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Elliptic Group Modulo p


E(a,b)/p denotes the elliptic group modulo p whose elements are 1 and infty together with the pairs of integers (x,y) with 0<=x,y<p satisfying

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

with a and b integers such that

 4a^3+27b^2≢0 (mod p).
(2)

Given (x_1,y_1), define

 (x_i,y_i)=(x_1,y_1)^i (mod p).
(3)

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, although this formula quickly becomes impractical. However, it has been proven that

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

Furthermore, for p a prime >3 and integer n in the above interval, there exists a and b such that

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

and the orders of elliptic groups mod p are nearly uniformly distributed in the interval.


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Cite this as:

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

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