Musings on $\Q(1/4)$: Arithmetic spin structures on elliptic curves

We introduce and study arithmetic spin structures on elliptic curves. We show that there is a unique isogeny class of elliptic curves over $\F_{p^2}$ which carries a unique arithmetic spin structure and provides a geometric object of weight $1/2$ in the sense of Deligne and Grothendieck. This object is thus a candidate for $\Q(1/4)$.

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Published 1 January 2010