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Elliptic Group Modulo p


An elliptic group modulo a prime p>3 is the abelian group of rational points on an elliptic curve over the finite field F_p. For integers a and b, let E(a,b)/p denote the group consisting of the pairs of integers (x,y) with 0<=x,y<p satisfying

 y^2=x^3+ax+b (mod p)
(1)

together with one point at infinity O, which is the identity element. Assume

 4a^3+27b^2≢0 (mod p).
(2)

For a positive integer i and P=(x_1,y_1), the notation [i]P denotes the sum of i copies of P under the group operation. When [i]P!=O, write

 [i]P=(x_i,y_i).
(3)

These are coordinates in F_p (Sutherland 2023a, p. 1). The group order h of E(a,b)/p is given by

 h=1+sum_(x=1)^p[((x^3+ax+b)/p)+1],
(4)

where ((x^3+ax+b)/p) is the Legendre symbol (Sutherland 2023b, p. 2). The group order satisfies

 p+1-2sqrt(p)<=h(E(a,b)/p)<=p+1+2sqrt(p).
(5)

For every integer n in this interval, there exist a and b such that

 h(E(a,b)/p)=n
(6)

(Sutherland 2023b, pp. 1-2).


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References

Sutherland, A. "Elliptic Curves as Abelian Groups." Lecture 2 in 18.783 Elliptic Curves. Sep. 12, 2023a. https://math.mit.edu/classes/18.783/2023/LectureNotes2.pdf.Sutherland, A. "Point Counting." Lecture 7 in 18.783 Elliptic Curves. Oct. 3, 2023b. https://math.mit.edu/classes/18.783/2023/LectureNotes7.pdf.

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Elliptic Group Modulo p

Cite this as:

Weisstein, Eric W. "Elliptic Group Modulo p." From MathWorld--A Wolfram Resource. https://mathworld.wolfram.com/EllipticGroupModulop.html

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