`2A = A + A = C`

`A = (x_a, y_a)`

`C = (x_c, y_c)`

`λ = (3x_a^2 + 2a_2x_a - a_1y_a + a_4)/(2y_a + a_1x_a + a_3)`

`x_c = λ^2 + λa_1 - a_2 -2x_a`

`y_c = -a_1x_c - a_3 - λx_c + λx_a - y_a`

$$Y^2 + a_1XY + a_3Y = X^3 + a_2X^2 + a_4X + a_6$$