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The Hodge realization of the polylogarithm and the Shintani generating class for totally real fields. (English) Zbl 07857681

The Beilinson conjecture for algebraic varieties and more general motives defined over algebraic number fields is a conjecture relating the special values of Hasse-Weil \(L\)-functions to the determinant of the regulator map from motivic cohomology to Deligne-Beilinson cohomology. This conjecture is a vast generalization of the Gross conjecture for special values of Artin \(L\)-functions B. H. Gross [Pure Appl. Math. Q. 1, No. 1, 1–13 (2005; Zbl 1169.11050)], based on the work by A. Borel [Ann. Sc. Norm. Super. Pisa, Cl. Sci., IV. Ser. 4, 613–636 (1977; Zbl 0382.57027)] on special values of Dedeking zeta functions. The general strategy for the proof of this conjecture is to construct explicit elements in motivic cohomology whose image by the regulator map is amenable to calculation. Beilinson originally proved the Beilinson conjecture for the noncritical values of Dirichlet \(L\)-functions by constructing the cyclotomic elements in the motivic cohomology, or equivalently \(K\)-groups, associated to the spectrum of cyclotomic fields [A. A. Beilinson, J. Sov. Math. 30, 2036–2070 (1985; Zbl 0588.14013); translation from Itogi Nauki Tekh., Ser. Sovrem. Probl. Mat. 24, 181–238 (1984); J. Neukirch, Perspect. Math. 4, 193–247 (1988; Zbl 0651.12009)]. Beilinson and Deligne provided a universal construction of the cyclotomic elements in terms of the motivic polylogarithm, a class constructed in the motivic cohomology of the projective line minus three points. This allows for a simplified and conceptual proof of the Beilinson conjecture for the Dirichlet \(L\)-function, by reducing the complicated calculation of the regulator to the calculation of the Hodge realization of the polylogarithm class [A. Beilinson and P. Deligne, Proc. Symp. Pure Math. 55, 97–121 (1994; Zbl 0799.19004); P. Deligne, Publ., Math. Sci. Res. Inst. 16, . 79–297 (1989; Zbl 0742.14022); A. Huber and J. Wildeshaus, Doc. Math. 3, 27–133 (1998; Zbl 0906.19004)]. Although perhaps not written explicitly in literature, similar strategy may be applied to the proof of the Beilinson conjecture for Hecke character associated to imaginary quadratic fields [C. Deninger, Symp. Math. 37, 99–137 (1997; Zbl 0888.14002)], originally proved by C. Deninger [Invent. Math. 96, No. 1, 1–69 (1989; Zbl 0721.14004); Ann. Math. (2) 132, No. 1, 131–158 (1990; Zbl 0721.14005)], using the elliptic polylogarithm constructed by A. Beilinson and A. Levin [Proc. Symp. Pure Math. 55, 123–190 (1994; Zbl 0817.14014)].
In this article, we prospect a similar strategy for Hecke \(L\)-functions of totally real fields, using an equivariant version of the polylogarithm for certain algebraic tori associated to totally real fields.
Let \(F\) be a totally real field and \(g=[F\colon\mathbb Q]\). We denote by \(\mathfrak{I}\) the set of nonzero fractional ideals of \(F\), and by the multiplicative group of totally positive elements of \(F\). For any \(\mathfrak{a}\in\mathfrak{I}\), we let \(\mathbb T^{\mathfrak{a}}=\mathrm{Hom}(\mathfrak{a},\mathbb G_m),\) which is a \(g\)-dimensional affine group scheme over \(\mathbb Q\). If we let \begin{align*} U&=\mathrm{coprod}_{\mathfrak{a}\in\mathfrak{I}}U^{\mathfrak{a}} \end{align*} for \(U^{\mathfrak{a}}=\mathbb T^{\mathfrak{a}}\setminus\{1\}\), then \(U\) has a natural action of \(F\times_+\). We regard the quotient stack \(U/F\times_+\) as a generalization of \(\mathbb P^1\setminus\{0,1,\infty\}\); indeed, when \(F=\mathbb Q\), we have \(U/F\times_+\cong\mathbb G_m\setminus\{1\}=\mathbb P^1\setminus\{0,1,\infty\}\).
The polylogarithm classes of general commutative group schemes were constructed by A. Huber and G. Kings [J. Algebr. Geom. 27, No. 3, 449–495 (2018; Zbl 1464.14012)]. We consider an equivariant version of their construction in the case of \(U/F\times_+\). More precisely, we construct an element of the equivariant Deligne-Beilinson cohomology \(H^{2g-1}_{\Delta}(U/F\times_+,\mathbb L\mathrm{og})\) with coefficients in the logarithm sheaf \(\mathbb L\mathrm{og}\), which is a certain equivariant pro-unipotent variation of mixed \(\mathbb R\)-Hodge structures on \(U\). Since the general theory of equivariant variations of mixed \(\mathbb R\)-Hodge structures and equivariant Deligne-Beilinson cohomology are not yet sufficiently developed, we give a definition of equivariant Deligne-Beilinson cohomology \(H^{m}_{\Delta}(U/F\times_+,\mathbb L\mathrm{og})\) specific to our case, using the logarithmic Dolbeault complex.
