Let $K$ be a finite extension of $\mathbb{Q}_p$ with absolute Galois group $G_K$. Let $A$ be an abelian variety defined over $K$. The (geometric) Tamagawa number is defined as the order of the quotient $$c(A/K)=A(K)/A_0(K)$$ where $A_0(K)$ denotes the $K$-points of $A$ that are sent to the identity component of the special fiber of the Néron model of $A$ via the natural reduction map.

On the other hand, following Fontaine and Perrin-Riou (in "Cohomologie galoisienne et valeurs de fonctions L", §4), starting with the $p$-adic Tate module $T_p=T_p(A)$, one may define the ($p$-adic) Tamagawa number ${\rm Tam}_\omega(T_p)$. The latter depends on a choice of a generator $\omega\in\det(t_{V_p})^*$, where $V_p=T_p\otimes\mathbb Q_p$ and $t_{V_p}=(B_{\rm dR}/B_{\rm dR}^+\otimes V_p)^{G_K}$.

As suggested by the vocabulary, can one expect a direct relation between $c(A/K)$ and ${\rm Tam}_\omega(T_p)$ to hold? Is there a proof of this?

The number ${\rm Tam}_\omega(T_p)$ is a power of $p$, so one might expect it to be comparable with the $p$-part of $c(A/K)$. However, one would need to have an almost cannonical choice for $\omega$.


1 Answer 1


Denote by $\Phi$ the quotient of $\mathcal A^\vee$, the special fiber of the smooth (but not necessarily proper) model of the dual abelian variety $A^\vee$, by the connected component of $0$ of $\mathcal A^\vee$. Then it is indeed true that the Tamagawa number $c_p$ as defined in $L$-functions and Tamagawa number of motives (Bloch, Kato) or in Autour des conjectures de Bloch-Kato (Fontaine, Perrin-Riou) is then equal up to a $p$-adic unit to $|\Phi[p^\infty]|$, but this is not quite obvious.

The main step of the proofs are as follows:

  • First, relate $\Phi$ to the formal group of $A^\vee$, and then through the $\log$ map to $\operatorname{Lie}_{\mathbb Z_p}A^{\vee}$ (which is also the tangent space of the formal group with values in $\mathbb Z_p$).
  • Then notice that this induces isomorphisms between the determinant of $\operatorname{Lie}_{\mathbb Z_p}A^{\vee}$ and the determinant of the first Bloch-Kato cohomology group $H^1_f(\mathbb Q_p,T_pA)$ of the $p$-adic Tate module of $A$. This isomorphism involves finite corrective terms which come from the fact that the formal group is a subgroup with finite index of $H^1_f(\mathbb Q_p,T_pA)$ (and similarly for the image of the $\log$ with respect to $\operatorname{Lie}_{\mathbb Z_p}A^{\vee}$).
  • Now the Tamagawa number $c_p$ arises as an alternate product of these corrective terms and of the Euler factor at $p$ (because the exponential map of Bloch-Kato coincide in this case with $\log$).
  • Finally, relate this alternate product to $\Phi$. This involves the classical description of $\Phi$ in terms of the Neron model as well as a study of the reduction of $A^{\vee}$.

Personally, I don't find any of these steps particularly easy, and the last one certainly isn't. I don't have a good reference to mention that would cover all of them. A good but hard reference for the hardest part is SGA7, Exposé IX.

One thing that I would mention is that, as you say, the computations of the proof above involve in several steps the choice of a $\mathbb Z_p$-basis, but as this plays a role both in the normalization of maps with target $\operatorname{Lie}A^\vee$ as well as in the definition of $c_p$, the resulting computation does not depend on the choice of the basis (up to a $p$-adic unit, which plays no role in the Tamagawa number conjectures of Bloch-Kato).


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