaces, combining the orientations for the space of discs with the complex orientation of the determinant line bundle over the moduli of closed curves. Thus no additional obstructions to orientation appear.Technically To be precise, one what goes into this is using a gluing theorem for the excision properties index bundle, which is part of elliptic operatorsthe linear analysis that underpins Floer theory and Gromov-Witten theory. It implies that the determinant index line over a connected sum is canonically isomorphic to the tensor product of the determinant index lines on the summands. Again, see Seidel's book, section 11 for the argument. For the underlying analysis, I'd recommend Donaldson's Floer homology book, chapter 3. 3) I don't know, but there are further concrete calculations for real loci in the work of Welschinger and also Solomon. 2 a minor correction 1) The problem of orienting moduli spaces of pseudo-holomorphic discs with totally real boundary conditions is really a problem in index theory. It was solved Vin de Silva in his (unpublished) D. Phil. thesis, using Atiyah's Real K-theory, and independently by FOOO. There's an excellent account in Seidel's book (section 11). A totally real Cauchy-Riemann problem is by definition a loop in the totally real Grassmannian$\mathrm{Gr}(V)$of a complex vector space$V$. V$ with a given real structure. The resulting space $L\mathrm{Gr}(V)$ parametrizes a family of Fredholm operators (the Cauchy-Riemann operator for functions on the closed disc, valued in $V$, with boundary conditions specified by the loop). Hence there is a determinant index bundle $$\underline{det} \to L \mathrm{Gr}(V),$$ and the basic orientation problem is to describe $w_1(\underline{det}) \in H^1(L \mathrm{Gr}(V);\mathbb{Z}/2)$. For the component $L_k\mathrm{Gr}(V)$ of Maslov index $k$ loops, $H^1(L_k \mathrm{Gr}(V);\mathbb{Z}/2)$ is 2 dimensional, as one calculates using a homotopy equivalence $\mathrm{Gr}(\mathbb{C}^n) \simeq U(n)/O(n)$, so there are just four possible answers to the orientation question for each $k$.

The answer is simpler to state assuming $k$ is even. It is then as follows (I learned this from de Silva's thesis): Take a loop $\gamma\colon S^1\to L_k\mathrm{Gr}(V)$. Then $\langle w_1(\underline{det}), \gamma\rangle = \langle w_2, T_\gamma \rangle$, where $w_2$ is the second SW class of the universal totally real bundle on $\mathrm{Gr}(V)$, and $T_\gamma\colon S^1\times S^1\to \mathrm{Gr}(V)$ is the torus of boundary values swept out by $\gamma$.

So, in a space of pseudo-holomorphic discs in a symplectic manifold $X$ attached to an orientable Lagrangian $\Lambda$, $w_1$ of the determinant bundle evaluates on a loop $\gamma$ by evaluating the torus of boundary values $T_\gamma$ on $w_2(T\Lambda)$. Essentially for this reason, it's natural to trivialize the determinant bundle by trivializing $w_2(\Lambda)$, i.e. specifying a Pin structure (though this isn't quite right because $TX$ isn't trivialized; one actually needs a Pin structure relative to $TX$). This is perhaps the only uniform way to orient moduli space spaces of pseudo-holomorphic discs.

2) For Cauchy-Riemann operators on other curves, one can degenerate to a nodal union of discs and closed Riemann surfaces, combining the orientations for the space of discs with the complex orientation of the determinant line bundle over the moduli of closed curves. Thus no additional obstructions to orientation appear. Technically, one is using the excision properties of elliptic operators.

3) I don't know, but there are further concrete calculations for real loci in the work of Welschinger and also Solomon.

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