Is hasse bound of elliptic curve larger when we discard smoothness

algebraic-geometryarithmetic-geometryelliptic-curvesnumber theory

Let $E$ be an elliptic curve over finite field $\Bbb{F}_q$.

The inequality $\mid 1+q-\sharp E(\Bbb{F}_q)\mid\leq 2\sqrt{q}$・・・① is well known.

My question is, does this inequality still hold if we discard smoothness of $E$, in other words, does this inequality hold for singular genus $1$ curves?

The proof of this theorem seems invalid if we discard smoothness, but I'm stucking with finding counterexamples of ①, that is,
I want to know an example of singular genus $1$ curve $C$ and prime $p$ which satisfies $\mid 1+p-\sharp C(\Bbb{F}_p)\mid > 2\sqrt{p}$.

Best Answer

There is the paper "A Weil Theorem for Singular Curves" by Y Aubry and M Perret in the monograph Arithmetic Geometry and Coding Theory, Pelikaan et al, de Gruyter, 1996.

We generalize Weil's theorem on the number of rational points of smooth curves over a finite field to singular ones

If you google by title you can find a scan of the paper. I am not an expert in this but it seems that $$ | 1+q-\#E(\mathbb{F}_q) | \leq 2 g \sqrt{q}+\Delta_E\leq 2 g \sqrt{q}+\pi-g\leq2 \pi \sqrt{g} $$

where $\pi$ is the arithmetic genus and $g$ the geometric genus of the curve, $$ \Delta_x=\#\{\tilde{E}(\mathbb{F}_q)\setminus E(\mathbb{F}_q)\}, $$ and $\tilde{E}$ is the normalization of the curve $E$ with an explicit product of cyclotomic polynomials.

Also, Bach, E. (1996). "Weil bounds for singular curves", Applicable Algebra in Engineering, Communication and Computing 7(4), 289–298. doi:10.1007/bf01195534

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