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The $j$-invariant of an elliptic curve $E$ defined over $\mathbb{Q}$ is an invariant of the isomorphism class of $E$ over $\overline{\mathbb{Q}}$ . If the Weierstrass equation of $E$ is \[y^2+a_1xy+a_3y=x^3+a_2x^2+a_4x+a_6,\] then its $j$-invariant is given by \[j= \frac{c_4^3}{\Delta}\] where \[\begin{aligned} b_2 &= a_1^2 + 4 a_2\\ b_4 &= 2a_4 + a_1 a_3\\ b_6 &= a_3^2 + 4 a_6 \\ b_8 &= a_1^2 a_6 + 4 a_2 a_6 - a_1 a_3 a_4 + a_2 a_3^2 - a_4^2\\ c_4 &= b_2^2 - 24b_4 \end{aligned}\] and \[\Delta=-b_2^2b_8 - 8 b_4^3 -27 b_6 ^2 + 9 b_2 b_4 b_6\] is the discriminant of $E.$

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  • Last edited by Andrew Sutherland on 2018-12-13 05:49:35
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