Formal Rings

Abstract

A notion of one-dimensional formal ring is presented. It consists of a triple $(A,\Phi,\Psi)$ where $A$ is a unital ring and $\Phi$ and $\Psi$ are two formal power series in $2$ variables ${\Phi(x,y),\Psi(x,y)\in A\llbracket x,y\rrbracket}$, the first one defining a one-dimensional formal group law over $A$ and the second one providing a second composition law satisfying axiomatic properties of compatibility with the first one. For a characteristic-zero ring $A$, a large class of one-dimensional formal rings can be obtained by constructing a new composition law, defined in terms of the group logarithm associated with a given formal group law $\Phi(x,y)$, and associative and distributive with respect to it. A natural $n$-dimensional generalization of the previous construction is also proposed; curves on $n$-dimensional formal rings are introduced. The higher-dimensional theory allows us to define a generalization of the ring $W(A)$ of Witt vectors over a ring $A$, which is recovered by means of a specific choice of the associated group logarithm. The composition laws of our generalized Witt ring are defined in terms of an underlying formal ring structure. Examples of formal rings related to Hirzebruch's theory of genera are explicitly computed. Finally, we also propose the examples of Euler's and Abel's formal rings.