We define the polylogarithm class in equivariant Deligne-Beilinson cohomology as follows.
Definition polylog. Let \(\mathrm{Cl}^+_F\) denote the narrow ideal class group of \(F\). We define the polylogarithm class \[ \operatorname{pol}\in H^{2g-1}_{\Delta}(U/F\times_+,\mathbb L\mathrm{og}) \] to be the class in the equivariant Deligne-Beilinson cohomology of \(U\) with coefficients in \(\mathbb L\mathrm{og}\) that maps to \((1,\ldots,1)\in \bigoplus_{\mathrm{Cl}^+_F}\mathbb R\) through the isomorphism \(H^{2g-1}_{\Delta}(U/F\times_+,\mathbb L\mathrm{og})\cong\bigoplus_{\mathrm{Cl}^+_F}\mathbb R\) given in Proposition (key).
When \(F=\mathbb Q\), the polylogarithm class \(\operatorname{pol}\) coincides with the Hodge realization of the polylogarithm class constructed by Beilinson and Deligne on the projective line minus three points.
Previously in [K. Bannai et al., Asian J. Math. 24, No. 1, 31–76 (2020; Zbl 1457.14020)], we constructed the Shintani generating class, which is a certain equivariant cohomology class on \(U\) generating the values of the Lerch zeta function of \(F\) at nonpositive integers. Our main theorem relates our polylogarithm class with the Shintani generating class.
Theorem 1. Under a natural inclusion \[ H^{2g-1}_{\Delta}(U/F\times_+,\mathbb L\mathrm{og})\hookrightarrow H^{2g-1}_{\mathrm{dR}}(U/F\times_+,\mathbb L\mathrm{og}\otimes\mathscr O_U), \] the polylogarithm class \(\operatorname{pol}\) maps to the de Rham Shintani class \(\mathcal S\), constructed from the Shintani generating class.
Theorem 1 indicates that our polylogarithm class is related to the critical values of Hecke \(L\)-functions, since these values are written in terms of the Lerch zeta values at nonpositive integers. We further expect that our polylogarithm class is related to the Lerch zeta values at positive integers, and hence to the noncritical Hecke \(L\)-values. Let \(\xi\) be a nontrivial torsion point in \(U\), and let \(\xi\Delta\) be the \(\Delta\) orbit of \(\xi\) with respect to the action of \(\Delta=\mathcal{O}_{F+}^\times\). Assuming further the existence of a conjectural equivariant plectic Hodge theory, fitting in with the formalism of the category of mixed plectic \(\mathbb R\)-Hodge structures \(\mathrm{MHS}^{\boxtimes I}_{\mathbb R}\) defined by J. Nekovář and A. J. Scholl [Contemp. Math. 664, 321–337 (2016; Zbl 1402.11092)], we may define the specialization \[ i_{\xi\Delta}^*\colon H^{2g-1}_{\Delta}(U/F\times_+,\mathbb L\mathrm{og}) \rightarrow H^{2g-1}_{\Delta^I}(\xi\Delta/\Delta,i_{\xi\Delta}^*\mathbb L\mathrm{og}) \cong\prod_{n=1}^\infty(2\pi i)^{(n-1)g}\mathbb R\tag{1} \] with respect to the equivariant morphism \(i_{\xi\Delta}\colon\xi\Delta\rightarrow U\). We propose the following:
Conjecture 1. Let \(\xi\) be a torsion point in \(U\), and we let \(\xi\Delta\) be the orbit of \(\xi\) in \(U\). Then \(i_{\xi\Delta}^*\operatorname{pol}\in H^{2g-1}_{\Delta^I}(\xi\Delta/\Delta,i_{\xi\Delta}^*\mathbb L\mathrm{og})=\prod_{n=1}^\infty(2\pi i)^{(n-1)g}\mathbb R\) satisfies \[ i_{\xi\Delta}^*\operatorname{pol}=(d_F^{1/2}\mathcal{L}^\infty(\xi\Delta,n))_{n=1}^\infty\in\prod_{n=1}^\infty(2\pi i)^{(n-1)g}\mathbb R. \] Here \(d_F\) denotes the discriminant of \(F\) and \[ \mathcal{L}^\infty(\xi\Delta,n)=\begin{cases} \mathrm{Re}\mathcal{L}(\xi\Delta,n) & \text{ if }(n-1)g\text{ is even}, \\ i\,\mathrm{Im}\mathcal{L}(\xi\Delta,n) & \text{ if }(n-1)g\text{ is odd}, \end{cases} \] where \(\mathcal L(\xi\Delta,s)\) is the Lerch zeta function corresponding to the point \(\xi\) (see Definition 1).
In a subsequent research, we will also consider a syntomic version of this conjecture.
If \(F=\mathbb Q\), then the Lerch zeta function for a nontrivial torsion point \(\xi\in\mathbb G_m(\mathbb C)\) is given as \(\mathcal L(\xi,s)=\sum_{n=1}^\infty \xi^n n^{-s}\), hence we have \(\mathcal L(\xi,k)=\mathrm{Li}_k(\xi)\), where \(\mathrm{Li}_k(\xi)\) is the value at \(\xi\) of the \(k\)-th polylogarithm function given by the series \(\mathrm{Li}_k(t)=\sum_{n=1}^\infty t^n n^{-k}\). Therefore Conjecture 1 is a direct generalization of the result of Beilinson and Deligne on the calculation of the specialization of \(\operatorname{pol}\) at nontrivial roots of unity. Recall that this calculation and the motivicity of the polylogarithm class lead to a conceptual proof of the Beilinson conjecture for the Dirichlet \(L\)-functions.

MSC:

11G55 Polylogarithms and relations with \(K\)-theory
14C30 Transcendental methods, Hodge theory (algebro-geometric aspects)
11M35 Hurwitz and Lerch zeta functions
11R42 Zeta functions and \(L\)-functions of number fields
11R80 Totally real fields

References:

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