Nonsingular plane cubic curves over finite fields
Journal of Combinatorial Theory Series A
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On average, there are q^r+o(q^r^/^2) F"q"^"r-rational points on curves of genus g defined over F"q"^"r. This is also true if we restrict our average to genus g curves defined over F"q, provided r is odd or r2g. However, if r=2,4,6,... or 2g then the average is q^r+q^r^/^2+o(q^r^/^2). We give a number of proofs of the existence of these q^r^/^2 extra points, and in some cases give a precise formula, but we are unable to provide a satisfactory explanation for this phenomenon.