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334 Chapter 7 ELLIPTIC CURVE ARITHMETIC<br />

generalized to fields F p k as follows:<br />

#E <br />

k<br />

Fpk = p +1+<br />

x∈F p k<br />

χ(x 3 + ax + b),<br />

where χ is the quadratic character for F p k. (That is, χ(u) = 1, −1, 0,<br />

respectively, depending on whether u is a nonzero square in the field, not<br />

a square, or 0.) A celebrated result of H. Hasse is the following:<br />

Theorem 7.3.1 (Hasse). The order #E of Ea,b(F p k) satisfies<br />

<br />

(#E) − (p k +1) ≤ 2 p k .<br />

This remarkable result strikes to the very heart of elliptic curve theory and<br />

applications thereof. Looking at the Hasse inequality for Fp, we see that<br />

p +1− 2 √ p

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