# Rendiconti del Seminario Matematico della Università di Padova

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**Volume 134, 2015, pp. 197–237**

**DOI: 10.4171/RSMUP/134-5**

Published online: 2015-11-17

The cyclic and epicyclic sites

Alain Connes^{[1]}and Caterina Consani

^{[2]}(1) Institut des Hautes Études Scientifiques, Bures-Sur-Yvette, France

(2) The Johns Hopkins University, Baltimore, USA

We determine the points of the epicyclic topos which plays a key role in the geometric encoding of cyclic homology and the lambda operations. We show that the category of points of the epicyclic topos is equivalent to projective geometry in characteristic one over algebraic extensions of the infinite semifield of "max-plus integers" ${\mathbb F}_{\rm max}$. An object of this category is a pair $(E,K)$ of a semimodules $E$ over an algebraic extension $K$ of ${\mathbb Z}_{\rm max}$. The morphisms are projective classes of semilinear maps between semimodules. The epicyclic topos sits over the arithmetic topos $\widehat{{\mathbb N}^\times}$ of [6] and the fibers of the associated geometric morphism correspond to the cyclic site. In two appendices we review the role of the cyclic and epicyclic toposes as the geometric structures supporting cyclic homology and the lambda operations.

*Keywords: *Grothendieck topos, cyclic category, groupoids, characteristic one, projective geometry

Connes Alain, Consani Caterina: The cyclic and epicyclic sites. *Rend. Sem. Mat. Univ. Padova* 134 (2015), 197-237. doi: 10.4171/RSMUP/134-5