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Let $A$ be a complex abelian variety of dimension $d$. Let $\omega_1, \ldots, \omega_j \in H^0(A, \Omega^1_A)$ be linearly independent (so $j \leq d$) and consider $\gamma_1, \ldots, \gamma_j \in H_1(A(\mathbb{C}), \mathbb{Q})$. Look at the matrix

$M=(\int_{\gamma_i} \omega_j)$

Is it possible to express the determinant of $M$ as the integral of $\omega_1 \wedge \ldots \wedge \omega_j \in H^0(A, \Omega^j_A)$ against some element of $H_j(A(\mathbb{C}), \mathbb{Q})$ constructed out of $\gamma_1, \ldots, \gamma_j$?

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  • $\begingroup$ What if the dimension of $X$ is less than $n$? Do you want to assume that $X$ is a complex torus and $\omega_1,\dots,\omega_n$ form a basis? $\endgroup$ Commented May 27, 2014 at 13:59
  • $\begingroup$ In that case both sides are zero, no? But you are right that the case I have in mind is $X$ an abelian variety of dimension $n$ and $\omega_i$ a basis of $H^0(X, \Omega^1_X)$ $\endgroup$
    – detted92
    Commented May 27, 2014 at 14:01
  • $\begingroup$ "In that case both sides are zero, no?" Why? Certainly the integral you write is zero, but that does not imply that the determinant is zero. That is why I raised my objection. $\endgroup$ Commented May 27, 2014 at 14:04
  • $\begingroup$ What is $\gamma _1\cup\ldots \cup \gamma _n$? $\endgroup$
    – abx
    Commented May 27, 2014 at 14:04
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    $\begingroup$ You can't take the cup-product of homology classes! Cup-product is an operation on cohomology, so the cycle you're integrating over is nonsensical as soon as $n > 1$. $\endgroup$ Commented May 27, 2014 at 14:26

1 Answer 1

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You can form the $j$-fold self product, $A^j$, together with its addition morphism to $A$, $$\Sigma:A^j \to A.$$ Via Künneth, you have an inclusion $$\bigotimes_{i=1}^j H_1(A;\mathbb{Q}) \hookrightarrow H_j(A^j;\mathbb{Q}).$$ Now take the image of the cycle $\gamma_1\otimes \dots \otimes \gamma_j$ in $H_j(A^j;\mathbb{Q})$, and then take the pushforward via the morphism $\Sigma$ (proper and locally a fiber bundle) to get a class $\gamma\in H_j(A;\mathbb{Q})$. Similarly, given de Rham differentials, $$\omega_1,\dots,\omega_j \in H^{1,0}(A),$$ you can form $\omega= \omega_1\wedge \dots \wedge \omega_j$. Consider $\int_\gamma \omega$, or equivalently, the pairing, $$\int_\gamma \omega = \langle \Sigma^*(\omega_1\wedge \dots \wedge \omega_j),\gamma_1\otimes \dots \otimes \gamma_j\rangle.$$
Then you can ask whether or not, $$ \int_\gamma \omega = \text{det}\left[\int_{\gamma_\alpha} \omega_\beta\right]_{1\leq \alpha,\beta \leq j}?$$

Of course, ultimately, this has nothing to do with the differentials $\omega_\beta$ being contained in $H^{1,0}(A)$. Really this is a question about the singular cohomology of $A$: how does the Hopf algebra structure induced by addition behave? That question answers itself: the cohomology is a Hopf algebra. More precisely, it is the free exterior algebra on $H^1(A;\mathbb{Q})$ (with the usual trace on the top exterior power) equipped with its standard structure of cocommutative Hopf algebra. In particular, considered as an element in $H^1(A;\mathbb{Q})\otimes H^1(A;\mathbb{Q})$, $\Delta(\omega_\beta)$ equals $\omega_\beta\otimes 1 + 1\otimes \omega_\beta$, where $\Delta$ is pullback via addition, $$m:A\times A \to A,$$ together with the Künneth isomorphism, $$H^1(A\times A;\mathbb{Q}) = \left(H^1(A;\mathbb{Q})\otimes \mathbb{Q}\right) \oplus \left(\mathbb{Q}\otimes H^1(A;\mathbb{Q}) \right).$$ By my computation, this does imply that the Künneth component in $H^1(A;\mathbb{Q})\otimes \dots \otimes H^1(A;\mathbb{Q})$ of $\Sigma^*(\omega_1\wedge \dots \wedge \omega_j)$ is equal to $$\sum_{\sigma\in{\mathfrak{S}_j}} \text{sgn}(\sigma) \omega_{\sigma(1)}\otimes \dots \otimes \omega_{\sigma(j)}$$ (it would be wise for you to double-check that computation). Thus the pairing against $\gamma_1\otimes \dots \otimes \gamma_j$ equals $$\int_\gamma \omega = \sum_{\sigma\in{\mathfrak{S}_j}} \text{sgn}(\sigma) \langle \omega_{\sigma(1)}\otimes \dots \otimes \omega_{\sigma(j)}, \gamma_1\otimes \dots \otimes\gamma_j \rangle = \text{det}\left[\int_{\gamma_\alpha} \omega_\beta\right]_{1\leq \alpha,\beta \leq j}.$$

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